NUCLEIC ACIDS

From Big Medical Encyclopedia

NUCLEIC ACIDS — class of the biopolymers responsible for storage, transfer and embodiment of genetic information; universal components of all live organisms.

There are two types H. to.: deoxyribonucleic (DNA) and ribonucleic (RNA), carbohydrate components to-rykh are presented desoxyribose (see) and ribose (see) respectively. Biol, a role of these types H. to. is in what at the majority of live organisms of DNA bears function of storage and reproduction of hereditary information while RNA is responsible for the embodiment of this information in the course of protein synthesis (see. Deoxyribonucleic acid , RNA ).

Distinguish the following main types of RNA: information, or matrix (IRNK, or MRNK), edges serves as a matrix for protein synthesis; ribosomal (RRNK), being a component of the beloksinteziruyushchy device of a cell — ribosomes (see); transport (TRNK), function a cut consists in transfer of the activated amino acids to the place of synthesis of protein — a ribosome. RNA serves as genetic material of many viruses.

The main part of DNA of eukaryotes contains in chromosomes. Besides, DNA is present at chlorolayers of plants and mitochondrions of animals and plants. In nek-ry cells, napr, spermiya, the content of DNA can make about a half of weight of all cell. A convenient source for receiving DNA are lymphocytes of a thymus. RNA preferential is in cytoplasm. Content of RNA in cells strongly varies depending on intensity of proteinaceous synthesis in them. Many RNA contains, e.g., in a pancreas, and also in fast-growing embryonic and tumor cells. Highly specialized muscular and nervous fabrics contain few RNA.

N to. were discovered in 1868 by the Swiss chemist by Mischa rum (F. Miescher), to-ry showed that they are localized in kernels of a cell, have acid properties and unlike proteins contain phosphorus.

Chemically N. to. represent the polynucleotides consisting of monomer units — so-called mononucleotides (nucleotides). Each nucleotide contains one of four types of nitrogen bases: purines — adenine (A) and guanine (G) and pyrimidines — tsitozin (C) and thymine (T). At nucleotides of RNA instead of thymine there is an uracil (U). The basis connected to carbohydrate — pentose, forms a so-called nucleoside, fosforilirovanny derivative to-rogo is called a nucleotide. Nucleotides in N. to. are connected by means of phosphodiester communication where R — H (hydrogen) for DNA and — OH (hydroxyl) for RNA.

N.'s specificity to. is defined by sequence of nitrogen bases that defines so-called primary structure of N. to. Primary structure is established for many TRNK, RRNK, IRNK, and also a number of RNA - and the DNA-containing viruses and bacteriophages. Spatial structure of N. to. is defined by not covalent interactions: hydrogen bindings between the bases, hydrophobic interactions between the planes of couples of bases, electrostatic interactions with participation of negatively charged phosphatic groups and antiions.

The major achievement molecular biology (see) there was an opening in 1953 by J. Watson and T. Shout of a double helix of DNA, in a molecule a cut two antiparallelno the located sakharofosfatny chains are kept by hydrogen bindings between adenine and thymine or guanine and tsitoziny. The sequence of nitrogen bases in one chain defines the sequence of the bases in another. The sizes of complementary couples And — T and G — C are identical. It allows the nucleotide chain consisting of complementary couples of the bases in any sequence to be curtailed in the correct double helix (secondary structure of N. to.). In fiziol, conditions the double helix of DNA is close to a so-called B-form, in a cut of couple of bases, lying one over other (pile), axes of a spiral are perpendicular.

10 couples of the bases are the share of one spiral turn. In saline solution of N. to. the number of the base pairs falling on one spiral turn varies, decreasing at increase in concentration of salt in solution and fall of temperature.

