GENETIC CODE

From Big Medical Encyclopedia

GENETIC CODE (grech, genetikos relating to an origin; synonym: code, biological code, amino-acid code, proteinaceous code, code nucleinic to - t) — system of record of hereditary information in molecules of nucleic acids of animals, plants, bacteria and viruses alternation of the sequence of nucleotides.

the Scheme of transfer of hereditary information (continuous shooters designated ways of information transfer): on molecule DNA creation of three RNA types is carried out: RRNK ribosomal (soluble), IRNK — information, TRNK — transport, participating in synthesis of protein and proteinaceous components of other substances of a cell (dotted shooters showed participation of separate components of a cell in regulation of information transfer); in the course of reproduction the copy of DNA (DNK1) for which transfer of hereditary information and its regulation are identical to this scheme is formed.

Genetic information (fig.) from a cell in a cell, from generation to generation, except for the RNA-containing viruses, is transferred by reduplication of molecules DNA (see. Replication ). The realization of hereditary information of DNA in the course of cell activity is enabled through 3 RNA types: information (IRNK or MRNK), ribosomal (RRNK) and transport (TRNK) which by means of enzyme of a RNA polymerase are synthesized on DNA as on a matrix. At the same time the sequence of nucleotides in molecule DNA unambiguously defines the sequence of nucleotides in all three RNA types (see. Transcription ). Information gene (see), coding a proteinaceous molecule, bears only IRNK. An end product of implementation of hereditary information is synthesis of proteinaceous molecules which specificity is defined by the sequence of the amino acids entering them (see. Broadcasting ).

As as a part of DNA or RNA only about 4 different nitrogen bases [are presented (C) to DNA — adenine (A), thymine (T), guanine (G), tsitozin; in RNA — adenine (A), uracil (U), tsitozin (C), guanine (G)] which sequence defines the sequence of 20 amino acids as a part of protein, arises a problem G. to., i.e. a problem of transfer of the 4-alphabetic alphabet nucleinic to - t in the 20-alphabetic alphabet of polypeptides.

For the first time the idea of matrix synthesis of proteinaceous molecules with the correct prediction of properties of a hypothetical matrix was formulated by N. K. Koltsov in 1928. In 1944 Mr. of Avery (O. of Avery) with soavt, established that molecules DNA are responsible for transfer of ancestral features at transformation at pneumococci. In 1948 E. Chargaff showed that in all molecules DNA quantitative equality of the corresponding nucleotides takes place (A-T, G-c). In 1953 F. Shout, J. Watson and Wilkins (M. H. F. Wilkins), proceeding from this rule and data X-ray crystallographic analysis (see), came to a conclusion that molecules and DNA represents the double helix consisting of two polynucleotide threads connected among themselves by hydrogen bindings. And against And one chain in the second there can be only a T, against — only C. This complementarity leads to the fact that the sequence of nucleotides of one chain unambiguously defines the sequence another. The second essential conclusion following from this model — molecule DNA is capable to self-reproduction.

In 1954. G. Gamow formulated a problem G. to. in its modern look. In 1957 F. Shout stated the Hypothesis of the adapter, having assumed that amino acids interact with nucleinic to - that not directly, and through intermediaries (known under the name TRNK now). In the years next after that all basic links of the general scheme of transfer of genetic information, in the beginning gipotetichny, were confirmed experimentally. In 1957 IRNK [A.S. Spirin, A. N. Belozersky et al. were open; Folkin and Astrakhan (E. Volkin, L. Astrachan)] and TRNK [Houglend (M. V. Hoagland)]; in 1960 DNA out of a cell with use as a matrix of the existing macromolecules DNA is synthesized (A. Kornberg) and DNA-dependent synthesis of RNA is open [Weiss (S. Century of Weiss) et al.]. In 1961 the acellular system was created, in a cut in the presence of natural RNA or synthetic polyribonucleotides synthesis of belkovopodobny substances was carried out [M. Nirenberg and J. H. Matthaei]. Problem of knowledge of G. to. consisted of a research of the general properties of a code and actually its interpretation, i.e. examination what combinations of nucleotides (codons) code certain amino acids.

