Functional genetic organization of DNA. Structural and functional levels of organization of hereditary material

Molecular basis heredity All prokaryotes and eukaryotes have a special class of bioorganic substances - nucleic acids, divided according to their chemical composition and biological role into deoxyribonucleic acids (DNA) and ribonucleic acids (RNA).

Both types of nucleic acid acids are thread-like molecules consisting of individual structural units - nucleotides, connected into a multi-unit polynucleotide chain. Each nucleotide consists of the following three chemically distinct parts: I) 5-carbon sugar residues, deoxyribose (in DNA) and ribose (in RNA), forming the “backbone” of the polynucleotide strand; 2) four nitrogenous bases adenine (A), guanine (G), cytosine (C) and thymine (T) (in the RNA molecule the last base is replaced by uracil U), and each nitrogenous base is covalently connected to the first carbon atom of the sugar through glycosidic bond; 3) a phosphate group that connects neighboring nucleotides into a single chain through the formation of phosphodiester bonds between the 5" carbon atom of one sugar and the 3 carbon atom of another.

Genetic recording information is carried out linearly from the 5" end to the 3" end of the nucleic acid molecule. One such molecule can contain up to many millions of nucleotides.

Molecules in a cell DNA exist in the form of a helical double chain (double helix), the threads of which are antiparallel, i.e. have the opposite orientation. The double strand of DNA is formed due to weak hydrogen bonds between complementary bases: adenine is strictly complementary to thymine, and cytosine is strictly complementary to guanine.

Under certain conditions These hydrogen bonds can be broken, leading to the appearance of single-stranded molecules (DNA denaturation), and subsequently formed again between the same complementary regions (renaturation, or DNA hybridization). During the hybridization process, the original DNA double helix is accurately restored. It is the presence of complementarity that ensures both the accuracy of DNA self-reproduction in each cycle of cell division (this process is called replication) and the restoration of the damaged nucleotide composition of the DNA molecule. Due to the complementarity of nucleotides in the double helix, the length of the DNA molecule is usually expressed in base pairs (bp), as well as thousands of base pairs (kilobases, kb) and millions of base pairs (megabases, mb). The DNA of humans as a biological species includes about 3 billion bp.

Directed DNA molecule synthesis in the cell is carried out by a special enzyme - DNA polymerase. This process involves the “unwinding” of the double helix at the synthesis site and the formation of a special protein-nucleic structure - a replication fork; the gradual advancement of the replication fork along the double helix is accompanied by the sequential addition of bases complementary to the single-stranded DNA template to the newly formed chain (synthesis of the growing DNA chain always proceeds strictly in the direction from 5" to 3").

Complementary DNA synthesis requires the presence in the environment of individual “building blocks” for elongation of the growing molecule - four types of deoxyribonucleotide triphosphate molecules (dATP, dTTP, dCTP and dGTP). The entire process is initiated by special seeds - primers, which are short oligonucleotide molecules complementary to a specific starting section of the DNA template.

Based on the above definitions of heredity and variability, we can assume what requirements the material substrate of these two properties of life must meet.

First, the genetic material must have ability to reproduce itself, to in. in the process of reproduction, transmit hereditary information on the basis of which the formation of a new generation will be carried out. Secondly, to ensure stability of characteristics over a number of generations, the hereditary material must keep your organization constant. Thirdly, the material of heredity and variability must have the ability acquire changes and reproduce them, providing the possibility of the historical development of living matter in changing conditions. Only if the specified requirements are met, the material substrate of heredity and variability can ensure the duration and continuity of the existence of living nature and its evolution.

Modern ideas about the nature of the genetic apparatus allow us to distinguish three levels of its organization: genetic, chromosomal And genomic. Each of them reveals the basic properties of the material of heredity and variability and certain patterns of its transmission and functioning.

