Block 2. DNA. Questions 5,6,7.

DNA structure. Model J. Watson and F. Cry. Properties and functions of hereditary material.

Self-reproduction of genetic material. DNA replication.

Organization of hereditary material in pro- and eukaryotes. Classification of nucleotide sequences in the eukaryot genome (unique, medium-reserving, high-fuel).

In 1868, the Swiss chemist F. Misher discovered in cell nuclei isolated from pus, and later from sperm salmon, which he called "nuclei" (from Lat. Nucleus - core). Subsequently, R. Altmann (1889) reported that the "Nuclein" allocated by F. Misher consists of two fractions - protein and nucleic acids. Nucleic acids, like proteins, possess the primary structure (under which their nucleotide sequence is meant) and a three-dimensional structure. Interest in DNA structure intensified when at the beginning of the XX century. There was an assumption that DNAmay be a genetic material. In 1952, Chargeff was opened by the Communication Rule, appointed later by the name of the Creator. It lies in the fact that:

1. The amount of adenine is equal to the amount of thymine, and guanin - cytosine: a \u003d t, r \u003d c.
2. The number of purines is equal to the amount of pyrimidines: a + r \u003d t + c.
3. The number of bases with amino groups in position 6 is equal to the amount of bases with keto groups in position 6: a + c \u003d r + t.

Subsequently, Wilkinson obtained a radiograph of DNA. And a few later, Watson and Creek in 1953 offered their own DNA model for which the Nobel Prize in 1962 was awarded with Wilkinson.

The basic principles of the structure of DNA.

1. DNA-nucleotide monomer consisting of a nitrogen base, deoxyribose and phosphoric acid residue. Nitrogen bases can be purinovye A, g or pyrimidine C, t.

2. Nitrogen bases are connected to C1 carbon atom in the pentose molecule, and phosphate is joined to C5. The third atom always has a group IS HE.

3. When the phosphate interacts with one nucleotide with hydroxyl deoxyribose of the other is established phosphodieter communication.

4. The connection of nucleotides occurs through it pentoses to the C3 position and the phosphate of the subsequent nucleotide.

5. DNA is a double polynucleotide chain. Two polynucleotide chains are interconnected by hydrogen bonds the principle of complimentality, A - T and Mr. Between a and t two hydrogen bonds, between T and C are three hydrogen bonds.

6. Anti-parallelity.5 The end of one chain is connected to the end of another chain.

7.Diamet DNA helix is \u200b\u200b2 nm, and the length of the step is 3.4 nm. For each round there are 10 pairs of nucleotides.

8. Primary structure- polynucleotide chain.

Secondary structure- Two complimentary to each other anti-parallel polynucleotide chains.

Tertiary structure- Three-dimensional spiral.

9. DNA has the ability to replicate.

Replication.

1 - DNA matrix chains; 2 - Helpasis enzyme separating the chains of matrix DNA; 3 - DSB proteins that prevent the reunification of DNA chains; 4 - Praimaz; 5 - RNA seed (synthesized RNA polymerase - pricymia); 6 - DNPOLIMOSAZ, synthesizing subsidiaries; 7 - DNA leading daughter chain; 8 - ligase connecting fragments of the lagging chain of DNA; 9 - a fragment of the provision (150-200 nucleotides); 10 - Topoisomeraza

The synthesis of the new DNA molecule is carried out by a semi-reparvative way. This means that the daughter molecule will contain one maternal and one newly synthesized chain. Since DNA synthesis occurs on a single-stranded matrix, it is preceded by a mandatory temporary separation of two chains, with the formation of a replicative fork. With the help of an electron microscope, they revealed that the replication area is of the eye of the eye inside the non-reclied DNA (replication peephole consisting of about 300 nucleotides).

Replicon - DNA fragment on the start point of replication to the point of its end.

To cut the DNA spiral need special enzymes (proteins).Several enzymes take part in replication, each of which performs its function.

DNA HELIKAZ (Helikaza) Bare hydrogen bonds between the bases, share the chains and promote a replicative fork.

Destabilizing proteins Hold chains.

DNA -Topoisomerase. Recall that DNA is a spiral.Accordingly, that the plug can move forward, the spiral must quickly spin. But it will require a lot of energy loss. In fact, this still does not happen. DNA topoisomerases contribute to this. They contribute to chain and double-stranded gaps that allow the chains to divide, and then eliminate these breaks. Thanks, one of the chains of DNA begins to rotate around the second chain. They also participate in the trip of the rings formed when ring DNA replication.

Synthesis of DNA chains is due to DNA polymerase. But this enzyme has a feature. It is able to add nucleotides to 3 end of the already existing chain. Such a pre-educated chain called seed which synthesizes praimaz. RNA seed differs from the rest of the DNA chain, as it has ribose. The size of the seed is small. The seed function has been removed by a special enzyme, and the formation form is eliminated. DNA polymerace(In this case, instead of the seed, it uses a 3D DNA fragment of the DNA fragment).

DNA replication suggests that the synthesis of two chains occurs simultaneously. But in fact, everything happens not quite so. Recall that chains anti-parallel.And the synthesis of the new chain can occur only in the direction from 5 end to 3. Therefore, continuously synthesis occurs only on one chain (leading).On the second (lagging), it occurs fragments of the provision. The synthesis of each fragment is carried out using a RNA seed. Then the seeds are removed, the bars are filled with DNA polymerase and fragments are sewn by the enzyme ligase .

Structural and functional organization DNA in pro- and eukaryotes

Examine the tables, rewrite them into the workbook.

Structural and functional organization of genetic material

4.2 DNA properties as a substance of heredity and variability

4.2.3 Changes in nucleotide DNA sequences.

4.2.4 Elementary units of variability of genetic material. Muton. Recon.

4.2.6 Mechanisms that reduce the adverse effect of gene mutations

4.3 Use of genetic information in life processes

4.3.2 Features of the organization and expression of genetic information from pro - and eukaryot

1. Heredity and variability - the fundamental properties of the living

Life as a special phenomenon is characterized by the duration of existence in time (on Earth, it originated more than 3.5 billion years ago), which is ensured by the continuity of generations of living systems. Changes generations of cells in the body, change of generations of organisms in populations, change of species in the bioscenosis system, the change of biocenoses forming the biosphere. The continuous existence of life in time is the ability of living systems to self-reproduction. The preservation of life in changing conditions is possible due to the evolution of living forms, in the process of which they appear changes, providing adaptation to a new habitat. The continuity of the existence and the historical development of wildlife is due to two fundamental properties of life: heredity and variability.

In training courses, the properties of heredity and variability are traditionally considered regarding the cell and the body. In fact, they are also manifested at the overshoot levels. On cellular and organized (ontogenetic) levels of the organization of a living under heredity understand the property of cells or organisms during the self-reproduction process to transmit the new generation to a certain type of metabolism and individual development, during which they form general features and properties of this type of cells and the type of organisms, As well as some individual peculiarities of parents. In the population-species level of organization of life, heredity is manifested in maintaining the constant relationship of various genetic forms in a number of generations of organisms of this population (species). At the biocenotic level, the prolonged existence of biocenosis is ensured by the preservation of certain ratios of the types of organisms forming this biocenosis.

In the course of the emergence and development of life on Earth, heredity played a decisive role, since it was fixed in a number of generations, biologically useful evolutionary acquisitions, providing certain conservatism of the organization of living systems. Heredity is one of the main factors of evolution.

The prolonged existence of wildlife in time against the background of changing conditions would be impossible if living systems did not have the ability to acquire and maintain some changes beneficial in the new environment environments. The property of living systems acquire changes and exist in various versions is called variability.

In individual cells and organisms of one type, the variability, affecting their individual development, is manifested in the emergence of differences between them. In the population-species level of life organization, this property is manifested in the presence of genetic differences between individual populations of the species, which underlies the formation of new species. The emergence of new species makes changes to intervidal relationships in biocenoses. The variability in a certain sense reflects the dynamism of the organization of living systems and along with heredity is the leading factor in evolution. Despite the fact that in terms of its results, the heredity and variability of multidirectional, in the wilderness of these two fundamental properties form inseparable unity, which is achieved at the same time preservation in the process of the evolution of the existing biologically expedient qualities and the emergence of new ones that make it possible to exist in a variety of conditions.

2. The history of the formation of ideas about the organization of the material substrate of heredity and variability

Heredity and variability as the most important properties of any living system are ensured by the functioning of a special material substrate. In the course historical Development The biological science of the idea of \u200b\u200bits properties, organizations and chemical nature is constantly expanding and complicated.

In the 60s. XIX century The founder of genetics (the science of heredity and variability) Mendel (1865) expressed the first assumptions about the organization of hereditary material. Based on the results of their experiments on the pea, he concluded that hereditary material was discretened, i.e. represented by individual hereditary deposits responsible for the development of certain signs of organisms. According to Mendel, in the hereditary material of organisms that breed sexually, the development of a separate feature is provided by a pair of allelic deposits that came with sex cells from both parents. When adhere to Games, only one of the pair of allelic deposits falls into each of them, so the gamets are always "clean". In 1909 Johansen called "hereditary deposits" Mendel genes.

80s. XIX century marked by important achievements in the field of cytology: mitosis and meyosis were described - division of respectively somatic and genital cells, during which nuclear structures are distributed naturally between daughter cells (V. Wolteer, 1888).

Data on the nature of the chromosome distribution in the process of cell division was allowed at the beginning of XX V.T. Bovers (1902-1907) and W. Sethore (1902-1903) conclude that the continuity of properties in a number of generations of cells and organisms is determined by the continuity of their chromosomes. Chromosome began to be considered as material carriers of the hereditary program.

The further development of the chromosomal theory of heredity, uniting the idea of \u200b\u200bhereditary deposits and chromosomes, was carried out at the beginning of the XX century. T. Morgan and his staff. In the experiments performed on Drozophile, a previously expressed assumption of the role of chromosomes in ensuring heredity was confirmed. It is established that the genes are placed in chromosomes, located in linear order. The genes of each chromosome form a clutch group, the number of which is determined by the amount of chromosome in the genital cells. The genes of one clutch group are inherited, as a rule, together. However, in some cases, their recombination occurs in connection with the crossliner, the frequency of which depends on the distance between the genes.

Thus, in the chromosomal theory, one of the most important principles of genetics was reflected - the unity of discreteness and continuity of hereditary material.

It should be noted that also at the beginning of the XX century. Facts that proved in the cells of an extrachromic hereditary material in various cytoplasmic structures in various cytoplasmic structures and determining a special cytoplasmic heredity (K. Korrens, 1908) were found.

At about the same time, X. de Frize (1901) found the foundations of the teaching on mutational variability associated with suddenly arising amendments in hereditary deposits or chromosomes, which leads to changes in certain signs of the body. In subsequent years, a mutagenic effect on chromosomes and x-rays genes, radiation radiation, certain chemicals and biological agents was discovered.

As a result of these studies, it became obvious that heredity and variability were due to the functioning of the same material substrate.

In the first decades of the XX century. Data was obtained in favor of the dependence of the status of the characteristics of the nature of the interaction of genes, which went beyond the relationship between the dominance and recession described by Mendel. From here there was an idea of \u200b\u200bthe genetic apparatus as a system of interacting genes - genotype, which is concentrated in a chromosomal kit - a karyotype.

The study of chemical composition chromosomes revealed two main types of compounds forming these structures - proteins and nucleic acids. In the first half of the XX century. The researchers resolved the question of the chemical nature of the substrate of heredity and variability. Originally expressed assumptions in favor of proteins. In 1928 Griffith was raised by experience on pneumococci, in which the change (transformation) of some hereditary properties of one bacterial strain was influenced by the material obtained from the killed cells of another strain. The chemical nature of the substance transforming the hereditary properties of bacteria was established only in 1944. Avery, who proved its belonging to nucleic acids (DNA).

Other evidence of DNA participation in ensuring heredity and variability are:

1) the constancy of the DNA content in all types of somatic cell cells;

2) the correspondence of the DNA of the cell dam cell (in somatic cells it is twice as much as in the genital, in polyploid cells, it corresponds to the number of chromosome sets);

3) the phenomenon of genetic recombination in bacteria in their conjugation, during which the part of the DNA is penetrated from one cell to another and changing the properties of the latter;

4) a change in the hereditary properties of bacterial cells by transferring DNA from one strain to another with the help of DNA phage - a transduction phenomenon;

5) infecting activity of insulated virus nucleic acid.

An important result of a targeted study of nucleic acids was the creation of J. Watson and F. Creek (1953) of the spatial model of the DNA molecule.

In the second half of the XX century. The efforts of scientists are aimed at studying the properties of nucleic acids that make up the basis of their genetic functions, the methods of recording and reading the hereditary information, the nature and structure of the genetic code, the mechanisms for regulating the activity of genes in the process of forming individual signs and phenotype as a whole. In the 60s. Works M. Nirenberg, S. Ochoa, X. The Quran and others were performed complete decoding of the genetic code, the correspondence of nucleotide triplets in the nucleic acid molecule with certain amino acids was established. In the 70s Gender engineering methods began to be actively developed, allowing to target the hereditary properties of living organisms.

By the end of the XX century, due to the new molecular genetic technologies, it became possible to determine the sequences of nucleotides in the DNA DNA molecules of various organisms (reading DNA texts). The DNA texts of the human genome, represented in general, 3 billion pairs of nucleotides are mainly read by 2001. The scientific and practical direction of molecular biology, aimed at determining the nucleotide sequences of DNA molecules, was called genomics.

