From Big Medical Encyclopedia

TRANSCRIPTION in biology (a synonym matrix synthesis of RNA) — synthesis of RNA on a matrix of deoxyribonucleic acid. T., occurring in living cells, represents the initial stage of implementation of the genetic signs concluded in DNA (see. Deoxyribonucleic acid). As a result of T. RNA (see RNA) — the exact copy of one of DNA threads on the sequence of nitrogen bases in a polynucleotide chain is formed. T. it is catalyzed DNK-za-visimymi by RNA polymerases (see Polymerases) and provides synthesis of RNA of three types: template-RNA (MRNK), coding primary structure of protein, i.e. the sequence of the amino-acid remains in an iolipeptidny chain under construction (see Proteins, biosynthesis); the ribosomal RNA (RRNK) which are a part of the ribosomes (see), and acceptor RNA (TRNK) participating in the course of synthesis of protein as the component which is carrying out «code conversion» of information concluded in MRNK.

T. at microorganisms it is studied more stoutly, than at the higher organisms (see Bacteria, the geneticist). Process of T., catalyzed RNK-polimera-zoy, is divided into 4 stages: linkng of a RNA polymerase with DNA, the beginning — initiation — synthesis of a chain of RNA, actually process of synthesis of a polynucleotide chain — elongation and completion of this synthesis — termination.

The RNA polymerase has the greatest affinity to the certain sites of a DNA matrix containing the specific sequence of nucleotides (so-called pro-motor sites). Svyazyva

ny enzyme with such site is followed by partial local melting of DNA threads and their discrepancy. At a stage of initiation there is an inclusion of the first nucleotide — usually adenosine (A) or guanine riboside (G) — in molecule RNA. During elongation the RNA polymerase locally untwines a double helix of DNA and copies one of its chains according to the principle of a complementarity (see the Remark

a tion). In process of advance of a RNA polymerase along DNA the growing

chain of RNA departs from a matrix, and the double-helix structure of DNA after passing of enzyme is recovered. Termination of synthesis of RNA happens also on specific sites of DNA. In nek-ry cases additional proteins are necessary for recognition of signals of termination, one of to-rykh is

the r-factor representing protein with ATF-aznoy activity, in other cases it can be the modified nitrogen bases. At achievement by a RNA polymerase of the terminatorny site the synthesized RNA thread finally separates from a DNA matrix.

Functional transkriptsionny unit at microorganisms is the operon (see) including one promoter, one operator and a number of the genes coding polppeptidny chains (see the Gene). T. an operon begins with a stage of linkng RNK-po-limerazy with promoter — the site which is in the beginning of an operon. At once behind promoter the operator — the site of DNA capable to contact protein-repressorom is located. If the operator is free, then there is a T. all operon, but if the operator is connected with protein-repressorom, T. it is blocked. All well studied turnips springs represent the proteins capable to be exposed to allosteric changes (see Conformation). The structure of proteins - a represso-ditch is coded by the regulatory genes located either just before an operon or at considerable distance from it. Synthesis and activity of repressor are defined by conditions out of - and the intracellular environment (concentration of metabolites, ions, etc.) *

The transcription of DNA at the higher organisms is carried out by the certain sites called by units of T. — transkryptons. In structure of unit of T. DNA of the corresponding gene and the sites adjoining it enters. Ideas of structure of units of T. gained essential development in connection with detection of functional inadequacy of the sequence of sites of genes of eukaryotes. It turned out that in structural genes of the higher organisms there are so-called introns — the inserted sequences of DNA which do not have a direct bearing on coding of this protein. The number and the size of introns of different genes strongly vary, in many cases the total length of all nitron considerably exceeds length of the coding part of genes (exon). Clarification of a role of introns — one of urgent problems of molecular genetics (see).

In the course of T. the RNA which is the copy of all transkriptsionny unit is formed. When genes code protein synthesis, primary product of T. is called the nuclear predecessor of MRNK (about-MRNK), by the size it several times exceeds MRNK. The sequences transcribed on the coding sites (exons), introns and, perhaps, contiguous zones of DNA are a part about-MRNK. In a cellular kernel about-MRNK turns into mature MRNK, there is a so-called processing, or maturing. At the same time specific enzymes interact with about-MRNK and selectively delete the excess sequences, in particular, those, to-rye are synthesized on introns. At the same stage nek-ry modifications of RNA, such as methylation, addition of specific groups, etc. are carried out. The mature MRNK leaving in cytoplasm contains nevertheless the excess sites which are not relating directly to coding of structure of protein and necessary as believe for the correct interaction of RNA with ribosomes, proteinaceous factors of broadcasting (see), etc.

Disturbances of process of T. can change metabolism of cells considerably. Defects of the enzymes which are taking part in synthesis of RNA can cause decrease in intensity of T. a large number of genes and to lead to considerable disturbance of functioning of a cell up to her death.

Genetic defects in structure of separate unit of T. are a cause of infringement of synthesis of this RNA (and the protein corresponding to it) and by that can be a basis of monogenic hereditary pathology (see. Hereditary diseases).

There is the return T. — synthesis of DNA on a matrix of RNA, at Krom transfer of information happens not to DNA on RNA, as in the course of direct T., and in the opposite direction. Return T. it was for the first time established at the RNA-containing oncogenous viruses after in mature virus particles found the RNA-dependent DNA polymerase called by the return transcriptase, or revertazy (see). With the participation of this enzyme in the cell infected with viruses on a matrix of RNA DNA, further capable to serve as a matrix for formation of RNA of new virus particles is synthesized. Synthesized by the return T. virus DNA can join in DNA of a host cell and by that to be the cause of malignant transformation of cells. Return T. in vitro is usually used in researches on genetic engineering (see) for synthesis on matrixes of any RNA of structural zones of the corresponding genes.

See also Broadcasting.

Bibliograshmarin And * P., Molecular biology, page 70, L., 1974;

3 e N of at sh the Item. Molecular and cellular biology, the lane with it., t. 1, page 135, M., 1982; Kiselyov L. L. RNA - on - pravlyaemy synthesis of DNA. (Reverse transcription), M., 1978, bibliogr.; At about t -

with about N D. Molecular biology of a gene, the lane with English, page 268, M., 1978.

S. A. Limborskaya.