3.4. GENE LEVEL OF ORGANIZATION OF THE GENETIC APPARATUS

The elementary functional unit of the genetic apparatus, which determines the possibility of developing a separate characteristic of a cell or organism of a given species, is gene(hereditary deposit, according to G. Mendel). By transferring genes over a series of generations of cells or organisms, material continuity is achieved - the inheritance of the characteristics of their parents by descendants.

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 by which they differ from each other.

Most of the features of organisms or cells listed above fall into 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. The properties of a protein molecule are determined by the amino acid sequence of its polypeptide chain, which is directly determined by the sequence of nucleotides in the DNA of the corresponding gene and is elementary, or a simple sign.

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

3.4.1. Chemical organization of the gene

Research aimed at elucidating the chemical nature of the hereditary material has irrefutably proven that the material substrate of heredity and variability is nucleic acids, which were discovered by F. Miescher (1868) in the nuclei of pus cells. Nucleic acids are macromolecules, i.e. have a high molecular weight. These are polymers consisting of monomers - nucleotides, including three components: sugar(pentose), phosphate And nitrogenous base(purine or pyrimidine). A nitrogenous base (adenine, guanine, cytosine, thymine or uracil) is attached to the first carbon atom in the C-1 pentose molecule, and a phosphate is attached to the fifth carbon atom C-5 using an ester bond; the third carbon atom C-3" always has a hydroxyl group - OH (Fig. 3.1).

The joining of nucleotides into a nucleic acid macromolecule occurs through the interaction of the phosphate of one nucleotide with the hydroxyl of another so that a phosphodiester bond(Fig. 3.2). As a result, a polynucleotide chain is formed. The backbone of the chain consists of alternating phosphate and sugar molecules. One of the nitrogenous bases listed above is attached to the pentose molecules at position C-1 (Fig. 3.3).

Rice. 3.1. Nucleotide structure diagram

See text for explanation; the nucleotide component designations used in this figure are retained in all subsequent nucleic acid diagrams

The assembly of a polynucleotide chain is carried out with the participation of the enzyme polymerase, which ensures the attachment of the phosphate group of the next nucleotide to the hydroxyl group located in position 3", of the previous nucleotide (Fig. 3.3). Due to the noted specificity of the action of the named enzyme, the growth of the polynucleotide chain occurs only at one end: there , where the free hydroxyl is at position 3". The beginning of the chain always carries a phosphate group at position 5". This allows us to distinguish 5" and 3" - ends.

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

3.4.1.1. DNA structure. J. Watson and F. Crick model

DNA consists of nucleotides, which include sugar - deoxyribose, phosphate and one of the nitrogenous bases - purine (adenine or guanine) or pyrimidine (thymine or cytosine).

A feature of the structural organization of DNA is that its molecules include two polynucleotide chains connected to each other in a certain way. In accordance with the three-dimensional model of DNA, proposed in 1953 by the American biophysicist J. Watson and the English biophysicist and geneticist F. Crick, these chains are connected to each other by hydrogen bonds between their nitrogenous bases according to the principle of complementarity. Adenine of one chain is connected by two hydrogen bonds to thymine of another chain, and three hydrogen bonds are formed between guanine and cytosine of different chains. This connection of nitrogenous bases ensures a strong connection between the two chains and maintaining an equal distance between them throughout.

Rice. 3.4. Diagram of the structure of a DNA molecule

Arrows indicate anti-parallelism of targets

Another important feature of the combination of two polynucleotide chains in a DNA molecule is their antiparallelism: the 5" end of one chain is connected to the 3" end of the other, and vice versa (Fig. 3.4).

X-ray diffraction data showed that a DNA molecule, consisting of two chains, forms a helix twisted around its own axis. The helix diameter is 2 nm, the pitch length is 3.4 nm. Each turn contains 10 pairs of nucleotides.

Most often, double helices are right-handed - when moving upward along the helix axis, the chains turn to the right. Most DNA molecules in solution are in the right-handed - B-form (B-DNA). However, left-handed forms (Z-DNA) also occur. How much of this DNA is present in cells and what its biological significance is has not yet been established (Fig. 3.5).