3. General properties of genetic material and levels of the genetic apparatus

Based on the above definitions of heredity and variability, it can be assumed what the requirements should be the material substrate of these two properties of life.

First, the genetic material must have a self-reproduction ability to in. The process of reproduction transmit inheritance information on the basis of which the formation of a new generation will be implemented. Secondly, to ensure the stability of the characteristics in a number of generations, the hereditary material must maintain its organization constant. Thirdly, the material of heredity and variability should have the ability to acquire changes and reproduce them, ensuring the possibility of historical development of living matter in changing conditions. Only in case of compliance with the specified requirements, the material substrate of heredity and variability can ensure the duration and continuity of the existence of wildlife and its evolution.

Modern ideas about the nature of the genetic apparatus allow you to allocate three levels of its organization: gene, chromosomal and genomic. Each of them shows the basic properties of the material of heredity and variability and certain patterns of its transfer and functioning.

4. Generic Level Organization of the Genetic Apparatus

The elementary functional unit of the genetic apparatus, which determines the possibility of developing a separate feature of a cell or the body of this species, is the gene (hereditary deposit, in Mendel). Transmission of genes in a number of generations of cells or organisms is achieved material continuity - inheritance with descendants of signs of parents.

Under the sign, they understand the unit of morphological, physiological, biochemical, immunological, clinical, and any other discreteness of organisms (cells), i.e. A separate quality or property for which they differ from each other.

Most of the features listed above the characteristics of organisms or cells refers to the category of complex signs, the formation of which requires the synthesis of many substances, primarily proteins with specific properties - enzymes, immunoproteins, structural, contractile, transport and other proteins. Properties of the protein molecule are determined by the amino acid sequence of its polypeptide chain, which is directly defined by the sequence of nucleotides in the DNA of the corresponding gene and is elementary, or simple, sign.

The main properties of the gene as a functional unit of the genetic apparatus are determined by its chemical organization,

4.1 Chemical organization of gene

Studies aimed at finding out the chemical nature of hereditary material, irrefutably proved that nucleic acids, which were found by F. Misher (1868) in the nuclei of the Poek cells are the material substrate of heredity and variability. Nucleic acids are macromolecules, i.e. Different with a large molecular weight. These are polymers consisting of monomers - nucleotides, including three components: sugar (pentose), phosphate and a nitrogen base (purmine or pyrimidine). To the first carbon atom in the pentose molecule of the P-1, a nitrogen base (adenine, guanine, cytosine, thymine or uracil) is joined, and to the fifth carbon C-5 carbon "with the help of essential communication - phosphate; The third carbon C-3 atom "always has a hydroxyl group - it is (Fig. 1).

The nucleotide compound in a nucleic acid macromolecule occurs by the interaction of the phosphate of one nucleotide with the hydroxyl of another so that a phosphodieter communication is established between them (Fig. 2). As a result, a polynucleotide chain is formed. The chain isza consists of alternating phosphate and sugar molecules. To the pentose molecules in the C-1 position, one of the above nitrogen bases is attached (Fig. 3).

Fig.1. The scheme of the structure of nucleotide

See the explanation in the text; Nucleotide components designations used in this figure are maintained in all subsequent nucleic acid schemes

The polynucleotide chain assembly is carried out with the participation of the polymerase enzyme, which ensures the addition of the phosphate group of the next nucleotide to the hydroxyl group facing 3 ", the previous nucleotide (Fig. 3.3). Thanks to the noted specific action of the name of the enzyme, the polynucleotide chain extension occurs only at one end: there where there is a free hydroxyl in position 3 ". The beginning of the chain always carries the phosphate group in position 5 ". This allows you to allocate in it 5" and 3 "-Cons.

Among nucleic acids, two types of compounds are distinguished: deoxyribonucleic (DNA) and ribonucleic acid (RNA) acid. The study of the composition of the main carriers of hereditary material - chromosomes - found that they are the most chemically stable component is DNA, which is a substrate of heredity and variability.

4.1.1 DNA structure. Model J. Watson and F. Cry

DNA consists of nucleotides, which includes sugar - deoxyribosis, phosphate and one of nitrogenous bases - Purin (adenin or guanine) or pyrimidine (thymine or cytosine). A feature of the DNA structural organization is that its molecules include two polynucleotide chains related to a certain way. In accordance with the three-dimensional DNA model proposed in 1953 by the American Biophysician J. Watson and the English biophysician and Genetic F. Screech, these chains are connected to each other with hydrogen bonds between their nitrogen bases on the principle of complementarity. Adenine of one chain is connected by two hydrogen bonds with a thymine of another chain, and three hydrogen bonds are formed between the guanin and cytosine of different circuits. Such a compound of nitrogen base provides a solid connection between two chains and maintain an equal distance between them all over.

Fig.4. The diagram of the structure of the DNA molecule. The arrows indicate the antilarality of the goals.

Another important feature of the combining of two polynucleotide chains in the DNA molecule is their anti-parallelity: 5 "Conference of one chain is connected to 3" - the other, and on the contrary (Fig.4).

Data x-ray structural analysis have shown that the DNA molecule consisting of two chains is formed a spiral twisted around its own axis. The diameter of the spiral is 2 nm, the length of the step is 3, 4 nm. Each round includes 10 pairs of nucleotides.

Most often, double spirals are human rights - when moving upwards along the axis of the circuit helix rotate to the right. Molel DNA molecules in the solution is in human rights - in-form (B-DNA). However, there are also left-handed forms (Z-DNA). What amount of this DNA is present in cells and what its biological significance is not yet established (Fig.3.5).

Fig.5. Spatial models of the left-handed Z-form (I) and human rights-based B-form (II) DNA

Thus, in the structural organization of the DNA molecule, a primary structure can be distinguished - a polynucleotide chain, a secondary structure, two complementary to each other and anti-parallel polynucleotide chains, connected by hydrogen bonds, and the tertiary structure is a three-dimensional spiral with the above spatial characteristics.

4.1.2 The method of recording genetic information in the DNA molecule. Biological code and its properties

Primary all variety of life is determined by a variety of protein molecules performing various biological functions in cells. The structure of proteins is determined by the set and order of the amino acids in their peptide chains. It is this sequence of amino acids in peptides encrypted into DNA molecules with a biological (genetic) code. The relative primitiveness of the DNA structure representing the alternation of only four different nucleotides, for a long time prevented researchers to consider this compound as a material substrate of heredity and variability in which extremely diverse information should be encrypted.

In 1954 Gamov was suggested that the coding of information in DNA molecules should be carried out by combinations of several nucleotides. In the manifold of proteins that exist in nature, about 20 different amino acids were found. To encrypt this number of numbers, a sufficient number of combinations of nucleotides can only provide a triplet code in which each amino acid is encrypted with three nucleotides. In this case, 4 3 \u003d 64 triplets are formed from four nucleotides. The code consisting of two nucleotides would give the opportunity to encrypt only 4 2 \u003d 16 different amino acids.

Complete decapitation of the genetic code was carried out in the 60s. of our century. Of the 64 possible DNA tripts 61 encodes various amino acids; The remaining 3 were called meaningless, or "nonsense-triplets". They do not encrypt amino acids and perform the feature of punctuation marks when reading hereditary information. These include ATT, ACS, ATC.

The obvious redundancy of the code appears that many amino acids are encrypted with several triplets (Fig. 6). This property of a triplet code, called degeneration, is very important, since the occurrence of changes in the DNA molecule in the type of replacement of one nucleotide in the polynucleotide chain may not change the meaning of the triplet. Thus, a new combination of three nucleotides encodes the same amino acid.

In the process of studying the properties of the genetic code, its specificity was discovered. Each triplet is capable of encoding only one specific amino acid. An interesting fact is the complete correspondence of the code in various types of living organisms. Such universality of the genetic code indicates the unity of origin of the whole manifold of living forms on Earth in the process of biological evolution. Minor differences in the genetic code are found in the Mitochondrial DNA of some species. This does not contradict the same provision on the universality of the code, but testifies in favor of a certain divergence in its evolution in the early stages of the existence of life. Deciphering the code in the DNA mitochondria of various species showed that in all cases in mitochondrial DNA there is a general feature: the ACC triplet is read as ACC, and therefore from the nonsense triplet turns into a tryptophan amino acid cipher.

Fig.6. Amino acids and coding their DNA thrills

Other features are specific to various types of organisms. In yeast triplet GAT and, perhaps, all family of ha encodes instead of amino acid leucine threonine. In mammals, tryptile tag has the same meaning as the TAC, and encodes the amino acid methionine instead of isoleucine. TCG and TCC trailelets in the Mitochondrial DNA of some species do not encode amino acids, being nonsense triplets. Along with triplength, degeneration, specificity and versatility, its continuity and non-reference of the codons at reading are the most important characteristics of the genetic code. This means that the sequence of nucleotides is read by a triplet for a triplet without skipping, while the adjacent pulls do not overlap each other, i.e. Each separate nucleotide is part of only one triplet at a given reading frame (Fig. 3.7). The proof of the non-replaceability of the genetic code is to replace only one amino acid in the peptide when replacing one nucleotide into DNA. In the case of the inclusion of a nucleotide into several overlapping triplets, its replacement entails a replacement for 2-3 amino acids in the peptide chain.

Fig.7. Continuity and continuity of genetic code when reading hereditary information.

Nucleotides are indicated by numbers

4.2 DNA properties as a substance of heredity and variability

4.2.1 Self-reproduction of hereditary material. DNA replication

One of the main properties of the material of heredity is its ability to self-copy - replication. This property is ensured by the peculiarities of the chemical organization of the DNA molecule consisting of two complementary chains. In the process of replication on each polynucleotide chain, the DNA Molecule is synthesized by the complementary chain. As a result, two identical double spirals are formed from one double helix of the DNA. Such a method of doubling molecules, in which each daughter molecule contains one maternal and one newly synthesized chain, is called a semi-seruous.

To replicate the maternal DNA chain must be separated from each other to become matrices on which complementary chains of subsidiaries will be synthesized.

Replication initiation is carried out in special areas of DNA denoted by ORI (from English origin - beginning). They include a sequence consisting of 300 nucleotide couples recognizable by specific proteins. The double DNA helix in these loci is divided into two chains, while, as a rule, on both sides of the point of return of replication, the areas of separation of polynucleotide chains are formed - replication forks that move in opposite directions from the locus. A structure is formed between replication forks, called the replication eye, where new polynucleotide chains are formed on two chains (Fig. 8, a).

With the help of the enzyme Helicase, tearing hydrogen bonds, the DNA double helix is \u200b\u200bbroken at replication points. The single DNA chains formed are associated with special destabilizing proteins, which stretch the circuit cores, making them nitrogenous bases available for binding to complementary nucleotides that are in nucleoplasm. At each of the chains formed in the replication plug, with the participation of the DNA polymerase enzyme, the synthesis of complementary circuits is carried out (Fig. 8, b).

Fig.8. Replication start area. Replication fork

A. The formation of replication eye.

B. Replication plug area in DNA molecule

In the process of synthesis, replication plugs move along the maternal spiral in opposite directions, capturing all new zones.

The separation of spiral swirling chains of parental DNA by the enzyme helicase causes the appearance of supervisors before the replication fork. This is explained by the fact that with the discrepancy between 10 pairs of nucleotides forming one round of the spiral, the parental DNA must make one full turn around its axis. Consequently, to promote the replication fork, the entire DNA molecule before it would have to rotate quickly, which would require high energy costs. In fact, this is not observed due to the special class of proteins called DNA topoisomerase. Topoisomerase breaks one of the DNA chains, which gives her the opportunity to rotate around the second chain. This weakens the accumulated voltage in the DNA double helix (Fig. 9).

The released hydrogen bonds of the nucleotide sequences of separated parent chains are joined by free nucleotides from nucleoplasm, where they are present in the form of: Datfe, DGTF, DCTF, DTTF. The complementary nucleosidththrifosphate forms hydrogen bonds with a certain base of the DNA maternal chain. Then, with the participation of the DNA-polymerase enzyme, it binds to phosphodieter bond with a preceding nucleotide of a newly synthesized chain, while giving an inorganic pyrophosphate (Fig. 10).

Since the DNA polymerase joins the next nucleotide to the one-group in 3 "position the preceding nucleotide, the chain is gradually lengthened on its 3" -Conacea.

A feature of the DNA polymerase is its inability to begin the synthesis of a new polynucleotide chain by simply binding two nucleosidatriphosphates: 3 "-on-end of any polynucleotide chain, paired with a DNA matrix chain, to which DNA polymerase can only add new nucleotides. Such a polynuk -Lotid chain is called seed or primer.

The role of the seed for the synthesis of polynucleotide DNA circuits during replication is performed by short RNA sequences formed with the participation of the RNA-Praimaz enzyme (Fig. 11). The specified feature of the DNA polymerase means that the matrix when replication can serve only the DNA circuit carrying a paired seed with it, which has a free 3 "-one-end.

