The existence of living matter is associated with the presence of nucleic acids. F. Misher first isolated these substances from leukocyte nuclei (1869). They were later found in all cells of living organisms (humans, animals, plants, bacteria and viruses).
Nucleic acids are prostatic groups of nucleoproteins. Hydrolysis of nucleic acids produces nitrogenous bases (adenine, cytosine, guanine, uracil and thymine), pentoses (deoxyribose, ribose) and phosphate acid. Given the biochemical composition, nucleic acids are classified into deoxyribonucleic (DNA) and ribonucleic (RNA) acids. The structure of human DNA includes deoxyribose, and RNA includes ribose. These acids differ in the structure of molecules, the composition of nitrogenous bases, cellular localization, and, of course, functions. Bio compounds whose molecule consists of a purine or pyrimidine base and a monose (ribose or deoxyribose) are called nucleosides. The name of the nucleoside is determined by the nitrogenous base that is contained in it. So, a nucleoside, which includes an adenine molecule in its structure, is called adenosine, thymine - thymidine, uracil - uridine, cytosine - cytidine, guanine - guanosine. Depending on the monosaccharides (pentoses), which are part of the molecules, they distinguish between ribonucleosides and deoxyribonucleosides.
Human DNA is the chemical basis of genes in which hereditary information of an organism is concentrated. It is localized mainly in the nuclei of cells, mainly in the chromosomes. During DNA hydrolysis, nucleotides are formed: deoxyadenylic (A), deoxyguanilic (G), deoxycytidyl (C) and thymidyl (T) acids. Sometimes in the nucleotide composition other derivatives of purines and pyrimidines are found in small quantities - minor bases: 5-hydroxymethylcytosine (in bacteriophages), 5-methylcytosine (in thymus tissues), and others. Nucleotides are connected into a polynucleotide chain (DNA) by oxygen bridges formed by hydroxyl - the phosphate acid residue of one nucleotide and the hydroxyl group at the third carbon atom of the deoxyribose residue of the second nucleotide.
The sequence of nucleotide placement in a human DNA molecule is studied by sequencing (eng. Seguence - sequence study). To do this, use a sequencer device, which, based on computer analysis, sets the sequence of nucleotide placement (up to one hundred units).
The number of nucleotides in the composition of a human DNA molecule is 25,000 - 35,000 or more, and the molecular weight is from several million to 2-5 billion. DNA molecules can be "seen" in an electron microscope. The absolute number of nucleotides of four species (A, G, C and T) in DNA molecules of different origin varies in wide ranges.
The primary structure of DNA
F. Crick and D. Watson in 1953 established that the deoxyribonucleic acid molecule is a double helix of polynucleotide chains twisted around its axis. The spiral resembles a spiral staircase in which the handrails are formed by deoxyribose residues interconnected by phosphorus-ether bonds of type 3 - 5, and the steps by nitrogenous bases. Adenine is connected by hydrogen bonds with thymine, guanine - with cytosine. The deoxyribonucleic acid configuration may be compressed and stretched. This is the secondary structure of DNA. A single-stranded DNA molecule was found in some bacteriophages. The deoxyribonucleic acid molecule is also characterized by a tertiary structure, which is formed due to the reunification of the double-stranded and ring forms of DNA with the subsequent formation of spiralized and supercoiled structures.