Unlike DNA consisting, as a rule, of two chains, molecules RNA usually contain one chain. This chain, being bent and being cast over itself, forms double-helix areas (often with defects), from to-rykh so-called tertiary structure of N. forms to. Double-helix sites of RNA considerably differ on the geometry from V-for-my of DNA and belong to a so-called A-form, edges can arise also at DNA, but at the reduced relative humidity of drug. The spiral of an A-form is more untwisted, 11 — 12 base pairs are the share of one its round. Couples are inclined and strongly shifted from an axis of a spiral to the periphery. As not covalent interactions between N.'s chains to. are weak, the highest structures of N. to. — tertiary and secondary collapse at temperature increase or at dissolution of drug in non-aqueous solvents. Process of destruction of double-helix structure is called transition a spiral — a ball or melting. In the course of melting the double helix is divided into the chains making it that leads to change of hydrodynamic and optical characteristics of N. to. At fall of temperature of a molecule H. to. are capable to recover secondary structure. This property is widely used for receiving the «hybrid» molecules containing chains of DNA and RNA. Distinctiveness of double-helix N. to. very big rigidity on a bend is thanks to what the pieces containing up to 100 couples of the bases are almost rectilinear. But since the number of base pairs in DNA is big, free DNA in solution forms a friable ball. From here need of the special mechanisms providing compact DNA folding in chromosomes follows. It is reached by formation of a superspiral from a double helix. In ring DNA of prokariot superspiralling arises at change of a tilt angle between base pairs. Super and spiral DNA have excess energy of elastic deformation, edges can be used in various biochemical, processes, in particular in regulation of gene activity, stimulation of nek-ry biosynthetic reactions. Special enzymes (topoisomerases) support required degree of a sverkhspi-ralization of ring DNA.

N.'s synthesis to. in a cell it is carried out by the principle of copying of a molecule matrix by proteins polymerases, at the same time there is polyreaction of nukleozidtrifosfa-t to eliminating of a pyrophosphate. The sequence of nitrogen bases in a molecule of a product of this enzymatic process is defined by the sequence of nitrogen bases in a molecule matrix. Synthesis of DNA is called replication (see) it is also carried out by the complex of proteins consisting of a DNA polymerase, the protein dividing DNA threads, nucleases, ligases etc. Synthesis of RNA — transcription (see) occurs on a matrix of DNA, and RNA polymerases read out one, so-called meaning, thread of a double helix. In the course of a transcription the RNA copy is formed gene (see). Information for synthesis of specific proteins is put into the sequences of the bases of MRNK. To each amino acid of protein there correspond certain three of nucleotides — the triplets forming genetic code (see). Therefore changes of the nucleotide sequence in DNA — mutations (see) affect structure of synthesizable proteins, influence their function and hereditary information. Distortion of hereditary information can be the cause hereditary diseases (see), napr, sickemia, fenilketonuriya, or hereditary anomalies, napr, albinism. Mutations can be picked up by natural selection and serve as a factor of evolution. Natural frequency of a mutirovaniye is very low, but it considerably raises at influence of the ionizing, ultra-violet radiations or chemical mutagens. During evolution live organisms developed effective mechanisms of a reparation (recovery) of N. to., the eliminating damages to their molecules (see. Reparation of genetic damages ).

According to a chemical structure of a polynucleotide chain there are three groups of methods of quantitative definition of N. to.: on the content of nitrogen bases [usually use determination of size of absorption in an ultra-violet part of a range, i.e. spektrofotometriya (see)], on the maintenance of a carbohydrate component [various staining reactions (see. Carbohydrates )], by amount of phosphorus. Spectrophotometric methods yield satisfactory results only at insignificant amount of impurity in the studied sample. Methods of the second group are specific to type H. to. also allow to distinguish DNA from RNA.


Bibliography: Organic chemistry of nucleic acids, under the editorship of N. K. Kochetkov and E. I. Budovsky, M., 1970; Chemistry and biochemistry of nucleic acids, under the editorship of I. B. Zbarsky and S. S. Debov, JI., 1968; Sh and and r about in and 3. And. and Bogdanov of A. A. Himiya of nucleic acids and their components, M., 1978; Chromatin, Cold Spr. Harb. Symp. quant. Biol., v. 42, pt 2, 1978; Handbook of biochemistry and molecular biology, nucleic acids, ed. by G. D. Fasman, v. 1, Cleveland, 1975.


V. I. Ivanov.

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