Fig.9. The gap of one of the DNA chains using the DNA Topoisomerase enzyme: I - DNA topoisomerase forms a covalent bond with one of the Phosphate groups of DNA (upper chain); Ii - as a result of the phosphodiester communication break in one polynucleotide chain around the corresponding connection of the other chain, rotation is carried out, which removes the voltage caused by the discrepancy between two DNA circuits in the replication plug; III - After removal of the voltage in the DNA helix, the spontaneous separation of DNA topoisomerase and the restoration of phosphodiester communication in the DNA chain

The ability of the DNA polymerase to assemble the polynucleotide in the direction of 5 "- to 3" - an end with an anti-parallel compound of two DNA circuits means that the replication process should flow on them differently. Indeed, if on one of the matrices (3 "→ 5"), the assembly of a new chain occurs continuously from 5 "- to 3" - Content and it is gradually lengthened on the 3 "-conception, then another chain synthesized on the matrix (5" → 3 "), I would have to grow from 3" - to 5 "- Content. This is contrary to the direction of the enzyme of the DNA polymerase.

Fig.10. Attaching the next nucleotide to the DNA subsidiary, synthesized with the participation of DNA polymerase: FF pyrophosphate

Currently, it has been established that the synthesis of the second DNA chain is carried out by short fragments (fragments of the provision) also in the direction of 5 "- to 3" -con (by the type of sewing "back needle"). Procarnitis, fragments contain from 1000 to 2000 nucleotides, in eukaryota, they are much shorter (from 100 to 200 nucleotides). The synthesis of each such fragment is preceded by the formation of a RNA seed of about 10 nucleotides. The newly formed fragment using the DNA ligase enzyme is connected to the preceding fragment after removing its RNA seed (Fig. 12, a).

In connection with the specified singularities, the replication plug is asymmetric. Of the two synthesized daughter chains, one is constructed continuously, its synthesis is faster and this chain is called the leading. The synthesis of another chain is slower, as it is assembled from individual fragments that require education, and then removing the RNA seed. Therefore, such a chain is called delay (lagging). Although individual fragments are formed in the direction 5 "→ 3", in general, this chain is growing in the direction 3 "→ 5" (Fig. 3.12, a). In view of the fact that two replication plugs going in opposite directions are as a rule, the synthesis of the leading chains in them goes on different chains of maternal DNA (Fig. 12, b). The final result of the replication process is the formation of two DNA molecules, the nucleotide sequence of which is identical to that in the maternal double spiral of DNA.

Fig.11. The diagram of the synthesis reaction of the short RNA-seed catalyzed RNA-Prica

The considered sequence of events occurring during replicative synthesis involves the participation of a whole system of enzymes: Helicases, topoisomerases, destabilizing proteins, DNA polymerases and other jointly acting in the field of replication plug (Fig. 13).

DNA replication in pro- and eukaryotes in basic features occurs similarly, however, the rate of synthesis in eukaryotes (about 100 nucleotides / s) is an order of magnitude lower than that of prokaryotes (1000 nucleotides / s). The reason for this may be the formation of DNA eukaryotes sufficiently strong compounds with proteins, which makes it difficult to despiralize it necessary to implement replicative synthesis.

The DNA fragment on the point of return to replication to the point of its end is formed by the replication unit - replicon. One day, starting at the point of the beginning (Locus ON), replication continues until the entire replicon is duplicated. DNA ring molecules of prokaryotic cells have one locus ON and are entirely individual replica. Eukaryotic chromosomes contain a large number of replicoes. In this regard, the doubling of the DNA molecule, located along the eukaryotic chromosome, starts at several points. In different replicles, doubling can go at different times or at the same time.

Fig. 12. Synthesis of two daughter DNA chains on different chains of the maternal molecule

A. Due to the anti-parallelism of DNA chains, the synthesis of child chains is different, on the top maternal chain, a continuously leading chain is synthesized, on the lower mother chain, a child chain is assembled from fragments of the catering - lagging chain.

B. Synthesis of leading chains in multidirectional forks occurs on different chains of maternal DNA

4.2.2 Mechanisms for preserving the nucleohydric sequence of DNA. Chemical stability. Replication. Repair

To maintain the main characteristics of the cell or the body throughout their life, as well as in a number of generations, the hereditary material should differ resistant to external influences or should exist mechanisms for the correction of changes arising in it. Both factors are used in wildlife. The third factor is the accuracy of copying the nucleotide sequences of maternal DNA in the process of its replication.

Fig.13. Proteins involved in the DNA replication process

DNA Helpaspa breaks a double DNA helix, separating its polynucleotide chains; destabilizing proteins straighten the DNA chain section; DNA topoisomerase breaks phosphodiester communication in one of the polynugleotide DNA chains, removing the voltage caused by the spiral schedule and the discrepancy between the chains in the replication fork; RNA-Prymaz synthesizes RNA seeds for a subsidiary chain and for each fragment of the provision; DNA polymerase carries out continuous synthesis of the leading chain and synthesis of fragments of the lagging chain; DNA ligase sews fragments after removing RNA seed

According to the reactivity of the DNA molecule refer to the category of chemically inert substances. It is known that the role of the substance of heredity can perform not only DNA, but also RNA (some viruses). It is believed that the choice in favor of DNA is due to its lower compared to RNA with reactivity.

The replication mechanism discussed above is the extremely high accuracy of the DNA structure. When doubling the DNA error occurs on average with a frequency of 1 · 10 -6 complementary pairs of bases.

In maintaining high replication accuracy, an important role belongs primarily by the DNA polymerase enzyme. This enzyme takes place of the necessary nucleotides from among the nucleosidththrifosphates available in the nuclear juice (ATP, TTF, GTF, CTF), accurately attaching them to the DNA matrix chain and inclusion in the growing child chain. The inclusion frequency of incorrect nucleotides at this stage is 1 · 10 -5 base pairs.

Such errors in the operation of DNA polymerase are associated with the occurrence of modified forms of nitrogenous bases, which form "illegal" couples with the bases of the mother chain. For example, the modified form of cytosine instead of guanin binds to hydrogen bonds with adenine. As a result, an erroneous nucleotide is included in the DNA growing chain. The normal transition of the modified form of such a base in the usual disrupts its binding to the matrix, the unpaired 3 "-on-end of the growing chain of DNA appears. In this situation, the self-correction mechanism carried out by the DNA polymerase (or closely associated with the enzyme - editing endonuclease). Self-correction It consists in the cleavage of a nucleotide erroneously included in the DNA chain, not paired with the matrix (Fig. 14). The consequence of self-correction is to reduce the frequency of errors 10 times (from 10 -5 to 10 -6).

Despite self-correction efficiency, during replication after doubling DNA, errors are detected in it. Especially often this is observed in violation of the concentration of four nucleosidththrifosphates in the surrounding substrate. A significant part of the changes also occurs in DNA molecules as a result of spontaneously occurring processes associated with the loss of purine bases - adenine and guanin (apurimation) - or deamination of cytosine, which turns into uracil. The frequency of recent changes reaches 100 per 1 genome / day.

The base-contained base may vary under the influence of reactive compounds that violate their normal pairing, as well as under the action of ultraviolet radiation, which can cause the formation of a covalent bond between the two adjacent balances in DNA (Dimmers of Timine). These changes in the next replication cycle should lead either to the fallout of the base pairs in subsidiary DNA, or to replace some pairs of others. These changes really accompany each DNA replication cycle, but their frequency is significantly less than it should be. This is explained by the fact that most changes in this kind are eliminated due to the action of the reparation mechanism (molecular reduction) of the initial nucleotide sequence of DNA.

The reparation mechanism is based on the presence of two complementary chains in the DNA molecule. The distortion of the nucleotide sequence in one of them is detected by specific enzymes. Then the corresponding site is removed and replaced by a new, synthesized on the second complementary chain of DNA. Such reparation is called the excisional, i.e. With "cutting" (Fig.15). It is carried out until the next replication cycle, therefore it is also corrective.

Fig.14. Correction process diagram with DNA synthesis:

I-inclusion in a nucleotide DNA chain with a changed (tautomeric) form of cytoaeine, which "illegally" paired with adenine; II - the rapid transition of cytosine in the usual form disrupts its pairing with adenine; The unpaired 3 "-one-end of the synthesized chain prevents its further elongation under the action of DNA polymerase; III - DNA polymerase removes illegal nucleotide, as a result of which a paired with a matrix 3" -on-end appears again; IV - DNA polymerase continues to increase the chain on the 3 "-on-end.

Restoration of the initial structure of DNA requires the participation of a number of enzymes. An important point In the start of the reparation mechanism is the detection of an error in the DNA structure. Often, such errors occur in a newly synthesized chain in the replication process. Reparation enzymes must detect this particular chain. In many types of living organisms, a newly synthesized DNA chain differs from the maternal degree of methylation of its nitrogenous bases, which is lagging behind the synthesis. Reparations at the same time unnewned chain. The object recognition by the enzymes of repair can also serve ruptures in the DNA chain. At the highest organisms where the synthesis of DNA does not happen continuously, but separate replicances, the newly synthesized DNA chain has a break, which makes it possible to recognize. Restoration of the DNA structure in the loss of purine bases of one of its chains involves the detection of a defect using an endonuclease enzyme that breaks the phosphoester in the area of \u200b\u200bdamage to the chain. The altered section with several adjacent nucleotides is removed by the enzyme with nucleotide, and in its place in accordance with the order of the bases of the complementary circuit, the correct nucleotide sequence is formed (Fig. 15).

Fig.15. Scheme of excision, efficient DNA reparation.

With a change in one of the bases in the DNA circuit in the restoration of the original structure, the enzymes of DNA glycosylase are taken by a number of about 20. They specifically recognize the damage due to deamination, alkylation and other structural reasons. Such modified bases are removed. Sections arise, devoid of grounds that are reparished, as with the loss of Purines. If the restoration of the normal structure is not carried out, for example, in case of deamination of nitrogenous bases, one pairs of complementary bases of the other - a pair of c-g can be replaced by a pair of T-A, and the like. .

Education in polynucleotide chains under the action of UV rays of thyminic dimers (Tt-T) requires the participation of enzymes, learning not separately modified bases, but more extended damage to the DNA structure. The reparative process in this case is also associated with the removal of a section carrying a dimer, and the restoration of a normal nucleotide sequence by synthesis on the complementary DNA chain.

In the case when the excision reparation system does not correct the changes arising in one DNA circuit, during replication, this change is fixed and it becomes the property of both DNA chains. This leads to a replacement of one pair of complementary nucleotides to another either to the appearance of breaks (shash) in a newly synthesized chain against the changed areas. The restoration of the normal DNA structure can occur after the replication.

Personalicative reparation is carried out by recombination (fragments exchange) between the two newly formed DNA double spirals. An example of such a post-solicative repair can be the restoration of the normal structure of DNA in the occurrence of thyminic dimers (Tt-T), when they are not eliminated spontaneously under the action of visible light (light reparation) or in the course of expecative excision reparation.

Covalent bonds arising between nearby Timine residues make them not capable of binding to complementary nucleotides. As a result, in a newly synthesized DNA chain, breaks (bars) recognizable by the enzymes of repair appear. The restoration of the integrity of the new polynucleotide chain of one of the subsidiary DNA is carried out due to the recombination with the other subsidiary DNA corresponding to it. The gap formed in the maternal chain is then filled with a synthesis by the complementary polynucleotide chain (Fig. 16). The manifestation of such post-solicative reparation carried out by recombination between the chains of two daughter DNA molecules can be considered often the observed exchange of material between nursing chromatids (Fig. 17).

Fig.16. Scheme of post-solicative DNA reparation:

I - the occurrence of the thyminic dimer in one of the DNA chains;

II - the formation of "bars" in a newly synthesized chain against the changed section of the mother molecule after replication (the arrow shows the subsequent filling of the "bare" by a section from the corresponding chain of the second subsidiary DNA molecule);

III - restoration of the integrity of the subsidiary of the upper molecule due to the recombination and in the lower molecule due to the synthesis on the complementary chain

Fig.17. Interchromatide exchanges (indicated by arrows)

During the expense and post-solicative reparation, most of the damage to the DNA structure is restored. However, if there is too much damage in the hereditary material of the cell and some of them are not eliminated, the system induced (motivated) reparation enzymes (SOS system) is included. These enzymes fill the bars, restoring the integrity of the synthesized polynucleotide chains without accurate compliance with the principle of complementarity. That is why the reparation processes themselves can serve as a source of persistent changes in the DNA structure (mutations). The named reaction also refers to the SOS system.

If in the cell, despite the projection, the amount of damage to the DNA structure remains high, the processes of DNA replication are blocked in it. Such a cell is not divided, which means it does not convey the changes that have arisen.

The stopping of the cell cycle caused by damage to DNA in combination with the impossibility of molecular reparation of the changed hereditary material can with the participation of the protein, the synthesis of which is controlled by the genome P53, lead to the activation of the process of self-destruction (apotposis) of the defective cell in order to eliminate it from the body.

Thus, an extensive set of different reparation enzymes carries out a continuous "inspection" of DNA, removing damaged areas from it and contributing to maintaining the stability of hereditary material. The joint action of replication enzymes (DNA polymerase and editing endonuclease) and repair enzymes ensures a sufficiently low frequency of errors in DNA molecules, which is maintained at 1 · 10 -9 pairs of altered nucleotides on the genome. With the size of the human genome 3 · 10 9 nucleotide pairs, this means the appearance of about 3 errors to the replicated genome. At the same time, even this level is sufficient for education during the existence of life on Earth significant genetic diversity in the form of gene mutations.

4.2.3 Changes in nucleotide DNA sequences.

Non-uncorrected changes in the chemical structure of genes reproduced in consecutive replication cycles and manifest themselves in the form of new features of signs are called gene mutations.