Rice. 3.5. Spatial models of left-handed Z-shape ( I)

And right-handed B-form ( II) DNA

Thus, in the structural organization of the DNA molecule we can distinguish primary structure - polynucleotide chain, secondary structure- two complementary and antiparallel polynucleotide chains connected by hydrogen bonds, and tertiary structure - a three-dimensional spiral with the above spatial characteristics.

3.4.1.2. A method of recording genetic information in a DNA molecule. Biological code and its properties

Primarily, the diversity of life is determined by the diversity of protein molecules that perform various biological functions in cells. The structure of proteins is determined by the set and order of amino acids in their peptide chains. It is this sequence of amino acids in peptides that is encrypted in DNA molecules using biological(genetic)code. The relative primitiveness of the DNA structure, representing the alternation of only four different nucleotides, has long prevented researchers from considering this compound as a material substrate of heredity and variability, in which extremely diverse information should be encrypted.

In 1954, G. Gamow suggested that the encoding of information in DNA molecules should be carried out by combinations of several nucleotides. In the variety of proteins that exist in nature, about 20 different amino acids have been discovered. To encrypt such a number of them, a sufficient number of nucleotide combinations can only be provided triplet code, in which each amino acid is encrypted by three adjacent nucleotides. In this case, 4 3 = 64 triplets are formed from four nucleotides. A code consisting of two nucleotides would make it possible to encrypt only 4 2 = 16 different amino acids.

The complete deciphering of the genetic code was carried out in the 60s. of our century. Of the 64 possible DNA triplets, 61 code for different amino acids; the remaining 3 were called meaningless, or “nonsense triplets.” They do not encrypt amino acids and act as punctuation marks when reading hereditary information. These include ATT, ACT, ATC.

Noteworthy is the obvious redundancy of the code, manifested in the fact that many amino acids are encrypted by several triplets (Fig. 3.6). This is a property of a triplet code called degeneracy, is very important, since the occurrence of changes in the structure of the DNA molecule such as the replacement of one nucleotide in a polynucleotide chain may not change the meaning of the triplet. The new combination of three nucleotides thus created 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 different types of living organisms. Such versatility The genetic code testifies to the unity of origin of the entire diversity of living forms on Earth in the process of biological evolution.

Minor differences in the genetic code have been found in the mitochondrial DNA of some species. This does not generally contradict the proposition that the code is universal, but it does testify to a certain divergence in its evolution in the early stages of the existence of life. Deciphering the code in the DNA of mitochondria of various species showed that in all cases, mitochondrial DNA has a common feature: the triplet ACC is read as ACC, and therefore turns from a nonsense triplet into a code for the amino acid tryptophan.

Rice. 3.6. Amino acids and DNA triplets encoding them

Other features are specific to different types of organisms. In yeast, the GAT triplet and possibly the entire GA family encodes threonine instead of the amino acid leucine. In mammals, the TAG triplet has the same meaning as TAC and encodes the amino acid methionine instead of isoleucine. TCG and TCC triplets in the mitochondrial DNA of some species do not encode amino acids, being nonsense triplets.

Along with triplicity, degeneracy, specificity and universality, the most important characteristics of the genetic code are its continuity And non-overlapping codons during reading. This means that the nucleotide sequence is read triplet by triplet without gaps, and neighboring triplets do not overlap each other, i.e. each individual nucleotide is part of only one triplet for a given reading frame (Fig. 3.7). Proof of the non-overlapping genetic code is the replacement of only one amino acid in the peptide when replacing one nucleotide in the DNA. If a nucleotide is included in several overlapping triplets, its replacement would entail the replacement of 2-3 amino acids in the peptide chain.

