Deoxyribonucleic Acid DNA

an acid present in every organism and in every living cell, mainly in the nucleus; a nucleic acid containing deoxyribose as the sugar and adenine, guanine, cytosine, and thymine as nitrogenous bases. DNA plays a very important biological role by preserving and transmitting (through heredity) genetic information concerning the structure, development, and individual characters of all living organisms. DNA preparations can be obtained from a variety of animal and plant tissues, as well as from bacteria and DNA-containing viruses.

DNA is an organic polymer consisting of many monomers—deoxyribonucleotides—linked by phosphoric acid residues in a certain sequence specific to each DNA molecule. The unique sequence of deoxyribonucleotides in a given DNA molecule is a coded record of biological information. Two such polynucleotide chains form a double helix in which complementary bases—adenine with thymine, and guanine with cytosine—are linked by hydrogen bonds and so-called hydrophobic interaction. This characteristic structure is responsible for both the biological and the physiochemical properties of DNA. The large number of phosphate residues makes DNA a powerful polybasic acid (polyanion), present in tissues in the form of salts. The presence of purine and pyrimidine bases accounts for the intensive absorption of ultraviolet rays (with a peak at a wavelength of about 260 mµ,). Heating DNA solutions weakens the bond between the bases, and at a certain temperature, characteristic of the particular DNA (usually 80°90°C), the two polynucleotide chains separate (denaturation).

The molecular weight of native DNA molecules reaches the hundreds of millions. Only in mitochondria and some viruses and bacteria is the molecular weight significantly lower. In such cases the DNA molecules are ring-shaped sometimes—for example, in the Ø×174 bacteriophage—a single strand) or, less commonly, linear in structure. DNA is found in the cell nucleus mainly in the form of DNA proteins—complexes with proteins (chiefly histones) that form the characteristic nuclear structures chromosomes and chromatin. The nucleus of each somatic cell (diploid) in an individual of a given species contains a constant amount of DNA, and the nucleus of a germ cell (haploid) contains half that amount. In polyploidy the amount of DNA is higher and proportional to the ploidy. During cell division the amount of DNA doubles in the interphase (in the so-called synthetic, or S, period, between the G₁ and G₂ periods of mitosis). The process of doubling (replication) consists of the unwinding of the double helix and the synthesis in each polynucleotide chain of a new chain complementary to it. Thus, each of the two new DNA molecules, identical to the old molecule, contains one old and one newly synthesized polynucleotide chain. DNA is synthesized in the organism from nucleoside triphosphates, rich in free energy, under the influence of the enzyme DNA polymerase. At first small portions of the polymer are synthesized, later combining into longer chains under the influence of the enzyme DNA polymerase. Outside the organism DNA is synthesized in the presence of all four types of deoxyribonucleoside triphosphates, the corresponding enzymes, and a DNA template (on which the complementary nucleotide sequence is synthesized). The American scientist A. Kornberg was the first to produce this reaction (1967). He was able to prepare biologically active viral DNA in vitro by enzymatic synthesis. In 1968, H. Khorana (United States) synthesized chemically a polydeoxyribonucleotide corresponding to the structural gene (cistron) of DNA.

DNA also serves as a template for the synthesis of ribonucleic acids (RNA), thereby determining their primary structure (transcription). Transfer RNA (t-RNA) mediates translation—the synthesis of specific proteins whose structure is assigned by DNA in the form of a particular nucleotide sequence. Thus, where RNA transports the biological information “recorded” in DNA molecules to the synthesized protein molecules, DNA preserves this information and transmits it by heredity. This role of DNA is demonstrated by the fact that purified DNA from a single bacterial strain can transmit to other strains the characters typical of the donor strain, and by the fact that the DNA of a virus latent in bacteria of one strain can transmit portions of the DNA of these bacteria to another strain when that strain is infected with this virus, reproducing the corresponding characters in the recipient strain. Thus, the hereditary elements (genes) are physically incorporated (in a particular sequence of nucleotides) in portions of the DNA molecule and can be transmitted with these portions from one individual to another. Hereditary changes in organisms (mutations) are caused by the change, loss, or inclusion in the polynucleotide chains of nitrogenous bases, these can be the result of physical or chemical influences. Elucidation of the structure of DNA molecules and the changes that take place in them will make it possible to induce hereditary changes in animals, plants, and microorganisms and to correct hereditary defects.