Changes in the structure of the DNA forming the gene can be divided into three groups. Mutations of the first group are to replace some reason by others. They constitute about 20% of spontaneously emerging genes. The second mutation group is due to the shift of the reading frame, which occurs when a change in the number of nucleotide pairs in the gene composition. Finally, the third group represent mutations associated with a change in the order of nucleotide sequences within the gene (inversion).

Mutations for the type of nitrogen base. These mutations occur due to a number of specific reasons. One of these may be randomly or under the influence of specific chemical agents change the structure of the base already included in the DNA helix. If such a modified form of base remains not seen by the reparation enzymes, then with the nearest replication cycle, it can attach another nucleotide to itself. An example is the deamination of cytosine, which turns into uracil spontaneously or under the influence of nitrate acid (Fig. 18). Forming uracil, not seen by the enzyme DNA glycosylase, is connected to adenine, which subsequently attaches a thymidyl nucleotide. As a result, a pair of C-Γ is replaced in the DNA pair T-A (Fig. 19, I). The deamination of methylated cytosine turns it into Timin (see Fig.3.18). The thimidyl nucleotide, being a natural component of DNA, is not detected by the enzymes of reparation as a change and the following replication attaches adenyl nucleotide. As a result, instead of a pair of C-G, a pair of T-A (Fig. 19, II) also appears in the DNA molecule.

Fig.18. Spontaneous deamination of cytosine

Another reason for replacing the base may be the erroneous inclusion in the synthesized Nucleotide DNA circuit, which bears a chemically changed form of the base or its analog. If this error remains not seen by replication and reparation enzymes, the modified base is included in the replication process, which often leads to a replacement of one pair to another. An example of this can be an accession during replication to the adenine of the nucleotide of the nucleotide with 5-bromuracyl (5-bu) similar to the thymidyl nucleotide. In case of subsequent replication, 5-bu is no longer adenine attached to himself, but guanine. Guanine in the course of further doubling forms a complementary pair with a cytosine. As a result, the pair A-T is replaced in the DNA molecule of Mr. (Fig. 20).

Fig. 19. Mutations on the type of base replacement (deamination of nitrogen bases in the DNA chain):

I - the transformation of cytosine in uracil, replacement of the c-g-pair on the T-A-couple;

II - the transformation of methyl-cytosine in Timin, replacing the c-g-pair on T-A-couple

From the above examples, it can be seen that changes in the structure of the DNA molecule by the base of base replacement arise either before or in the replication process originally in one polynucleotide chain. If such changes are not corrected during the reparation, then upon subsequent replication, they become the entrance to both DNA chains.

Fig. 20. Mutations on the type of substitution replacement (inclusion of an analogue of a nitrogen base under DNA replication)

The consequence of the replacement of one pair of complementary nucleotides to another is the formation of a new triplet in a nucleotide DNA sequence encoding the sequence of amino acids in the peptide chain. This may not affect the peptide structure in the event that the new triplet will be "synonymous" former, i.e. will encode the same amino acid. For example, Valin amino acid is encrypted by four triplets: CAA, TsAG, CA, Tsats. Replacing the third base in any of these triplets will not change its meaning (degeneracy of the genetic code).

In the case when the newly emerging triplet encrypts another amino acid, the structure of the peptide chain and the properties of the corresponding protein are changed. Depending on the nature and location of the replacement, the specific properties of the protein are changed to varying degrees. There are cases when the replacement of only one amino acid in the peptide significantly affects the properties of the protein, which is manifested in a change in more complex signs. An example is the change in the properties of the hemoglobin of a person during sulfur-cell anemia (Fig.21). In such hemoglobin- (HBS) (in contrast to normal HBA) - in p-globin chains in the sixth position, glutamic acid is replaced by valine. This is a consequence of the replacement of one of the bases in a triplet encrypting glutamic acid (CTT or CTCs). As a result, a triplet appears, encrypting valine (CAT or CACs). In this case, the replacement of one amino acid in the peptide substantially changes the properties of the globin included in the hemoglobin (its ability to bind to 02) is reduced, a person develops signs of sickle cell anemia.

In some cases, the replacement of one base to another can lead to the appearance of one of the nonsense-triplets (ATT, ATC, ACC), which does not encrypt any amino acid. The consequence of such a replacement will be the interruption of the synthesis of the peptide chain. It is estimated that the replacement of nucleotides in one triplet leads in 25% of cases to the formation of synonym triplets; In 2-3 - meaningless triplets, at 70 - 75% - to the emergence of true gene mutations.

Thus, mutations by the type of substitution of bases may occur as as a result of spontaneous changes in the base structure in one of the chains of an existing DNA double helix, and during replication in a newly synthesized chain. In the event that these changes are not corrected in the process of repair (or, on the contrary, they occur during repair), they are fixed in both chains and will continue to be reproduced in the following replication cycles. Consequently, an important source of such mutations are violations of replication and repair processes.

Mutations with the shift of the reading frame. This type of mutations is a significant proportion of spontaneous mutations. They occur due to loss or insertion into the nucleotide sequence of DNA of one or more pairs of complementary nucleotides. Most of the studied mutations causing a frame shift were found in sequences consisting of identical nucleotides.

The change in the number of nucleotide pairs in the DNA circuit contributes to the effects on the genetic material of some chemicals, such as acridine compounds. Deforming the structure of the double helix of DNA, they lead to the insertion of additional bases or their fallout when replication. An example is the mutations obtained from the Phage T4 under the influence of Proflowin. They are in the inclusion or removal of only one nucleotide pair. An important reason for the change in the number of nucleotide pairs in the gene by type of large divisions (extinction) can be x-ray irradiation. In a fruit flies, for example, a mutation of a gene controlling the color of the eye, which is caused by irradiation and consists in dividing the order of 100 nucleotide pairs.

Fig.21. Pleiotropic replacement effect of one amino acid in the human hemoglobin β-chain leading to the development of sickle-cell anemia

A large number of mutations in the type of insertion occurs due to the inclusion in the sequence of nucleotides of moving genetic elements - transposons. Transposons are quite extended nucleotide sequences, built into the EU and prokaryotic cell genomes, capable of spontaneously change their position. With a certain probability of insertion and division, as a result of recombination errors with an uneven intragenic crossing rider (Fig.22).

Fig.22. Mutations with the shift of the reading frame (unequal exchange with intragenic crossing rider):

I are the ruptures of allelpic genes in different areas and the exchange of fragments between them;

II - the loss of the 3rd and 4th pairs of nucleotides, shifting the reading frame;

III - doubling 3rd and 4th nucleotide pairs, shift reading frame

Fig.23. Consequence of changes in the number of nucleotide couples in the DNA molecule

The shift of the reading frame as a result of the insertion of one nucleotide into the coded chain leads to a change in the composition of the peptide encrypted in it

With continuity of reading and non-redeebles of the genetic code, the change in the number of nucleotides, as a rule, leads to a shift of the reading frame and changing the meaning of biological information recorded in this DNA sequence (Fig.23). However, if the number of inserted or lost nucleotides is multiple of three, the frame shift may not occur, but this will turn on additional amino acids or the fallout of the part of them from the polypeptide chain. A possible consequence of the frame shift is the occurrence of nonsenstriplets leading to the synthesis of shortened peptide chains.

Mutations by type of inversion of nucleotide sequences in the gene. This type of mutation occurs due to the rotation of the DNA section 180 °. Usually it is preceded by the formation of a DNA molecule loop, within which replication goes in the direction opposite.

Within the inverted portion, the reading of information is violated, as a result, the amino acid sequence of the protein changes.

4.2.4 Elementary variability units genetic material. Muton. Recon

The gene is an elementary unit of the function of hereditary material. This means that the DNA molecule fragment corresponding to a separate gene and determines due to the biological information contained in it the possibility of developing a specific feature, is further indivisible in functionality. Information about the gene mutations outlined above indicate the value of changes in the chemical structure affecting not the entire gene, and its individual sections, as a result of which new features appear.

The minimum amount of hereditary material capable, changing, lead to the appearance of the feature options corresponds to an elementary unit of the mutation process and is called Muton. The examples of gene mutations considered above indicate that it suffices to replace one pair of complementary bases in the gene to change the properties of the protein encoded by him. Thus, Mouton corresponds to one pair of complementary nucleotides.

Some of the gene mutations in the type of inserts and the fraudes of nucleotide pairs occur due to the unequal exchange between the DNA molecules at Cross Hopera, i.e. With a violation of recombination between them. This is accompanied by a shift of the reading frame and leads to a violation of the synthesis of the peptide chain with the specified properties. Observations show that in order to distort the biological information recorded in the gene or a single pair of nucleotides. It follows from the above that the elementary unit of recombination - reconna - at the molecular level corresponds to one pair of nucleotides.

Arriving spontaneously or influenced by various external influences of changes in nucleotide sequences lead to the fact that the same gene may exist in several versions that differ in the biological information contained in them. The specific form of the existence of a gene that determines the possibility of developing a specific variant of this feature is called the allele. Alleles gene are located in the same section-locus-specific chromosome, which in normal can simultaneously contain only one of the series of alleles. This makes alternative alternative (mutually exclusive) options for the existence of a gene.

Changes in the chemical structure may occur in various sections of the gene. If they are compatible with life, i.e. Do not lead to the death of cells or organisms - carriers of these mutations, they all persist in the gene pool form.

The presence in the gene pool of the form at the same time different alleles gene is called multiple allelism. An example of this is a different eye coloring options for fruit flies: white, cherry, red, apricot, eosinova, - caused by various alleles of the corresponding gene. In humans, like other representatives of the organic world, multiple allelism is inherent to many genes. Thus, three allele of gene I determine the group affiliation of blood according to the AV0 system (I A, I B, I 0). Two allele has a gene resulting reserves. More than one hundred alleles are the genes of α - and β-polypeptides of hemoglobin.

The cause of multiple allelism is random changes in the structure of the gene (mutation) stored in the process of natural selection in the gene pool population. The variety of alleles recombining during sexual reproduction determines the degree of genotypic diversity among representatives of this species, which has a large evolutionary value, increasing the viability of populations in the changing conditions of their existence. In addition to evolutionary and environmental importance, the allelic state of genes has a great influence on the functioning of genetic material. In the diploid somatic cells of eukaryotic organisms, most genes are represented by two alleles that jointly affect the formation of signs.

4.2.5 Functional classification of gene mutations

Changes in the structure of the gene, as a rule, are unfavorable, reducing the viability of the cell, the body (harmful mutations), and sometimes lead to their death (lethal mutations). Less frequently emerging mutations are not significantly reflected on the viability of their carriers, so they are considered as neutral. Finally, alleles that have a beneficial effect (useful mutations) occur extremely rare, providing their support for preferential survival. In most cases, the newly emerged allele of the gene acts as a recessive in relation to the "wild" type widespread allele, i.e. Not manifested in combination with him. But sometimes the mutant shape of the gene can be dominant, i.e. Purchase the manifestation of the "wild" allele, which is more often found in the gene pool population.

4.2.6 Mechanisms that reduce adverse effects gene mutations

As a result of gene mutations, the meaning of biological information changes. The consequences of this can be a two-way. In habitats, changing slightly, new information usually reduces survival. With a sharp change of the conditions of existence, when mastering a new ecological niche, the presence of a variety of information is useful. In this connection, the intensity of the mutation process in natural conditions is maintained at a level that does not cause a catastrophic reduction in the viability of the species. An important role in limiting the adverse effects of mutations belongs to antimutation mechanisms arising in evolution.

Some of these mechanisms are discussed above. We are talking about the features of the functioning of the DNA polymerase, selecting the required nucleotides in the process of DNA replication, as well as self-correction during the formation of a new DNA chain along with an editing endonuclease. Details disassembled various mechanisms for reparation DNA structure, the role of degeneracy of the genetic code. The solution of this problem is the triplet of the biological code, which allows the minimum number of replacements inside the triplet leading to the distortion of information. Thus, 64% of the substitution of the third nucleotide in triplets does not give changes to their semantic value. True, the replacement of the second nucleotide 100% lead to distortion of the meaning of the triplet.

The protection factor against the adverse effects of gene mutations is the pair of chromosomes in the diploid karyotype of somatic eukarot cells.

A carnival of alleles of genes prevents the phenotypic manifestation of mutations if they have a recessive nature.

A certain contribution to the decrease in the harmful effects of gene mutations is a phenomenon of extracting genes encoding vital macromolecules. It is in the presence of several dozen genotype, and sometimes hundreds of identical copies of such genes. An example is the RRNA, TRNA genes, histone proteins, without which the vital activity of any cell is impossible.

In the presence of extraction, mutational change in one or even several identical genes does not lead to a catastrophic for the cell consequences. Copies remaining unchanged is enough to ensure normal operation.

The functional unequivocability of amino acid substitutions in the polypeptide is also significant. If the new and changeable amino acids are similar in physicochemical properties, changes in the tertiary structure and biological properties of the protein are insignificant.

Thus, the mutant hemoglobins of HBS and human NW differ from the normal hemoglobin na replacement in the 6th position of the glutamic acid p-chain respectively to the valine or lysine. The first replacement change dramatically the properties of hemoglobin and leads to the development of severe disease - sickle-cell anemia.

With a second replacement, the properties of hemoglobin change to a much lesser extent.

The reason for these differences is that glutamic acid and lysine show similar hydrophilic properties, while the valine is a hydrophobic amino acid.

Thus, the listed mechanisms contribute to the preservation of genes selected during the evolution and at the same time accumulating in the gene pool population of various alleles, forming a reserve of hereditary variability. The latter determines the high evolutionary plasticity of the population, i.e. Ability to survive in a variety of conditions.