Rice. 3.7. Continuity and indisputability of the genetic code

When reading hereditary information

The numbers indicate nucleotides

DNA is a complex organic compound that is a material carrier of hereditary information. It is a double unbranched linear polymer, the monomers of which are nucleotides. A DNA nucleotide consists of a nitrogenous base, a phosphoric acid residue, and a deoxyribose carbohydrate. There are 4 types of nucleotides, differing in nitrogenous base: adenine, which includes adenine, cytosine - cytosine, guanine - guanine, thymine - thymine. The nitrogenous base of one DNA strand is connected by a hydrogen bridge to the base of another, so that A is connected to T, and G to C. They are complementary to each other. It is on this that the property of DNA is based, which explains its biological role: the ability to reproduce itself, i.e. to autoreproduction. Autoreproduction of DNA molecules occurs under the influence of polymerase enzymes. In this case, the complementary chains of DNA molecules unwind and diverge. Then each of them begins to synthesize a new one. Since each of the bases in nucleotides can attach another nucleotide only of a strictly defined structure, exact reproduction of the parent molecule occurs.
The main biological function of DNA is the storage, constant self-renewal and transmission of genetic information in the cell.
The genetic code is a system for the arrangement of nucleotides in a DNA molecule that controls the sequence of amino acids in the DNA molecule. The genes themselves are not directly involved in protein synthesis. The mediator between gene and protein is mRNA. The gene is the template for constructing the mRNA molecule. Encoding of information must be carried out by combinations of several nucleotides. 20 amino acids were found in the diversity of proteins. To encrypt such a number of them, a sufficient number of combinations of nucleotides can only be provided by a triplet code, in which each amino acid is encrypted by three adjacent nucleotides. In this case, 64 triplets are formed from 4 nucleotides. Of the 64 DNA triplets, 61 encode different amino acids, the remaining 3 are called meaningless, or nonsense triplets, they act as punctuation marks. The sequence of triplets determines the order of amino acids in the protein molecule.
Properties of the genetic code:
Degeneracy. It manifests itself in the fact that many amino acids are encrypted by several triplets.
Specificity. Each triplet can code for only one specific amino acid
Versatility. Evidence of the unity of origin of the entire diversity of living forms on Earth in the process of biological evolution.
Along with these properties, the most important characteristics of the genetic code are the continuity and indisputability of codons during reading. This means that the nucleotide sequence is read triplet by triplet without gaps, and adjacent triplets do not overlap each other.

Research aimed at elucidating the chemical nature of hereditary material has irrefutably proven that the material substrate of heredity and variability arenucleic acids, which were discovered by F. Miescher (1868) in the nuclei of pus cells. Nucleic acids are macromolecules, i.e. have a high molecular weight. These are polymers consisting of monomers - nucleotides, including three components: sugar(pentose), phosphate And nitrogenous base(purine or pyrimidine). A nitrogenous base (adenine, guanine, cytosine, thymine or uracil) is attached to the first carbon atom in the C-1 pentose molecule, and a phosphate is attached to the fifth carbon atom C-5 using an ester bond; the third carbon atom C-3" always has a hydroxyl group - OH ( see diagram ).

Rice. 3.1. Nucleotide structure diagram

DNA structure. Model by J. Watson et al. Scream

DNA consists of nucleotides, which include sugar - deoxyribose, phosphate and one of the nitrogenous bases - purine (adenine or guanine) or pyrimidine (thymine or cytosine).

Rice. 3.4. Diagram of the structure of a DNA molecule. The arrows indicate the antiparallelism of the circuits

Rice. 3.5. Spatial models of left-handed Z-shape ( I)

and right-handed B-form ( II) DNA

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. During the replication process, a complementary chain is synthesized on each polynucleotide chain of the parent DNA molecule. As a result, two identical double helices are formed from one DNA double helix. This method of doubling molecules, in which each daughter molecule contains one parent and one newly synthesized chain, is called semi-conservative(see Fig. 2.12).

For replication to occur, the chains of maternal DNA must be separated from each other to become templates on which complementary chains of daughter molecules will be synthesized.