The general properties of a code were found out irrespective of its interpretation and generally to it by the analysis of molecular patterns of formation of mutations (F. Shout and soavt., 1961; N. V. Luchnik, 1963). They are as follows:

1. The code is universal, i.e. is identical, at least generally for all living beings.

2. The code tripleten, i.e. each amino acid is coded by the three of nucleotides.

3. The code which is not blocked i.e. this nucleotide cannot be a part more than one codon.

4. The code is degenerated, i.e. one amino acid can be coded by several triplets.

5. Information on primary structure of protein is read out with IRNK consistently, since the fixed point.

6. The majority of possible triplets makes «sense», i.e. codes amino acids.

7. From three «letters» of a codon have preferential value only two (obligate), third (optional) bears considerably smaller information.

Direct interpretation of a code would consist in comparison of the sequence of nucleotides in a structural gene (or IRNK synthesized on it) with the sequence of amino acids in the corresponding protein. However such way is still technically impossible. Two other ways were applied: synthesis of protein in acellular system with use as a matrix of artificial polyribonucleotides of the famous structure and the analysis of molecular patterns of education mutations (see). The first brought positive takes earlier and historically played in G.'s interpretation to. big role.

In 1961 M. Nirenberg and Mateja applied as a matrix homopolymer — synthetic poliuridilovy to - that (i.e. artificial RNA of structure of UUUU...) also received polyphenylalanine. From this followed that the codon of phenylalanine consists of several At, i.e. in case of a triplet code it is deciphered as UUU. Later along with homopolymers the polyribonucleotides consisting of different nucleotides were used. At the same time only the composition of polymers was known, the arrangement of nucleotides in them was statistical therefore also the analysis of results was statistical and gave indirect conclusions. Quickly enough it was succeeded to find at least on one triplet for all 20 amino acids. It became clear that presence of organic solvents, change of pH or temperature, some cations and especially antibiotics do a code ambiguous: the same codons begin to stimulate inclusion of other amino acids, in certain cases one codon began to code up to four different amino acids. Streptomycin influenced reading of information as in acellular systems, and in vivo, and was effective only on streptomitsinchuvstvitelny strains of bacteria. At streptomitsinzavisimy strains it «corrected» reading from the codons which changed as a result of a mutation. Similar results gave the grounds to doubt correctness of interpretation of G. to. by means of acellular system; confirmation, and first of all was required by these in vivo.

A specification on G. to. in vivo are received in the analysis of amino-acid composition of proteins at the organisms processed mutagens (see) with the known mechanism of action, napr, nitrogenous to - that, edge causes in molecule DNA replacement of C by Wu Yi And for. Useful information is given also the analysis of the mutations caused by nonspecific mutagens, comparison of distinctions in primary structure of related proteins in different types, correlation between composition of DNA and proteins, etc.

G.'s interpretation to. on the basis of these in vivo and in vitro yielded coinciding results. Later three other methods of interpretation of a code in acellular systems were developed: binding aminoacyl-TRNK (i.e. TRNK with the attached activated amino acid) trinucleotides of the famous structure (M. Nirenberg and soavt., 1965), binding aminoacyl-TRNK the polynucleotides beginning with a certain triplet (Mateja et al., 1966), and use as IRNK of polymers in which not only the structure, but also an order of nucleotides is known (X. Koran and soavt., 1965). All three methods supplement each other, and results are according to the data obtained in experiences of in vivo.

In the 70th 20 century methods of especially reliable check of results of interpretation of G. appeared to. It is known that the mutations arising under the influence of proflavine consist in loss or an insert of separate nucleotides that leads to a frameshift. A number of mutations at which the structure of a lysozyme changed was caused in a phage of T4 by proflavine. This structure was analyzed and compared with those codons which had to turn out at a frameshift. Full compliance was obtained. In addition this method allowed to establish what triplets of a degenerate code code each of amino acids. In 1970 to Adams (J. The m of Adams) with employees was succeeded to carry out partial interpretation of G. to. by a direct method: at a phage of R17 defined the sequence of the bases in a fragment of 57 nucleotides and compared to the amino-acid sequence of protein of its cover. Results completely matched the received less direct methods. Thus, the code is deciphered completely and truly.