4.3 Use of genetic information in the processes of vital activity

4.3.1 The role of RNA in the implementation of hereditary information

Hereditary information recorded using a genetic code is stored in DNA molecules and multiplies in order to provide newly formed cells with the necessary "instructions" for their normal development and operation. At the same time, direct participation in the livelihood of DNA cells does not accept. The role of the mediator whose function is the translation of hereditary information stored in DNA into the operating form, ribonucleic acids play RNA.

Unlike DNA molecules, ribonucleic acids are represented by one polynucleotide chain, which consists of four varieties of nucleotides containing sugar, ribose, phosphate and one of four nitrogen bases - adenine, guanine, uracil or cytosine. RNA is synthesized on DNA molecules using RNA-polymerase enzymes with compliance with the principle of complementaryness and anti-parallelity, and the DNA adenine in RNA complementary Uracil. All variety of RNAs acting in a cell can be divided into three main types: mRNA, TRNA, RRNA.

Matrix, or information, RNA (mRNA, or IRNA). Transcription. In order to synthesize proteins with specified properties, the "instruction" on the order of inclusion of amino acids in the peptide chain is received to the place of their construction. This manual is enclosed in a nucleotide sequence of matrix, or information RNA (MRNA, IRNA) synthesized at the respective DNA sections. The process of mRNA synthesis is called transcription.

The synthesis of mRNA begins with the detection of the RNA polymerase of a special area in the DNA molecule, which indicates the starting place of the transcription - promoter. After connecting to the promoter, the RNA polymerase spoys the adjacent DNA spiral coil. Two DNA chains in this place are diverged, and on one of them the enzyme performs the synthesis of mRNA. The ribonucleotide assembly in the chain occurs in compliance with their complementarity of DNA nucleotides, as well as antipherally with respect to the DNA matrix chain. Due to the fact that RNA polymerase is capable of collecting polynucleotide only from 5 "-concar to 3" - only one of two DNA circuits can serve for transcription, namely, which faces an enzyme with its 3 "-concons ( 3 "→ 5"). Such a circuit is called codecine (Fig. 3.24). Anti-parallelity of the connection of two polynucleotide chains in the DNA molecule allows RNA polymerase to choose the matrix to correctly select the MRNA synthesis.

Moved along the Codec Chain DNA, the RNA polymerase performs a gradual accurate rewriting of information until it encounters a specific nucleotide sequence - transcription terminator. In this section, the RNA polymerase is separated from both the DNA matrix and from the newly synthesized mRNA (Fig. 25). The DNA molecule fragment, which includes a promoter, transcribed sequence and a terminator forms a transcription unit - transcripton.

In the process of synthesis, as the RNA polymerase is advanced along the DNA molecule, each single-chain dNA sections are rebounded into a double helix. The MRNA generated during the transcription contains an accurate copy of the information recorded in the corresponding DNA section. Troops nearby mRNA nucleotides, encrypting amino acids, are called codons. The sequence of codons of MRNA encrypts the sequence of amino acids in the peptide chain. The codons of mRNA correspond to certain amino acids (Table 1).

Table 1. Genetic MRNA code (emphasized codon-terminators). Second nucleotide

W.		C.		BUT		G.

Fig.24. MRNA synthesis scheme

The MRNA transcription matrix is \u200b\u200ba Codegenized DNA chain facing an enzyme with its 3rd

Fig. 25. The role of RNA polymerase in transcription:

I - detection of the promoter region in the DNA molecule and the spiral spiral spiral; II - initiation of the RNA chain synthesis by binding the first first ribonucleoside rifriffs; Iii - extension of the RNA circuit in the direction 5 "→ 3" by connecting ribonucleosidegroms; IV - release of 5 "-concar of the synthesized RNA and the restoration of the DNA double helix; V - the end of the RNA synthesis in the terminator area, the separation of the polymerase from the completed RNA circuit

Transport RNA (TRNA). Broadcast. An important role in the process of using the cell's hereditary information belongs to transport RNA (TRNA). By delivering the necessary amino acids to the place of assembly of peptide chains, TRNA performs the function of a broadcast mediator.

TRNA molecules are polynucleotide chains synthesized on certain DNA sequences. They consist of a relatively small number of nucleotides - 75-95. As a result of the complementary compound of the bases that are in different parts of the polynucleotide chain of the TRNA, it acquires a structure resembling a clover sheet (Fig. 26).

Fig.26. The structure of a typical TRNA molecule

It allocate four main parts that perform various functions. The acceptor "stem" is formed by two complementary connected terminal parts of TRNA. It consists of a seven pairs of grounds.3 "The monitor of this stem is somewhat longer and forms a single-chain area, which ends with a CCA sequence with a free ON-group. The transported amino acid is connected to this end. The remaining three branches are complementarly paired nucleotide sequences that end out of unpaired sequences. The media from these branches is anti-acid - consists of five pairs of nucleotides and contains anti-cymodone in the center of its loop. Anti-cymodón is three nucleotides, complementary codon of mRNA, which encrypts the amino acid transported by this TRNA to the peptide synthesis site.

There are two side branches between acceptor and antiquodonic branches. In its loops, they contain modified bases - dihydrouridine (D-loop) and triplet Tψc, where the pseudoroneine (T ^ C-loop). Between Aiticodone and T ^ C-branches, an additional loop contains from 3-5 to 13-21 nucleotides.

In general, various types of TRNA are characterized by a certain constancy of the nucleotide sequence, which most often consists of 76 nucleotides. Variation of their number is mainly due to a change in the number of nucleotides in an additional loop. Complementary sections supporting the structure of TRNA are usually conservative. The primary structure of TRNA, determined by the nucleotide sequence, forms the secondary structure of the TRNA, having the form of a clover sheet. In turn, the secondary structure determines the three-dimensional tertiary structure for which the formation of two perpendicularly located double spirals (Fig.27) is characterized. One of them is formed by acceptor and t ψs-branches, the other - anti-acid and D-branches.

At the end of one of the double spirals there is a transported amino acid, at the end of the other - antikodon. These sites are rendered as possible from each other. The stability of the tertiary structure of TRNA is maintained due to the occurrence of additional hydrogen bonds between the bases of the polynucleotide chain, which are in different areas, but spatially close in the tertiary structure.

Various types of TRNA have a similar tertiary structure, albeit with some variations.

Fig.27. Spatial organization TRNA:

I is the secondary structure of TRNA in the form of a "clover sheet", determined by its primary structure (a sequence of nucleotides in the chain);

II - two-dimensional projection of the tertiary structure of TRNA;

III - TRNA molecule laying scheme in space

One of the features of TRNA is the presence of unusual bases arising from the chemical modification after inclusion of a normal base in a polynucleotide chain. These modified bases cause a large structural manifold TRNA with a general plan of their structure. The greatest interest is the modifications of the bases forming antiquodon, which affect the specificity of its interaction with the codon. For example, an atypical base of Inozin, sometimes standing in the 1st position of the anti-cytone TRNA, is capable of complementary to connect with three different third bases of the codon of MRNA - y, C and A (Fig.3.28). Since one of the peculiarities of the genetic code is its degeneration (see Section.3.4.1.2), many amino acids are encrypted by several codons, which, as a rule, differ in their third base. Thanks to the nonspecification of the binding of the modified base of anti-cymodone, one TRNA recognizes several codon synonyms.

Fig.28. Connection of Inosina with hydrogen bonds with three different nitrogen bases The hydrogen bonds are indicated by points

There is also an existence of several types of TRNA, capable of connecting to the same codon. As a result, the cells in the cytoplasm occurs not 61 (by the number of codons), and about 40 different TRNA molecules. This amount is enough to transport 20 different amino acids to the site of the protein assembly.

Along with the function of accurate recognition of a specific codon in mRNA, the TRNA molecule delivers a strictly defined amino acid that is encrypted with this codon to the place of synthesis of the peptide chain. The specific compound of TRNA with "its" amino acid flows in two stages and leads to the formation of a compound called aminoacyl-TRNA (Fig.29).

Fig.29. Attaching amino acids to the corresponding TRNA:

I is the 1st stage, the interaction of amino acids and ATP with the release of pyrophosphate;

II - 2nd stage, accession of depleted amino acid to 3 "RNA -Con

At the first stage of the amino acid activated, interacting with its carboxyl group with ATP. As a result, an amypilated amino acid is formed.

At the second stage, this compound interacts with an ON-group located on the 3 "-conace of the corresponding TRNA, and the amino acid joins it with its carboxyl group, releaseing an AMP. Thus, this process proceeds with the considerable energy obtained by hydrolysis of ATP to AMP .

The specificity of the compound of amino acid and TRNA carrying the corresponding anti-cycle is achieved due to the properties of the aminoacyl-TRNA synthetase enzyme. In the cytoplasm there is a whole set of such enzymes that are capable of spatial recognition, on the one hand, its amino acid, and on the other - the corresponding anticodone TRNA (Fig.3.30). Hereditary information, "recorded" in DNA molecules and "rewritten" on mRNA, is deciphered during the broadcast due to two processes of specific recognition of molecular surfaces. At first, the aminoacyl-tall synthetase enzyme provides the TRNA connection with the amino acid transported by it. Then aminoacyl-TRNA complementarily pairing with mRNA due to the interaction of anti-cymodone with the codon. With the help of the TRNA system, the nucleotide chain of mRNA. translated into the language of the amino acid sequence of the peptide (Fig. 30).

Ribosomal RNA (RRNA). Ribosomal protein synthesis cycle. The process of interaction between mRNA and TRNA, which provides information translating information from the nucleotide language into the amino acid language, is carried out on ribosomes. The latter are complex RRNA complexes and a variety of proteins, in which the first form the framework. Ribosomal RNAs are not only a structural component of ribosomes, but also ensure that they are bonding with a certain nucleotide sequence of mRNA. This establishes the beginning and the reading frame in the formation of a peptide chain. In addition, they ensure the interaction of ribosomes and TRNA. Numerous proteins that are part of the ribosome along with RRNA are performed both a structural and enzymatic role.

Fig.30. The transmission scheme of the genetic code: I - the addition of amino acids (tryptophan) to the corresponding TRNA using the aminoacyl-tall synthetase enzyme; II - joining TRNA carrying its amino acid, to mRNA due to the binding of its anti-cycle with the codon of mRNA

Ribosomes Pro- and eukaryotes are very similar in structure and functions. They consist of two sub-interpreters: big and small. In eukaryota, a small sub-partition is formed by one RRNA molecule and 33 molecules of different proteins. The large sub-partition combines three RRNA molecules and about 40 proteins. Procarniotic ribosomes and ribosomes Mitochondria and plastids contain fewer components.

There are two grooves in ribosomes. One of them holds the growing polypeptide chain, the other - mRNA. In addition, in ribosomes, two plots connecting TRNA are distinguished. In the aminoacil, the Aminoacil-TRNA, which bears a certain amino acid. In the peptidal, the P-site is usually a TRNA, which is loaded with a chain of amino acids connected by peptide bonds. Education A - and P-sites is provided by both sub-displeteities of ribosomes.

At each moment of the ribosome, it shields a mRNA segment with a length of about 30 nucleotides. In this case, the interaction of only two TRNA with two adjacent mRNA codons (Fig.31) is ensured.

The broadcast of information on the "language" of amino acids is expressed in the gradual increase in the peptide chain in accordance with the instructions concluded in mRNA. This process proceeds on ribosomes that provide a sequence of decoding information using TRNA. During the broadcast, three phases can be distinguished: initiation, elongation and termination of the synthesis of the peptide chain.

Fig.31. Plots of binding of TRNA molecules and ribosomes:

I - unloaded ribosome, II - loaded ribosome; AK - amino acid

The initiation phase, or the beginning of the synthesis of the peptide, is to combine the two proclaims in the cytoplasm of the subcontizations of ribosomes on a certain part of mRNA and accession to it first aminoacyl-TRNA. This is also given a frame of reading the information concluded in MRNA (Fig. 32).

In a molecule of any mRNA near her 5 "- Conference there is a plot, complementary RDNA Male Ribosoma Sub-Fiction and Specificly recognizable it. Next to it, it is the initiating start code of the Out, encrypting the amino acid methionine. A small subpartigitis of ribosomes is connected to mRNA in such a way that the start code of Out is located In the region corresponding to the P-section. At the same time, only the initiating TRNA carrying methionine is able to take a place in the unfinished P-section of the Small Subcourse and complementarily connect with the starting codon. After the described event, the large and small subcontizations of ribosomes occur with the formation of its peptidyl and aminoacyl formation. plots (Fig.3.32).

Fig.32. Protein synthesis initiation:

I - the compound of a small subchasche ribosome with mRNA, connecting to the starting codon of carrier methionine TRNA, which is located in an unfinished P-site; II - the compound of the large and small subchalipcies of ribosomes with the formation of P - and A-sites; The next stage is associated with the accommodation in the Aminoacil-TRNA, corresponding to the Code of MRNA located in it, - initially elongation; AK - amino acid

By the end of the initiating phase, the P-section is occupied by an aminoacyl-trading associated with methionine, while in the A-section of the ribosome is located next to the starting codon.

The described translation initiation processes are catalyzed by special proteins - initiation factors that are movably connected with a small sub-displete ribosome. Upon completion of the initiation and formation phase of the Ribosoma complex - MRNA - initiating aminoacil-TRNA, these factors are separated from the ribosome.