Initiation of replication occurs in special regions of DNA called ori (from the English origin - beginning). They include a sequence of 300 nucleotide pairs that is recognized by specific proteins. The DNA double helix in these loci is divided into two chains, and, as a rule, areas of divergence of polynucleotide chains are formed on both sides of the origin of replication - replication forks, which move in opposite directions from the locus ori directions. Between replication forks a structure called replication eye, where new polynucleotide chains are formed on two strands of maternal DNA (Figure 3.8, A).

The end result of the replication process is the formation of two DNA molecules, the nucleotide sequence of which is identical to that of the parent DNA double helix.

DNA replication in prokaryotes and eukaryotes is basically similar; however, the rate of synthesis in eukaryotes (about 100 nucleotides/s) is an order of magnitude lower than in prokaryotes (1000 nucleotides/s). The reason for this may be the formation of eukaryotic DNA in fairly strong compounds with proteins (see Chapter 3.5.2.), which complicates its despiralization necessary for replicative synthesis.

In 1869, Swiss biochemist Friedrich Miescher discovered compounds with acidic properties and even higher molecular weights than proteins in the nucleus of cells. Altman called them nucleic acids, from the Latin word “nucleus” - nucleus. Just like proteins, nucleic acids are polymers. Their monomers are nucleotides, and therefore nucleic acids can also be called polynucleotides.

Nucleic acids have been found in the cells of all organisms, from the simplest to the highest. The most surprising thing is that the chemical composition, structure and basic properties of these substances turned out to be similar in a variety of living organisms. But if about 20 types of amino acids take part in the construction of proteins, then there are only four different nucleotides that make up nucleic acids.

Nucleic acids are divided into two types - deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). DNA contains nitrogenous bases (adenine (A), guanine (G), thymine (T), cytosine (C)), deoxyribose C5H10O4 and a phosphoric acid residue. RNA contains uracil (U) instead of thymine, and ribose (C5H10O5) instead of deoxyribose. The monomers of DNA and RNA are nucleotides, which consist of nitrogenous, purine (adenine and guanine) and pyrimidine (uracil, thymine and cytosine) bases, a phosphoric acid residue and carbohydrates (ribose and deoxyribose).

DNA molecules are found in the chromosomes of the cell nucleus of living organisms, in the equivalent structures of mitochondria, chloroplasts, in prokaryotic cells and in many viruses. The structure of the DNA molecule is similar to a double helix. Structural model of DNA in
form of a double helix was first proposed in 1953 by the American biochemist J. Watson and the English biophysicist and geneticist F. Crick, who together with the English biophysicist M. Wilkinson, who received the X-ray diffraction pattern of DNA, were awarded the 1962 Nobel Prize. Nucleic acids are biopolymers, the macromolecules of which consist of repeatedly repeating units - nucleotides. Therefore they are also called polynucleotides. The most important characteristic of nucleic acids is their nucleotide composition. The composition of a nucleotide - a structural unit of nucleic acids - includes three components:

nitrogenous base - pyrimidine or purine. Nucleic acids contain four different types of bases: two of them belong to the class of purines and two to the class of pyrimidines. The nitrogen contained in the rings gives the molecules their basic properties.

monosaccharide - ribose or 2-deoxyribose. The sugar that is part of the nucleotide contains five carbon atoms, i.e. is a pentose. Depending on the type of pentose present in the nucleotide, two types of nucleic acids are distinguished - ribonucleic acids (RNA), which contain ribose, and deoxyribonucleic acids (DNA), which contain deoxyribose.

phosphoric acid residue. Nucleic acids are acids because their molecules contain phosphoric acid.

The method for determining the composition of PC is based on the analysis of hydrolysates formed during their enzymatic or chemical breakdown. Three methods of chemical cleavage of NC are commonly used. Acid hydrolysis under severe conditions (70% perchloric acid, 100°C, 1h or 100% formic acid, 175°C, 2h), used for the analysis of both DNA and RNA, leads to the cleavage of all N- glycosidic bonds and the formation of a mixture of purine and pyrimidine bases.

