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CHEMICAL COMPOSITION AND STRUCTURE OF NUCLEIC ACIDS

Nucleic acids were first discovered by I.F. Misher in 1868. He isolated a special substance of an acidic nature from the nuclei of cells and called it nuclein. Subsequently, he was given the name "nucleic acid." Two types of nucleic acids have been discovered. They were named depending on the carbohydrate component in the composition. Nucleic acid, which contains the carbohydrate of deoxyribose, was called deoxyribonucleic acid (DNA), and which contains the carbohydrate of ribose, called ribonucleic acid (RNA). In the period from 1900 to 1932, the chemical composition of nucleic acids was determined. They include the following components:

RNA DNA

Purine bases Adenine, guanine Adenine, guanine

Pyrimidine bases cytosine, thymine cytosine, uracil

Carbohydrate component of Deoxyribose Ribose

Both nucleic acids include phosphoric acid residues. The difference lies in the fact that the RNA contains the nitrogenous base uracil instead of thymine and ribose instead of deoxyribose.

In 1936, A. N. Belozersky and I. I. Dubrovskaya first isolated pure DNA from plant material at the Department of Plant Biochemistry of Moscow University. By the mid-40s, it was found that DNA and RNA are simultaneously present in every living organism.

In the late 40s and early 50s, new physical and chemical methods of research began to be used in the study of nucleic acids. In 1950, E. Chargaff established the rules of nucleotide relations that underlie the structure of all DNA.

The rules of Chargaff are that in DNA the content of adenine is equal to the content of thymine (A = T), and the content of guanine is equal to the content of cytosine (G = H), hence A + G / T + C = 1; the sum of purine nucleotides is equal to the sum of pyrimidine nucleotides. In accordance with this rule, the nucleotide composition of different organisms can vary only in magnitude A + T / G + C.

By 1952, R. Franklin and M. Wilkins were able to obtain high-quality x-rays of DNA, which showed that it has a spiral shape and a dual structure.

In 1953, J. Watson and F. Crick, based on the data of X-ray analysis and the rules of Charguff, established the structure of DNA. According to their model, the DNA molecule has a double helix consisting of two polynucleotide chains with a common axis (Fig. 17). The diameter of the double helix of DNA is 2 nm, and the distance between the turns of 3.4 nm. For each coil of the helix there are 10 pairs of nucleotides, hence the distance between the nitrogenous bases is 0.34 nm.

The structural units of polynucleotide chains are nucleotides. The composition of the nucleotide includes: one of the nitrogenous bases - purine (adenine or guanine) or pyrimidine (thymine or cytosine), deoxyribose, phosphate residue. These components are connected to each other in the following order: nitrogen base - deoxyribose - phosphate residue.
The combination of one of the bases with deoxyribose leads to the formation of a nucleoside. When a phosphate group is attached to the carbohydrate moiety of a nucleoside, a nucleotide is formed.

Deoxyribose in nucleotides binds to the bases with a glycoside bond, and with phosphoric acid - through ether bonds. Therefore, in terms of chemical composition, any nucleotide is a phosphoric ester of nucleosides. Accordingly, nucleotides are called deoxyadenylic, deoxyguanilic, deoxycytidyl and thymidyl acids.

Along with the main nitrogenous bases, DNA also contains methylated bases, such as 5-methylcytosine, 5-hydroxymethylcytosine, etc. In animals, the amount of 5-methylcytosine in DNA usually does not exceed 1.5–2%.

In each of the DNA strands, the nucleotides are connected in series with each other using a phosphoric acid residue and a deoxyribose molecule. Deoxyribose binds to one molecule of phosphoric acid through carbon in the 3 'position, and on the other through carbon 5', forming a carbohydrate-phosphate backbone (Fig. 18).

Both chains in the DNA molecule have the opposite polarity. This means that the internucleotide bond in one chain

has a direction of 5 '-> 3', and in the other 3 '-> 5'.

The nitrogenous bases of the nucleotides of both DNA chains are enclosed internally between the turns of the helix and are connected by hydrogen bonds. In accordance with the rules of Chargaff, adenine of one chain is associated only with thymine of the other chain, and guanine is associated only with cytosine. The adenine-thymine pair is connected by two hydrogen bonds, and the guanine-cytosine pair by three. This order of compliance of nitrogen bases (A ** T and G ** D) is called

complementarity, and therefore the chains in DNA are complementary; they complement each other.

The carbohydrate-phosphate backbone along the entire length in all DNA molecules has the same structure and cannot carry genetic information. In contrast, the arrangement of purine and pyrimidine bases of nucleotides along the DNA chain is very variable and characteristic of each given type of DNA molecule. Hence, the hereditary information is encrypted with a different sequence of bases.

The nucleotide composition of DNA varies significantly depending on the membership of an organism to a particular systematic group (Table 7). The specificity of DNA is expressed by the ratio A + T / G + C, called the coefficient of species specificity.

In the DNA of animals there is an excess of A + T with respect to G + C. In fungi and bacteria, forms are found both rich in A + T and with a predominance of G + C, while at the same time there are close animals in terms of specificity. This suggests that the variability in the arrangement of the bases is already sufficient to ensure differences between the genes of these organisms.

DNA molecules consist of approximately 2–10–1–10 or more nucleotides and have a large relative molecular weight.
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CHEMICAL COMPOSITION AND STRUCTURE OF NUCLEIC ACIDS

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