The elongation phase, or the lengthening of the peptide, includes all reactions from the formation of the first peptide connection before the last amino acid attachment. It is cyclic repeating events under which there is a specific recognition of the aminoacyl-trading of the next codon, which is in the A-section, complementary interaction between the anti-cycodone and the codon.

Due to the peculiarities of the three-dimensional organization TRNA when it is connected to an antiquodone with the codon of mRNA. The amino acid transported by it is located in the A-site, close to the previously included amino acids located in the P-site. A peptide bond is formed between the two amino acids, catalyzed by special proteins that are part of the ribosome. As a result, the previous amino acid loses with its TRNA and joins the aminoacil-TRNA located in the A-site. At this point in the P-section of TRNA is released and goes to the cytoplasm (Fig.33). The movement of the TRNA, loaded by a peptide chain, from the A-site in the P-section is accompanied by the promotion of ribosomes on mRNA to a step corresponding to one codon. Now the next codon comes in contact with the A-site, where it will be specifically "identified" by the corresponding aminoacil-TRNA, which will place its amino acid here. Such a sequence of events is repeated until the Ribosomal Code will receive a code-terminator for which there is no corresponding TRNA.

Fig.33. Falganization phase in protein synthesis:

1st stage - aminoacil-TRNA joins the codon located in the A-site;

The 2nd stage - between amino acids located in A - and P-sites, a peptidium communications is formed: TRNA, located in the P-site, is exempt from its amino acid and leaves Ribosoma;

The 3rd stage - ribosome moves on mRNA to one codon so that the TRNA, loaded by a peptide chain, moves from the A-site to the P-section; Free A-Plot can be occupied by the corresponding aminoacil-TRNA

Fig.34. Termination of the synthesis of the peptide chain:

1st stage - joining the factor of release to stop codon;

2nd stage - termination, release of the completed peptide;

3rd stage - dissociation of ribosomes into two sub-obsits

Assembling the peptide chain is carried out with a sufficiently high speed depending on the temperature. The bacteria at 37 ° C is expressed in addition to the podpeptide from 12 to 17 amino acids in 1 seconds. In eukaryotic cells, this speed is lower and expressed in the addition of two amino acids in 1 seconds.

The termination phase, or the completion of the synthesis of the polypeptide, is associated with the recognition of a specific ribosomal protein of one of the termination codons (UAA, UAG or HA), when he is included in the zone of the Ribosoma A-section. At the same time, water is joined by the latter amino acid in the peptide chain, and its carboxyl end is separated from TRNA. As a result, the completed peptide chain loses relation to the ribosome, which disintegrates into two sub-disperses (Fig. 34).

4.3.2 Features of the organization and expression genetic information from pro- and eukaryotes

According to the chemical organization of the material of heredity and variability, eukaryotic and prokaryotic cells are not fundamentally different from each other. The genetic material is presented with DNA. The principle of recording genetic information is also common to them, as well as the genetic code. The same amino acids are encrypted in pro - and eukaritis the same codons. It is fundamentally the same way in the named cell types, the use of hereditary information stored in DNA is also carried out. At first, it is transcribed in the nucleotide sequence of mRNA molecule, and then translated into the amino acid sequence of the peptide on ribosomes with the participation of TRNA. However, some features of the organization of hereditary material, distinguishing eukaryotic cells from prokaryotic, determine differences in the use of their genetic information.

The hereditary material of the prokaryotic cell is mainly in a single ring DNA molecule. It is located directly in the cytoplasm of the cell, where the TRNA and enzymes are also necessary for the expression of genes, some of which are enclosed in ribosomes. Procarniot genes consist entirely of coding nucleotide sequences implemented during the synthesis of proteins, TRNA or RRNA.

The hereditary material of eukaryotes is larger in volume than the prokaryotes. It is located mainly in special nuclear structures - chromosomes that are separated from the cytoplasm with a nuclear shell. The device required for the synthesis of proteins, consisting of ribosomes, TRNA, a set of amino acids and enzymes, is in the cytoplasm of the cell.

Significant differences are available in the molecular organization of the genes of the eukaryotic cell. In most of them, the encoding sequences of exon are interrupted by intron sections that are not used in the synthesis of TRNA, RRNA or peptides. The number of such sites varies in different genes. It is established that the ovalbumin chic gene includes 7 introns, and the mammalian is punctured - 50. These areas are removed from the primary transcribed RNA, and therefore the use of genetic information in the eukaryotic cell occurs somewhat differently. In prokaryotic cell, where the hereditary material and the protein biosynthesis apparatus is not spatially disunity, transcription and translation occur almost simultaneously. In the eukaryotic cell, these two stages are not only spatially separated by a nuclear shell, but also in time they are separated by the process of ripening mRNA, from which non-informative sequences should be removed (Fig. 35).

Fig. 35. A generalized scheme of the process of expression of genetic information in a eukaryotic cell

In addition to these differences, at each stage of the expression of genetic information, some features of these processes of the pro - and eukaryota can be noted.

Transcription of pro - and eukaryotes. Transcription is the synthesis of RNA on the DNA matrix. In prokaryotes, the synthesis of all three types of RNA is catalyzed by one complex protein complex - RNA polymerase.

The transcriptional apparatus of eukaryotic cells includes three nuclear RNA polymerases, as well as RNA polymerase mitochondria and plastids. RNA polymerase I is detected in cell nuclei and is responsible for the transcription of RRNA genes. RNA polymerase II is localized in nuclear juice and is responsible for the synthesis of mRNA predecessor. RNA polymerase III is a small fraction located in nuclear juice and performing the synthesis of small RDNA and TRNA. Each of these enzymes has two large subunits to 10 small. RNA polymerase mitochondria and plastids differ from nuclear.

The RNA polymerase enzyme complex specifically recognizes a certain nucleotide sequence (often not one) located at a certain distance from the starting point of the transcription, the promoter. The starting point is the DNA nucleotide, which corresponds to the first nucleotide, included by the enzyme in the RNA transcript.

Prokaryotes usually not far from the starting point against the transcription movement there is a sequence of six nucleotides - Tatataat, called the Pribrnian block. This is an average sequence consisting of the most common foundations, the most conservative of which are 1.2 and the 6th base. The presence in this sequence of bases, preferably connected by double hydrogen bonds with complementary bases of another chain, obviously facilitates the local melting of the DNA double helix and the formation of two single-chain areas when contacting RNA polymerase. The Pribnn bloc is located in position from - 11 to - 5 or from - 14 to - 8, i.e. For several nucleotides before starting point of transcription (Fig. 36). Receiving this sequence, RNA polymerase firmly associated with it and starts the synthesis of RNA. As an important role in establishing the RNA polymerase contact with DNA belongs to another nucleotide sequence, the center of which is in position - 35. It is called the recognition area - Ttgatsa. Between the two specified areas, the distance is constantly constantly and ranges from 16 to 19 pairs of nucleotides (n. N).

The camomotors of eukaryotic genes also include at least two specific nucleotide sequences, the centers of which are in position - 25 and - 75 p. N.

At a distance of 19-27 nucleotides from the start point against the transcription progress, many eukaryotic genes found the average sequence of TAT A T A A T (TATA-Block, or Hognes Block), in which, as well as the procnite block, the bases prevail, Forming weak bonds. The second sequence found in many eukaryotic promoters and consisting of GG C TsAncet is denoted as CAAA block. It occupies a position between - 70 and - 80 nucleotides and is also an area recognizable by polymerase. In some genes, multicomponent promoters were found.

Thus, in separate genes of herpes virus, three DNA sequences located between - 19 and - 27 are needed to effectively initiate transcription, and between - 80 and 105 nucleotides.

Fig.36. Contact Points for RNA polymerase, located in the upper DNA chain (promoter)

The features of the promoter sites indicate that not only the combination of bases in certain areas of the promoter is important to initiate transcription in certain areas of the DNA of these regions, with which the RNA polymerase enzyme complex is associated.

After establishing the contact between the RNA polymerase and the promoter portion, the assembly of the RNA molecule begins, into which the nucleotide is first turned on, carrying a purine base (usually adenine) and containing three 5 "phosphate residues.

Further, as the RNA polymerase moves along the DNA molecule, there is a gradual elongation of the RNA chain, which continues to the enzyme meeting with the terminator area. The terminator is a plot where the further growth of the RNA circuit ceases and its release occurs from the DNA matrix. RNA polymerase is also separated from DNA, which restores its two-stranded structure.

Fig.37. DNA area with double symmetry - Palindrome:

I - Palindrome, in which there is a sequence equally when reading in opposite directions;

II - Palindrome, in which the shaded inverted repeat is at a distance from the axis of symmetry

In prokaryotic cells, terminators necessarily contain palindromes - double-stranded sequences of DNA nucleotides, which are equally read in both directions (Fig. 37). The RNA section transcribed from such a sequence is capable of forming double-stranded studs due to complementary pairing of palindrome nucleotides. It is possible that this is a signal to complete the transcription, recognizable RNA polymerase (Fig.3.38). The emerging studs apparently stop the polymerase on the terminator. Following the stud in the RNA molecule, a sequence of nucleotides containing uracil (polyu), which probably takes part in the release of RNA from the DNA matrix. Indeed, the RNA polyuz-sequence connected to the polyadenyl (poly) sequence of DNA is characterized by weak interaction. Attention is drawn to the fact that the DNA section rich in pairs AA is found not only at the site of transcription initiation (Siberian Block), but also in the Terminator region.

Bacterial terminators differ significantly in their effectiveness. Some of them, as if not coming RNA polymerase, and it continues transcription outside the Terminator. Such support of the terminator during the transcription of bacterial genes is observed as a result of the prevention of termination by specific proteins - antiterly factors. The consequence of antiterly is the synthesis of polycistron mRNA, which includes information written off with several sequentially located structural genes.

Terminators of eutricogical genes are learned to a lesser extent than in spakari, but they also found areas rich in Mr. couples connected by triple hydrogen bonds in which the site with A-T pairs. On this section, the transcript includes a polyu-sequence, which is weakly interacting with the DNA matrix poly.

Perhaps the area of \u200b\u200bthe terminator rich in Mr. couples plays a certain role in the RNA-polymerase stop, and the RNA portion containing the UUU provides a transcript separation from the DNA matrix.

Eukariot has no formation of structures similar to heels in prokaryotic RNA. Therefore, how they are carried out by the transcription termination, it remains unclear.

As part of all mRNA, you can select coding areas representing a set of codons that encrypt the sequence of amino acids in the peptide. As a rule, these sites begin with the starting codon Aug, but sometimes the bacteria uses Code Gut. At the end of the coding sequence is the terminal codon. In addition to encoding sections in mRNA, additional sequences can be placed at both ends. On the 5th "-conace, this is a leader area located in front of the starting codon. On the 3" -conception - the trailer next to the codon-terminator.

Fig.38. Education of the heel section of RNA during transcription termination in prokaryot

Palindrome RNA area forms a complementary pairing structure - hairpin (inverted repeats are shaded)

In polycistron mRNA prokaryotes between encoding areas there are intercouthry areas, varying in size (Fig.3.39).

Fig.39. Polycistron matrix RNA prokaryotic:

1 - non-coding areas, 2 - intercreant regions, 3 - coding areas, 4 - Terminating codons

Due to the fact that prokaryotic genes are entirely consisting of the nucleotide sequences involved in the encoding of information transcribed from them RNA immediately after their synthesis are able to perform the function of translation matrices. Only in exceptional cases their preliminary maturation is required - processing.

In contrast to prokaryotic genes, most of the genes of eukaryotic cells are intermittent, since they carry non-informative nucleotide sequences in their composition - intons that are not involved in the encoding of information. In this regard, the primary transcripts, synthesized RNA polymerase II, have large than necessary for broadcast, sizes and are less stable. In the aggregate, they form the so-called heterogeneous nuclear RNA (TRNA), which is before exiting the kernel and begin to actively function in the cytoplasm, is subjected to processing and turns into mature mRNA.

Processing eukaryotic mRNA. Ripening, or processing, mRNA implies modification of the primary transcript and removing non-corrective intron sections from it, followed by a compound (splicing) of coding sequences - exons. Modifying the primary transcript of eukaryotic mRNA begins shortly after the synthesis of its 5 "-concar containing one of the purine bases (adenine or guanine). At this end, the cap is formed - the CEP blocks 5" -Conal mRNA by attaching to the first nucleotide of the trifosphosposphonucleoside transcript containing Guanin, bond 5 "-5".

HFFF + FFAFN ... → Goffafn. + FF + F The result is a sequence of Hoffffchm ..., in which the remnant of touanine is in reverse orientation relative to other mRNA nucleotides. Modification of 5 "- Conference MRNA also involves methylation of the attached guanine and the first two to three bases of the primary transcript (Fig. 8.40). Caps formed on 5" - the ends of the CEPA mRNA ensure recognition of molecules by mRNA with small sub-displeteers ribosomes in the cytoplasm. Caching is carried out before the end of the primary transcript synthesis.

Fig. 40. Education of mature MRNA eukaryotes during processing:

1 - Inexient sequences, 2 - exons, 3 - Introns, 4 - Code Terminator

After completion of the transcription, the nucleotide part of the nucleotide is removed on the 3 "-conace of the primary transcript and the addition to it consisting of 100-200 residues of adenyl acid (poly) (Fig. 8.40). It is believed that this sequence contributes to the further processing and transport of mature mRNA from Nuclei. After the release of mRNA in the cytoplasm, its poly-sequence is gradually shortening under the action of enzymes that swipe nucleotides on 3 "-Conacea. Thus, along the length of poly-sequence, it is possible to indirectly judge the time of residence of mRNA in the cytoplasm. It is possible that the addition of poly-sequence during processing increases the stability of mRNA. However, about a third of MRNA does not contain a poly-segment. These include, for example, histone mRNA.