Nucleotides are linked into a chain through covalent bonds. The nucleotide chains formed in this way are combined into one DNA molecule along the entire length by hydrogen bonds: the adenine nucleotide of one chain is connected to the thymine nucleotide of the other chain, and the guanine nucleotide to the cytosine one. In this case, adenine always recognizes only thymine and binds to it and vice versa. A similar pair is formed by guanine and cytosine. Such base pairs, like nucleotides, are called complementary, and the principle of the formation of a double-stranded DNA molecule is called the principle of complementarity. The number of nucleotide pairs, for example, in the human body is 3 - 3.5 billion.

DNA is a material carrier of hereditary information, which is encoded by a sequence of nucleotides. The location of the four types of nucleotides in the DNA chains determines the sequence of amino acids in protein molecules, i.e. their primary structure. The properties of cells and the individual characteristics of organisms depend on the set of proteins. A certain combination of nucleotides that carry information about the structure of the protein and the sequence of their location in the DNA molecule form the genetic code. A gene (from the Greek genos - genus, origin) is a unit of hereditary material responsible for the formation of any trait. It occupies a section of the DNA molecule that determines the structure of one protein molecule. The set of genes contained in a single set of chromosomes of a given organism is called the genome, and the genetic constitution of the organism (the set of all its genes) is called the genotype. Violation of the nucleotide sequence in the DNA chain, and therefore in the genotype, leads to hereditary changes in the body - mutations.

DNA molecules are characterized by the important property of duplication - the formation of two identical double helices, each of which is identical to the original molecule. This process of doubling a DNA molecule is called replication. Replication involves the breaking of old and the formation of new hydrogen bonds that unite nucleotide chains. At the beginning of replication, the two old strands begin to unwind and separate from each other. Then, according to the principle of complementarity, new chains are attached to the two old chains. This creates two identical double helices. Replication ensures accurate copying of genetic information contained in DNA molecules and passes it on from generation to generation.

DNA composition

DNA (deoxyribonucleic acid)- a biological polymer consisting of two polynucleotide chains connected to each other. The monomers that make up each DNA strand are complex organic compounds containing one of four nitrogenous bases: adenine (A) or thymine (T), cytosine (C) or guanine (G); pentaatomic sugar pentose - deoxyribose, from which DNA itself is named, as well as a phosphoric acid residue. These compounds are called nucleotides. In each chain, nucleotides are joined by forming covalent bonds between the deoxyribose of one nucleotide and the phosphoric acid residue of the next nucleotide. Two chains are combined into one molecule using hydrogen bonds that arise between nitrogenous bases that are part of the nucleotides that form different chains.

By examining the nucleotide composition of DNA of various origins, Chargaff discovered the following patterns.

1. All DNA, regardless of their origin, contains the same number of purine and pyrimidine bases. Consequently, in any DNA there is one pyrimidine nucleotide for every purine nucleotide.

2. Any DNA always contains equal amounts in pairs of adenine and thymine, guanine and cytosine, which are usually denoted as A=T and G=C. The third follows from these regularities.

3. The number of bases containing amino groups in position 4 of the pyrimidine nucleus and 6 of the purine nucleus (cytosine and adenine) is equal to the number of bases containing an oxo group in the same positions (guanine and thymine), i.e. A+C=G+T . These patterns are called Chargaff's rules. Along with this, it was found that for each type of DNA the total content of guanine and cytosine is not equal to the total content of adenine and thymine, i.e. that (G+C)/(A+T), as a rule, differs from unity (maybe both more and less of it). Based on this feature, two main types of DNA are distinguished: A T-type with a predominant content of adenine and thymine and G C-type with a predominant content of guanine and cytosine.