Results of interpretation are tabulated. In it the structure of codons and RNA is specified. The structure of anti-codons of TRNK is complementary to codons of IRNK, i.e. instead of At is in them And, instead of And — At, instead of C — G and instead of G — C, and corresponds to codons of a structural gene (the DNA that threads, about a cut information is read out) with only that difference that the place of thymine is taken by uracil. From 64 triplets which can be formed by a combination of 4 nucleotides 61 it makes «sense», i.e. codes amino acids, and 3 are the «nonsense» (deprived of sense). Between structure of triplets and their sense there is quite accurate dependence, edges was found in the analysis of the general properties of a code. In some cases the triplets coding a certain amino acid (e.g., proline, alanine), are characterized by the fact that the two first a nucleotide (obligate) at them are identical, and third (optional) can be any. In other cases (during the coding, e.g., of asparagine, a glutamine) the same sense is made by two similar triplets at which match the two first a nucleotide, and the third any purine or any pyrimidine stands still.

A nonsense codons, 2 of which the special names corresponding to designation of phage mutants (UAA-ochre, UAG-amber, UGA-fell down) have, though do not code any amino acids, but are of great importance during the reading of information, coding the end of a polypeptide chain.

Reading of information happens in the direction from 5 1 -> 3 1 - by the end of a nucleotide chain (see. Deoxyribonucleic acid ). At the same time synthesis of protein goes from amino acid with a free amino group to amino acid with a free carboxyl group. The beginning of synthesis is coded by triplets of AUG and GUG which in this case include specific starting aminoacyl-TRNK, namely N-formilmetio-Nile-TRNK. The same triplets at localization in a chain code respectively methionine and valine. Ambiguity is removed the fact that the beginning of reading is preceded by a nonsense. There are data which are speaking well for the fact that the border between the sites of IRNK coding different proteins consists more than of two triplets and that secondary structure of RNA in these parts changes; this question is in a stage of a research. If the nonsense codon arises in a structural gene, then the corresponding protein is under construction only to the location of this codon.

Opening and interpretation of a genetic code — outstanding achievement of molecular biology — exerted impact on everything biol, sciences, having laid in some cases the foundation for development of special large sections (see. Molecular genetics ). Effect of opening of G. to. and the related researches compare to that effect which rendered on biol, sciences Darwin theory.

G.'s universality to. is the direct proof of universality of the main molecular mechanisms of life at all representatives of the organic world. Meanwhile big differences of functions of the genetic device and its structure upon transition from prokariot to eukaryotes and from one-celled to multicellular are probably connected also with molecular distinctions which research — one of problems of the future. As researches G. to. — business only of the last years, value of the received results for applied medicine has only Indirect character, allowing to understand the nature of diseases, the mechanism of operation of causative agents of diseases and medicinal substances so far. However opening of such phenomena, as transformation (see), transduction (see), suppression (see), points to a basic possibility of correction patholologically of the changed hereditary information or its correction — so-called. genetic engineering (see).


Table. The GENETIC CODE


  • Codes the end of a chain.
    • Codes also the beginning of a chain.



Bibliography: Ichas M. A biological code, the lane with English, M., 1971; Archer N. B. Biofizika of cytogenetic defeats and genetic code, L., 1968; Molecular genetics, the lane with English, under the editorship of. A. N. Belozersky, p.1, M., 1964; Nucleic acids, the lane with English, under the editorship of. A. N. Belozersky, M., 1965; Watson J. D. Molecular biology of a gene, the lane with English, M., 1967; Physiological genetics, under the editorship of M. E. Lobashev S. G., Inga-Vechtomo-va, L., 1976, bibliogr.; &Desoxyribonuc-leinsure, Schlttssel des Lebens, hrsg. v \E. E. E. Geissler, B., 1972; The genetic code, Gold Spr. Harb. Symp. quant. Biol., v. 31, 1966; W o e s e C. R. The genetic code, N. Y. a. o., 1967.

H. B. Archer.

Яндекс.Метрика