The formation of Caps on the 5 "-conace and poly-sequence of 3" is characteristic only for RNA processing, synthesized RNA polymerase II. In addition to methylation in the formation of the CEPs in the MRNA of higher eukaryotes, methylation of a small part of internal nucleotides with a frequency is approximately one per thousand grounds of mRNA.

Along with the modification of MRNA eukaryotes, the processing assumes the removal of non-informative transcripts for this protein of intron sections, the size of which varies from 100 to 10,000 nucleotides and more. Introns account for about 80% of the entire Garnna. The removal of introns with the subsequent compound of exon areas is called splicing (Fig. 40).

Splasing is a mechanism that should be removed from the primary transcript of strictly defined intron sections. The violation of this process can lead to a shift of the reading frame when broadcasting and the impossibility of the synthesis of a normal peptide. The pattern of cutting the introns is obviously ensured due to the presence at their ends of specific nucleotide sequences that serve as a signal for splicing.

Currently, several probable splicing mechanisms that ensure the accuracy of this process are currently described. It is possible that it is achieved by the action of some enzymes specifying the end portions of the intron and catalyzing the tear of phosphodiesfire bonds on the border of the exon - intron, and then the formation of connections between the two exons.

It has been actively involved in the splicing of special small, nuclear RNA (meroNNA), forming complexes with proteins (moonbrip). Obviously, with its nucleotide sequences, it interacts with its nucleotide sequences with end portions of introns, which form closed loops. RNA splitting at the mouth of an intron loop leads to the removal of non-informative sequence and compound (splicing) of the nasty ends of exons.

The autocatalytic ability of RNA transcript to splicing is also discussed. The described splicing methods indicate the absence of a universal mechanism of this process, but in all cases an accurate removal of introns is achieved with the formation of a certain mRNA, which ensures the synthesis of the required protein cell.

Currently, the possibility of alternative (mutually exclusive) splicing is proved, in which various nucleotide sequences can be removed from the same primary transcript and different mature mRNAs are formed. As a result, the same DNA nucleotide sequence can serve as information for the synthesis of different peptides. Alternative splaxing is probably very characteristic of the system of immunoglobulin genes in mammals, where it allows you to form on the basis of a single MRNA transcript for synthesis different species antibodies

Thanks to the transformations occurring with a RNA transcript during processing, mature eukaryotes mRNA are characterized by greater stability compared to prokaryotic mRNA.

Upon completion of the processing, the mature mRNA passes the selection before going into the cytoplasm, where only 5% garnnek falls. The rest is split, without leaving the kernel.

Thus, the transformations of primary transcripts of eukaryotic genes caused by their exonitron organization and the need to transition mRNA from the nucleus to cytoplasm, determine the features of the implementation of genetic information in the eukaryotic cell.

Translation from pro - and eukaryot. In prokaryotic cells, the transmission process is associated with the synthesis of mRNA: they occur almost simultaneously. To a large extent, this is due to the short-life of bacterial mRNA, which is quickly resolved. The interconnectedness of transcription and broadcast in the bacteria is manifested in consistency of the speeds of these processes. At 37 ° C, transcription comes at a speed of 2500 nucleotides / min (14 codons / s), and the broadcast is carried out at a speed of 15 amino acids / s.

Translation in prokaryott begins shortly after formation 5 "- Conference MRNA, earlier than its synthesis ends. As a result, ribosomes engaged by assembling peptide chains (Fig.41) are moving after RNA polymerase on mRNA. After some time after the start of the transcription (about 1 min) and until the conversion of the 3 "Conference of the matrix begins the degradation of its 5" - Conference. Due to the fact that the lifetime of different mRNA is not the same, the amount of protein synthesized on different matrices is different.

One of the features of the transmission in prokaryotes is the inclusion in the peptide chain as the first amino acid of the modified methionine - formylmineine, from which all the newly synthesized peptides begin. Even in the case when the role of the starting codon performs the google code, under normal conditions encrypting valine, in the first position of the peptide is formylmethionine. The starting codon of AUG or GUG follows the leader site that is shielded by the ribosome at the time of the initiation of the broadcast.

The mRNA ribosome compound is due to the complementary interaction of nucleotides of one of the RRNA with the nucleotide sequence of the MRNA leader.

This sequence (Chayne-Dalgalo) is located at a distance of 4-7 bases before the AUG codon and is found everywhere in the leader sections in prokaryotes.

When connecting 5 "-confaction of mRNA with a small sub-displeteness of the ribosome, the starting codon is usually almost in the middle of the ribosome fragment of mRNA, in the region corresponding to its P-section.

In eukaryota, the broadcast is carried out in the cytoplasm, which hits the nucleus of mature mRNA. The copied end of the MRNA is recognized by the small sub-displeteness of the ribosome, then the leading sequence containing up to 100 nucleotides interacts with RRNA. At the same time, the starting codon of AUG is in an unfinished P-section of the Ribosoma. After connecting to the starting codon of aminoacyl-trading carrier of methionine, the reunion of two sub-interpreters of ribosomes is reunited and its A - and P-sections are formed. The protein synthesis in the eukaryotic cell, carried out on monocystron mRNA, is completed after passing the ribosome throughout the mRNA, up to the recognition of the codon-terminator, which ceases to form peptide bonds.

Postranslation transformations proteins. Peptide chains synthesized during the transmission, based on its primary structure, acquire secondary and tertiary, and many - and a quaternary organization formed by several peptide chains. Depending on the functions performed by proteins, their amino acid sequences can undergo various transformations, forming functionally active protein molecules.

Many membrane proteins are synthesized in the form of predelkov, having a leader sequence on the N-terminus, which provides the HimDoys of the membrane. This sequence is cleaved when ripening and embedding a protein into a membrane. Secretor proteins also have a leader sequence at the N-terminus, which provides them with transport through the membrane. Some proteins immediately after the broadcast carry additional amino acid procedures, which determine the stability of the precursors of active proteins. When the protein is ripening, they are removed, providing a transition of an inactive spacing to an active protein. For example, insulin is initially synthesized as preproinsulin. During secretion, the pre-sequence is cleaved, and then the proinsulin is subjected to a modification in which part of the chain is removed from it and it turns into mature insulin.

Fig.41. Transcription, translation and degradation of mRNA in prokaryotm:

I - RNA polymerase binds to DNA and begins to synthesize mRNA in the direction 5 "→ 3";

II - as RNA polymerase moves to 5, the ribosomes, beginners of the protein synthesis, are attached to the 5 "

III - Ribosoma Group follows RNA polymerase, on 5 "-Concert MRNA begins its degradation;

IV - the degradation process takes place slower than transcription and broadcast;

V - After the end of the transcription of mRNA is released from DNA, broadcasting and degradation on 5 "

Forming a tertiary and quaternary organization during post-translation transformations, proteins acquire the ability to actively function, including certain cellular structures and exercising enzymatic and other functions.

The considered features of the implementation of genetic information in pro- and eukaryotic cells are found to be the fundamental similarity of these processes. Consequently, the gene expression mechanism associated with transcription and subsequent broadcast of information, which is encrypted with the help of biological code, has developed in general even before these two types of cell organization were formed. The divergent evolution of genomes of pro - and eukaryota led to the emergence of differences in organizing their hereditary material, which could not but affect the mechanisms of expression.

The continuous improvement of our knowledge of the organization and functioning of the material of heredity and variability determines the evolution of the representations of the gene as a functional unit of this material.

G E N E T and KA

Genetics - science, which studies the patterns of heredity and variability.

Heredity – the property of all living organisms to transmit features of their structure and the development of descendants.

Variability–the property of all living organisms to change the hereditary information received from parents, as well as the process of its implementation during individual development (ontogenesis).Variability is a property opposite to heredity.

These two concepts are closely connected with each other.

The term "genetics" was first proposed in 1906 by the English scientist W. Baton, however, the history of the development of this science is rooted in the distant past.

The entire history of the development of genetics can be divided into four stages:

The existence of a speculative hypotheses about the nature of heredity.

Opening of the basic laws of heredity.

Study of heredity at the cellular level.

Study of heredity at the molecular level.

Structural and functional levels of the organization of hereditary material

In the hereditary structure of the cell and the body as a whole, there are three levels of the organization of genetic material: gene, chromosomal and genomic.

Gene level

The smallest (elementary) unit of hereditary material is the gene.

The gene is part of a DNA molecule having a certain sequence of nucleotides and is a unit of functioning of hereditary material.

The gene carries information on a specific feature or property of the body.

A person has about 30 thousand genes.

Change in the gene structure leads to a change in the corresponding feature. Consequently, an individual inheritance and individual variability of signs are provided at the gene level.

Chromosomal level

All genes in the cage are combined into groups and are located in chromosomes in linear order. Each chromosome is unique in the set of genes included in it. The chromosome includes DNA, proteins (histone and non-district), RNA, polysaccharides, lipids and metal ions.

The chromosomal level in eukaryotic cells ensures the nature of the functioning of individual genes, the type of their inheritance and the regulation of their activity. It allows you to naturally reproduce and transmit hereditary information in the process of dividing the cell.

Genomic level

Genome – the combination of all genes in the haploid set of chromosomes. In fertilization, the two genomes of parent gamets merge and form the genotype.

Genotype – a combination of all genes enclosed in the diploid set of chromosomes, or a karyotype. The karyotype is a complete set of chromosomes, characterized in each type of them strictly defined by the number and structure.

The genomic level is highly stable. It provides a complex system of interaction of genes. The result of the interaction of genes with each other and with the factors of the external environment is the phenotype.

Molecular bases of heredity

The gene as an elementary unit of hereditary information performs certain functions and has certain properties.

Genov functions:

storage of hereditary information;

protein biosynthesis and other substances in the cell;

control over the development and aging of the cell.

Genov Properties:

discreteness: one gene controls one sign;

specificity: Each gene answers strictly for its sign;

stability Stability: Genes are transmitted from generation to generation without changing;

action dosage: one gene defines one dose of phenotypic manifestation;

ability to mutating (change in structure);

ability to replicate (self-defense);

the ability to recombination (transition from one homologous chromosome to another).

Functional classification of genes

All genes are divided into three groups:

structural - control the development of signs by synthesizing the corresponding enzymes;

regulatory - manage the activities of structural genes;

modulatory - Displays the process of manifestation of signs towards its strengthening or weakening, up to full blocking.

Features of the structure of genes

in prokaryotic and eukaryotic cells

Cells in nature are divided into prokaryotic and eukaryotic. Prokaryota gene has a continuous structure, i.e. It is a part of the DNA molecule.

In eukaryota, the gene consists of alternating sites: exons and intron . EXECE - informative site, intron - non-informative. The number of introns in different genes is irregular (from 1 to 50).

Expression (manifestation of action) gene in the process of protein synthesis

The whole protein synthesis process is conditionally divided into three stages: transcription,

processing and broadcast.

Transcription

Transcription – the process of rewriting information from the DNA molecule on and-RNA. Proceeds in the kernel.

The DNA molecule consists of two spiral twisted threads. Each thread is represented by a sequence of nucleotides, and each nucleotide consists of a carbohydrate (pentose), a nitrogenous base and a phosphoric acid residue.

Each thread of DNA molecules has two ends - hydroxyl (3) and phosphate (5). Threads are located in relation to each other antipherally.

Synthesis and RNA in the cell always comes from phosphate end to hydroxyl. Therefore, a transcription matrix is \u200b\u200bone DNA thread facing the synthesizing enzyme with its hydroxyl end; it is called codec or informative (and the other thread, respectively, non-coherent, or non-informative).

Transcription is divided into three periods:

initiation

elongation,

termination.

Based on the above definitions of heredity and variability, it can be assumed what the requirements should be the material substrate of these two properties of life.

First, the genetic material must have ability to self-reproduction To. The process of reproduction transmit inheritance information on the basis of which the formation of a new generation will be implemented. Secondly, to ensure the stability of the characteristics in a number of generations, the hereditary material should keep a constant organization. Thirdly, the material of heredity and variability should have the ability acquire changes and reproduce them By providing the possibility of historical development of live matter in changing conditions. Only in case of compliance with the specified requirements, the material substrate of heredity and variability can ensure the duration and continuity of the existence of wildlife and its evolution.

Modern ideas about the nature of the genetic apparatus allow you to allocate three levels of its organization: gene, chromosomaland genomic. Each of them shows the basic properties of the material of heredity and variability and certain patterns of its transfer and functioning.

^

3.4. Gene level of the organization of the genetic apparatus

The elementary functional unit of the genetic apparatus, which determines the possibility of developing a separate feature of a cell or the body of this species is gene (Hereditary deposit, in Mendel). Transmission of genes in a number of generations of cells or organisms is achieved material continuity - inheritance with descendants of signs of parents.

Under sign understand the unit of morphological, physiological, biochemical, immunological, clinical, and any other discreteness of organisms (cells), i.e. A separate quality or property for which they differ from each other.

Most of the features listed above the features of organisms or cells belongs to the category complex signs The formation of which requires the synthesis of many substances, primarily proteins with specific properties - enzymes, immunoproteins, structural, contractile, transport and other proteins. Properties of the protein molecule are determined by the amino acid sequence of its polypeptide chain, which is directly defined by the sequence of nucleotides in the DNA of the corresponding gene and is elementary or simple, sign.