The ratio of the content of the sum of guanine and cytosine to the sum of the content of adenine and thymine, characterizing the nucleotide composition of a given type of DNA, is usually called specificity coefficient. Each DNA has a characteristic specificity coefficient, which can vary from 0.3 to 2.8. When calculating the specificity coefficient, the content of minor bases is taken into account, as well as the replacement of major bases with their derivatives. For example, when calculating the specificity coefficient for wheat germ EDNA, which contains 6% 5-methylcytosine, the latter is included in the sum of the content of guanine (22.7%) and cytosine (16.8%). The meaning of Chargaff's rules for DNA became clear after its spatial structure was established.

Macromolecular structure of DNA

In 1953, Watson and Crick, relying on known data on the conformation of nucleoside residues, the nature of internucleotide bonds in DNA and the regularities of the nucleotide composition of DNA (Chargaff's rules), deciphered x-ray diffraction patterns of the paracrystalline form of DNA [the so-called B-form, formed at a humidity above 80 % and at a high concentration of counterions (Li+) in the sample]. According to their model, the DNA molecule is a regular helix formed by two polydeoxyribonucleotide chains twisted relative to each other and around a common axis. The diameter of the helix is almost constant along its entire length and is equal to 1.8 nm (18 A).

Macromolecular structure of DNA.

(a)-Watson-Crick model;

(6) parameters of the B-, C- and T-form DNA helices (projections perpendicular to the helix axis);

(G)-parameters of the DNA helix in A-form;

(d)- cross section of a DNA helix in A-shape.
The length of the helix turn, which corresponds to its identity period, is 3.37 nm (33.7 A). For one turn of the helix there are 10 base residues in one chain. The distance between the base planes is thus approximately 0.34 nm (3.4 A). The planes of the base residues are perpendicular to the long axis of the helix. The planes of carbohydrate residues deviate somewhat from this axis (initially Watson and Crick suggested that they were parallel to it).

The figure shows that the carbohydrate-phosphate backbone of the molecule faces outward. The spiral is twisted in such a way that two grooves of different sizes can be distinguished on its surface (they are often also called grooves) - a large one, about 2.2 nm wide (22 A), and a small one, about 1.2 nm wide (12 A). The spiral is dextrorotatory. The polydeoxyribonucleotide chains in it are antiparallel: this means that if we move along the long axis of the helix from one end to the other, then in one chain we will pass phosphodiester bonds in the 3"à5" direction, and in the other - in the 5"à3 direction ". In other words, at each end of a linear DNA molecule there is a 5" end of one strand and a 3" end of another strand.

The regularity of the helix requires that a purine base residue on one chain be opposite a pyrimidine base residue on the other chain. As already emphasized, this requirement is implemented in the form of the principle of the formation of complementary base pairs, i.e., adenine and guanine residues in one chain correspond to thymine and cytosine residues in the other chain (and vice versa).

Thus, the sequence of nucleotides in one chain of a DNA molecule determines the nucleotide sequence of the other chain.

This principle is the main consequence of the Watson and Crick model, since it explains in surprisingly simple chemical terms the main functional purpose of DNA - to be the storehouse of genetic information.

Concluding the consideration of the Watson and Crick model, it remains to add that neighboring pairs of base residues in DNA, which is in the B-form, are rotated relative to each other by 36° (the angle between the straight lines connecting the C 1 atoms in adjacent complementary pairs).
4.1 Isolation of deoxyribonucleic acids
Living cells, with the exception of sperm, normally contain significantly more ribonucleic acid than deoxyribonucleic acid. Methods for isolating deoxyribonucleic acids have been greatly influenced by the fact that, while ribonucleoproteins and ribonucleic acids are soluble in a dilute (0.15 M) solution of sodium chloride, deoxyribonucleoprotein complexes are actually insoluble in it. Therefore, the homogenized organ or organism is thoroughly washed with a dilute saline solution, and deoxyribonucleic acid is extracted from the residue using a strong saline solution, which is then precipitated by adding ethanol. On the other hand, elution of the same residue with water gives a solution from which the deoxyribonucleoprotein precipitates when salt is added. Cleavage of the nucleoprotein, which is basically a salt-like complex between polybasic and polyacid electrolytes, is easily achieved by dissolution in a strong saline solution or treatment with potassium thiocyanate. Most protein can be removed either by adding ethanol or by emulsifying with chloroform and amyl alcohol (the protein forms a gel with chloroform). Detergent treatments were also widely used. Later, deoxyribonucleic acids were isolated by extraction with aqueous n-aminosalicylate-phenolic solutions. Using this method, deoxyribonucleic acid preparations were obtained, some of which contained residual protein, while others were virtually free of protein, indicating that the nature of the protein-nucleic acid association differs in different tissues. A convenient modification is to homogenize the animal tissue in 0.15 M phenolphthaleine diphosphate solution, followed by the addition of phenol to precipitate DNA (free of RNA) in good yield.

Deoxyribonucleic acids, no matter how they are isolated, are mixtures of polymers of different molecular weights, with the exception of samples obtained from certain types of bacteriophages.
4.2 Fractionation
An early separation method involved the fractional dissociation of deoxyribonucleoprotein (eg, nucleohistone) gels by extraction with aqueous solutions of increasing molarity sodium chloride. In this way, deoxyribonucleic acid preparations were divided into a number of fractions characterized by different ratios of adenine and thymine to the sum of guanine and cytosine, with fractions enriched in guanine and cytosine being more easily isolated. Similar results were obtained by chromatographic separation of deoxyribonucleic acid from histone adsorbed on kieselguhr using gradient elution with sodium chloride solutions. In an improved version of this method, purified histone fractions were combined with n-aminobenzylcellulose to form diazo bridges from the tyrosine and histidine groups of the protein. Fractionation of nucleic acids on methylated serum albumin (with diatomaceous earth as a carrier) has also been described. The rate of elution from the column with saline solutions of increasing concentration depends on the molecular weight, composition (nucleic acids with a high guanine content with cytosine elute more easily) and secondary structure (denatured DNA is more firmly retained by the column than native DNA). In this way, a natural component, polydeoxyadenylic-thymidylic acid, was isolated from the DNA of the sea crab Cancer borealis. Fractionation of deoxyribonucleic acids was also carried out by gradient elution from a column filled with calcium phosphate.

Functions of DNA

In the DNA molecule, the sequence of amino acids in peptides is encrypted using a biological code. Each amino acid is encoded by a combination of three nucleotides, in this case 64 triplets are formed, of which 61 encode amino acids, and 3 are meaningless and serve as punctuation marks (ATT, ACT, ATC). The encryption of one amino acid by several triplets is called triplet code degeneracy. Important properties of the genetic code are its specificity (each triplet is capable of encoding only one amino acid), universality (indicating the unity of origin of all life on Earth) and non-overlapping codons when read.

DNA performs the following functions:

storage of hereditary information occurs with the help of histones. The DNA molecule folds, first forming a nucleosome, and then heterochromatin, which makes up chromosomes;

transmission of hereditary material occurs through DNA replication;

implementation of hereditary information in the process of protein synthesis.

Which of the above structural and functional features of the DNA molecule allow it to store and transmit hereditary information from cell to cell, from generation to generation, to provide new combinations of characteristics in the offspring?

1. Stability. It is provided by hydrogen, glycosidic and phosphodiester bonds, as well as by the mechanism of repair of spontaneous and induced damage;

2. Replication ability. Thanks to this mechanism, the diploid number of chromosomes is maintained in somatic cells. All of the listed features of DNA as a genetic molecule are shown schematically in the figure.

3. Presence of genetic code. The sequence of bases in DNA is converted through the processes of transcription and translation into the sequence of amino acids in a polypeptide chain;
4. Capacity for genetic recombination. Thanks to this mechanism, new combinations of linked genes are formed.