The main properties of the gene as a functional unit of the genetic apparatus are determined by its chemical organization,

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3.4.1. Chemical organization Gena

Studies aimed at finding out the chemical nature of hereditary material, irrefutably proved that the material substrate of heredity and variability is nucleic acids, which were found by F. Misher (1868) in the nuclei of the cells of the pus. Nucleic acids are macromolecules, i.e. Different with a large molecular weight. These are polymers consisting of monomers - nucleotides including three components: sugar(pentose), phosphate and nitrogenous base (Pourin or Pyrimidine). To the first carbon atom in the pentose molecule of the P-1, a nitrogen base (adenine, guanine, cytosine, thymine or uracil) is joined, and to the fifth carbon C-5 carbon "with the help of essential communication - phosphate; The third carbon atom C-3 "always has a hydroxyl group - it is (Fig. 3.1).

The nucleotide compound in a nucleic acid macromolecule occurs by the interaction of the phosphate of one nucleotide with the hydroxyl of the other so that it is established between them phosphodieter communication (Fig. 3.2). As a result, a polynucleotide chain is formed. The chain isza consists of alternating phosphate and sugar molecules. The pentose molecules in the C-1 position "is attached one of the above nitrogen bases (Fig. 3.3).

Fig. 3.1. The scheme of the structure of nucleotide

See the explanation in the text; Nucleotide components designations used in this figure are maintained in all subsequent nucleic acid schemes

The polynucleotide chain assembly is carried out with the participation of the polymerase enzyme, which ensures the addition of the phosphate group of the next nucleotide to the hydroxyl group facing 3 ", the previous nucleotide (Fig. 3.3). Thanks to the noted specific action of the name of the enzyme, the polynucleotide chain extension occurs only at one end: there where there is a free hydroxyl in position 3 ". The start of the chain always carries the phosphate group in position 5. This allows you to allocate in it 5 "and 3" - ends.

Among nucleic acids distinguish two types of compounds: deoxyribonucleinova(DNA) I. ribonucleinova(RNA) Acids. The study of the composition of the main carriers of hereditary material - chromosomes - found that they are the most chemically stable component is DNA, which is a substrate of heredity and variability.

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3.4.1.1. DNA structure. Model J. Watson and F. Cry

DNA consists of nucleotides, which includes sugar - deoxyribosis, phosphate and one of nitrogenous bases - Purin (adenin or guanine) or pyrimidine (thymine or cytosine).

A feature of the DNA structural organization is that its molecules include two polynucleotide chains related to a certain way. In accordance with the three-dimensional DNA model proposed in 1953 by the American Biophysician J. Watson and the English biophysician and Genetic F. Screech, these chains are connected to each other with hydrogen bonds between their nitrogen bases on the principle of complementarity. Adenine of one chain is connected by two hydrogen bonds with a thymine of another chain, and three hydrogen bonds are formed between the guanin and cytosine of different circuits. Such a compound of nitrogen base provides a solid connection between two chains and maintain an equal distance between them all over.

Fig. 3.4. DNA molecule structure

Arrows indicated antilarality purposes

Another important feature of the combining of two polynucleotide chains in the DNA molecule is their anti-parallelity: 5 "Conference of one chain is connected to 3" - Conference to another, and vice versa (Fig. 3.4).

Data x-ray structural analysis have shown that the DNA molecule consisting of two chains is formed a spiral twisted around its own axis. The diameter of the spiral is 2 nm, the length of the step is 3, 4 nm. Each round includes 10 pairs of nucleotides.

Most often, double spirals are human rights - when moving upwards along the axis of the circuit helix rotate to the right. Molel DNA molecules in the solution is in human rights - in-form (B-DNA). However, there are also left-handed forms (Z-DNA). What amount of this DNA is present in cells and what its biological value is not yet established (Fig. 3.5).

Fig. 3.5. Spatial models of the left-eacked Z-form ( I.)

And human rights in-form ( II.) DNA

Thus, in the structural organization of the DNA molecule, you can allocate primary structure - polynucleotide chain, secondary structure- two complementary each other and anti-parallel polynucleotide chains connected by hydrogen bonds, and tertiary structure - Three-dimensional spiral with the above spatial characteristics.

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3.4.1.2. The method of recording genetic information in the DNA molecule. Biological code and its properties

Primary all variety of life is determined by a variety of protein molecules performing various biological functions in cells. The structure of proteins is determined by the set and order of the amino acids in their peptide chains. It is this sequence of amino acids in peptides encrypted into DNA molecules using biological(genetic) Code. The relative primitiveness of the DNA structure representing the alternation of only four different nucleotides, for a long time prevented researchers to consider this compound as a material substrate of heredity and variability in which extremely diverse information should be encrypted.

In 1954, Gamov, it was suggested that the coding of information in DNA molecules should be carried out by combinations of several nucleotides. In the manifold of proteins that exist in nature, about 20 different amino acids were found. For the encryption of these numbers, a sufficient number of combinations of nucleotides can only provide triplet code In which each amino acid is encrypted by three nucleotides standing nearby. In this case, 4 3 \u003d 64 triplets are formed from four nucleotides. The code consisting of two nucleotides would give the opportunity to encrypt only 4 2 \u003d 16 different amino acids.

Complete decapitation of the genetic code was carried out in the 60s. of our century. Of the 64 possible DNA tripts 61 encodes various amino acids; The remaining 3 were called meaningless, or "nonsense-triplets". They do not encrypt amino acids and perform the feature of punctuation marks when reading hereditary information. These include ATT, ACS, ATC.

The explicit redundancy of the code appears that many amino acids are encrypted with several triplets (Fig. 3.6). This property of a triplet code called degenerate It has very important importance, since the occurrence of changes in the DNA molecule by the type of substitution of one nucleotide-otide in the polynucleotide chain may not change the meaning of the triplet. Thus, a new combination of three nucleotides encodes the same amino acid.

In the process of studying the properties of the genetic code, it was discovered specificity. Each triplet is capable of encoding only one specific amino acid. An interesting fact is the complete correspondence of the code in various types of living organisms. Such universality The genetic code indicates the unity of origin of the entire variety of living forms on Earth in the process of biological evolution.

Minor differences in the genetic code are found in the Mitochondrial DNA of some species. This does not contradict the same provision on the universality of the code, but testifies in favor of a certain divergence in its evolution in the early stages of the existence of life. Deciphering the code in the DNA mitochondria of various species showed that in all cases in mitochondrial DNA there is a general feature: the ACC triplet is read as ACC, and therefore from the nonsense triplet turns into a tryptophan amino acid cipher.

Fig. 3.6. Amino acids and coding DNA thrills

Other features are specific to various types of organisms. In yeast triplet GAT and, perhaps, all family of ha encodes instead of amino acid leucine threonine. In mammals, tryptile tag has the same meaning as the TAC, and encodes the amino acid methionine instead of isoleucine. TCG and TCC trailelets in the Mitochondrial DNA of some species do not encode amino acids, being nonsense triplets.

Along with triplet, degeneration, specificity and universality, its most important characteristics of the genetic code are continuity and unfolding codons when reading. This means that the sequence of nucleotides is read by a triplet for a triplet without skipping, while the adjacent pulls do not overlap each other, i.e. Each individual nucleotide is part of only one triplet at a given reading frame (Fig. 3.7). The proof of the non-replaceability of the genetic code is to replace only one amino acid in the peptide when replacing one nucleotide into DNA. In the case of the inclusion of a nucleotide into several overlapping triplets, its replacement entails a replacement for 2-3 amino acids in the peptide chain.

Fig. 3.7. Continuity and continuity of genetic code

When reading hereditary information

Nucleotides are indicated by numbers

According to the chemical organization of the material of heredity and variability, eukaryotic and prokaryotic cells are not fundamentally different from each other. The genetic material is presented with DNA. The principle of recording genetic information is also common to them, as well as the genetic code. The same amino acids are encrypted in pro- and eukaritis the same codons. It is fundamentally the same way in the named cell types, the use of hereditary information stored in DNA is also carried out. However, some features of the organization of hereditary material, distinguishing eukaryotic cells from prokaryotic, determine differences in the use of their genetic information.

The hereditary material of the prokaryotic cell is mainly in a single ring DNA molecule.

The hereditary material of eukaryotes is larger in volume than the prokaryotes. It is located mainly in chromosomeswhich are separated from the cytoplasm of a nuclear shell.

Significant differences are available in the molecular organization of the genes of the eukaryotic cell. Most of them coding sequences exonsions Interrupted intron plots that are not used in the synthesis of TRNA, RRNA or peptides. These areas are removed from the primary transcribed RNA, and therefore the use of genetic information in the eukaryotic cell occurs somewhat differently. In prokaryotic cell, where the hereditary material and the protein biosynthesis apparatus is not spatially disunity, transcription and translation occur almost simultaneously. In the eukaryotic cell, these two stages are not only spatially separated by a nuclear shell, but in time they are separated by the process of ripening mRNA, from which non-informative sequences should be removed.

Chemical organization of genetic material.

Gene level.

The elementary functional unit of the genetic apparatus, which determines the possibility of developing a separate feature of a cell or the body of this species is gene (Hereditary deposit, in Mendel). Transmission of genes in a number of generations of cells or organisms is achieved material continuity - inheritance with descendants of signs of parents.

Under sign understand the unit of morphological, physiological, biochemical, immunological, clinical, and any other discreteness of organisms (cells), i.e. A separate quality or property for which they differ from each other.

Chromosomal level.

Eucariot cell genes are distributed by chromosomes, forming a chromosomal level of organization of hereditary material. This level of the organization serves as a necessary condition for the adhesion of genes and the redistribution of parental genes in the descendants during sexual reproduction (crosslinker).

Chromosomes - Nucleoprotein structures in the kernel of the eukaryotic cell, in which most of the hereditary information is concentrated and which are intended for its storage, implementation and transmission.

Genomic level.

Genome - The whole set of hereditary material concluded in the haploid set of chromosomes of cells of this type of organisms. The species-specific genome, as it is the necessary set of genes, which ensures the formation of the species characteristics of organisms during their normal ontogenesis.

Gena structure.

Studies aimed at finding out the chemical nature of hereditary material, irrefutably proved that the material substrate of heredity and variability is nucleic acids, which were found by F. Misher (1868) in the nuclei of the cells of the pus. Nucleic acids are macromolecules, i.e. Different with a large molecular weight. These are polymers consisting of monomers - nucleotidesincluding three components: sugar(pentose), phosphate and nitrogenous base (Pourin or Pyrimidine). To the first carbon atom in the pentoses molecule, the C-1 'is joined by a nitrogen base (adenine, guanine, cytosine, thymine or uracil), and to the fifth carbon atom C-5' using the essential communication - phosphate; The third carbon atom C-3 'always has a hydroxyl group - it.

The nucleotide compound in a nucleic acid macromolecule occurs by the interaction of the phosphate of one nucleotide with the hydroxyl of the other so that it is established between them phosphodieter communication. As a result, a polynucleotide chain is formed. The chain isza consists of alternating phosphate and sugar molecules. The pentose molecules in the C-1 position are attached one of the above nitrogen bases.

DNA structure, properties and functions.

DNA consists of nucleotides, which includes sugar - deoxyribosis, phosphate and one of the nitrogen bases - adenine, guanine, thymine, cytosine. DNA molecules include two polynucleotide chains associated with a certain way. Watson and Creek suggested that these chains are connected to each other with hydrogen bonds between their nitrogen bases on the principle of complementarity. Adenine of one chain is connected by two hydrogen bonds with a thymine of another chain, and three hydrogen bonds are formed between the guanin and cytosine of different circuits. Such a compound of nitrogen base provides a solid connection between two chains and maintain an equal distance between them all over. Another important feature of two polynucleotide chains in the DNA molecule is their anti-parallelity: the 5th end of one chain is connected to the 3rd one other and vice versa. Data x-ray diffraction analysis has shown that the DNA molecule consisting of two chains is formed a spiral twisted around its axis. The diameter of the helix 2 nm, the length of the step is 3.4 nm. Each round includes 10 pairs of nucleotides. So In the structural organization of the DNA molecule, you can select the primary structure - polynucleotide chain, secondary - two complementary and anti-parallel chains and tertiary

The structure is a three-dimensional spiral.

DNA is capable of self-copying - replication. In the process of replication on each polynucleotide chain, the DNA Molecule is synthesized by the complementary chain. As a result, two identical double spirals are formed from one double helix of the DNA. This method of doubling molecules, in which each daughter molecule is one maternal and one newly synthesized chain is called half-party. To perform the replication of maternal DNA, should be separated from each other to become matrices on which complementary chains of subsidiaries will be synthesized. With the Helicase enzyme, the DNA double helix in separate zones is broken. Single-chain areas formed at the same time are associated with special destabilizing proteins. The molecules of these proteins are built along polynucleotide chains, stretching their caskets and making nitrogenous bases available for binding to complementary nucleotides. The areas of discrepancy between polynucleotide chains in replication zones are called replication forks. In each such area, the DNA of two new subsidiaries is synthesized with the participation of the DNA polymerase enzyme. In the process of synthesis, the replication plug moves along the maternal spiral, capturing all new zones. The final result of replication is the formation of two DNA molecules, the nucleotide sequence of which is identical to that in the maternal double helix of DNA,