GENE

From Big Medical Encyclopedia

GENE (grech, genos a sort, an origin) — unit of structural and functional heredity representing a piece of a molecule deoxyribonucleic to - you, at some viruses — ribonucleic to - you.

The fact of existence of G. was determined by G. Mendel in the experiments on crossing of various grades of peas. Its classical work «Experiences with vegetable hybrids» was published («Versuche uber Pflanzen-Hybriden») in 1866. Studying hybrids of plants which parent forms differed from each other on one, two or three signs, Mendel came to conclusion that any signs of an organism are defined by factors which are transmitted from parents to descendants through sex cells and that these factors during the crossing are not split up, and are transferred as something whole independently from each other., Emergence of different signs is caused by various combination of hereditary factors, and the frequency of emergence of each sign can be predicted, knowing as it is inherited.

However Mendel's opening took place unnoticed. Only in 1900 a goal. biologist of X. de Fris is also mute. the selector Korrens (To. Correns) published articles in which confirmed results and theoretical conclusions of Mendel.

In 1909 dates. the biologist W. Johannsen offered for the hereditary factors opened by Mendel the term «gene», for all G.' set of an organism — the term «genotype», for a sign which is defined by one G. — the term «hair dryer», and for set of all signs of an organism — the term «phenotype» (see. Genotype ).

In the last quarter of 19 century it was suggested that an important role in transfer of hereditary factors is played chromosomes (see), and in 1902 — 1903 an amer. the cytologist W. S. Sutton is also mute. biologist Boveri (Th. Boveri) presented tsitol. proofs that the laws of transfer and filial segregation established by Mendel can be explained with a recombination of maternal and fatherly chromosomes during the crossing.

In 1911 T. Morgan's works with sotr. it was shown that G. represents a part of a chromosome (see. Chromosomal theory of heredity ). G. concentrated in one chromosome are transferred from parents to descendants jointly as one linked group. The number of linkage groups for any normal organism is constant also to equally haploid number (i.e. to unary set) chromosomes in its sex cells. After it was succeeded to show that homologous chromosomes in a crossing-over, i.e. in exchange of homologous sites of homologous chromosomes (see. Recombination , chromosomes), are capable to exchange with each other the sites — blocks of genes, T. Morgan carried out the analysis of intra chromosomal localization of G. and showed that G. are located in a chromosome linearly and that each of them takes strictly certain place in the corresponding chromosome. After the card of an arrangement of G. on length of a chromosome (see. Chromosome map ) were made for a number of animals (a mouse, a chicken, a drosophila, etc.), plants (corn, tomatoes, etc.), bacteria and viruses. Due to the development of methods of genetics of somatic cells drawing up chromosome maps of the person which continue to be developed intensively and until now became possible.

In the first quarter of 20 century considered that G. is final indivisible unit of heredity, edges at mutations (see) changes jump, entirely passing into other, so elementary state i.e. that this process reminds the phenomenon of an isomerism. Was considered also that the crossing-over is a mechanical exchange of homologous sites of chromosomes and that the rupture of chromosomes can occur only in intergene space, and intragenic gaps are impossible. The next G.' functions in a chromosome were considered independent from each other. No elements of the biochemical, functional organization in a chromosome were noted. Association G. in chromosome complexs was considered as a selection response on the basis of perfection of transfer of G. at nuclear fission. Thus, idea of G. during this period came down to the fact that G. is elementary genetic unit of structure, function, a recombination and a mutation.

In 1929 — 1934 N. P. Dubinin, A.S. Serebrovsky, etc. for the first time put forward and experimentally confirmed the idea about complex structure of G., according to G.'s cut represents a complex system with the special internal organization and complexity of functions. Earlier was considered that at interaction of two alleles (see), i.e. two various forms of the same G., manifestation of one allele can be defined as dominant, as intermediate or as recessive (see. Dominance ). During the studying of interaction of alleles in the site of a chromosome occupied by G. scute-achaete (locus) authors found out in drosophilas that on one group of signs this allele retsessiven, and on another — dominanten.

For designation of this phenomenon the term «step allelism» because authors of a research believed that they in heterozygotes of an allele block each other as steps of a ladder was offered. In the subsequent this phenomenon received the name of complementation (see. Mutational analysis ).

In 1957 amer. the geneticist S. Benzer, using a method of complementation, made an attempt of mapping of mutations in two nearby located genes And yes in the field of r II of a phage T4. The mutants of r (English rapid bystry) allocated in 1948 with researchers of A. Hershi and R. Rotman, differed from normal forms of phages in the big size of phage plaques on a lawn of E. coli To. In turn all mutants were subdivided into the areas rI, rII, rIII. During the crossing of two different mutants of owing to a crossing-over occasionally there were normal particles of a phage. Mutants in itself of r II were not capable to develop on a lawn of E. coli To while normal particles of a phage bred on a lawn of this strain. This feature was also used by S. Benzer for selection of recombinants. As a result of the done work S. Benzer defined 200 mutational points within a gene And yes 180 points within a gene of Century. Mutational points in G. it designated the term «website» (English site the place, the site). All 380 websites of both G. were located in a linear order.

All this demonstrated that G. consists of the separate parts located on its length in a linear order. The sites located within G. can independently mutate, and mutate different parts with a different frequency, some of them have the maximum mutability, being in G. as if «hot spots». Therefore, G. is not unit of a mutation. Besides, it became clear that G. is not unit of a recombination as the crossing-over can pass in. Thus, G., being unit of function in metabolism of a cell, at the same time it is initially difficult as G.'s action in general is caused by integration of functions of its separate parts.

Because such concepts as a mutation, a recombination and function used earlier for definition by G., do not match with each other, S. Benzer suggested to enter new designations. Elementary unit, not divisible by a recombination, he suggested to designate the term «recon»; the smallest site G., change to-rogo can lead to emergence of a mutation, to call «muton». The sizes of a muton and recon at a phage of T4 correspond to one couple of nucleotides. S. Benzer suggested to designate the site of a chromosome which defines one function the term «cistron» (the word comes from a combination of terms cis-and trans-situation). However with development of molecular genetics the term «cistron» received more specific definition. It designates the functional unit of genetic substance managing synthesis of a certain protein or its subunits (a polypeptide chain). Complex structure of G. is obvious. However G. as unit of hereditary information remains functionally. indivisible, i.e. it is discrete unit of functional and structural heredity.

Till 1944 dominating in science was the opinion that G. are molecules of protein, In 1944 Mr. of Avery (Oh, T. Avery, 1877 — 1955), Mac-Laud (S. M. of Macleod) and IAC Carti (M. of McCarty), studying. the reason of emergence of virulence in avirulent strains of Diplococcus pneumoniae at simultaneous infection of mice with the inactivated heat, virulent and live avirulent strains of diplococcuses (see. Transformation ), came to a conclusion that the material basis of heredity of Diplococcus pneumoniae is made by molecules deoxyribonucleic acid (see). The subsequent researches showed that nucleinic to - you, DNA (or for the RNA some viruses) make a material basis of heredity for all organisms. In 1953 the English scientist F. Shout and amer. the scientist J. Watson offered the model of a structure of molecule DNA later confirmed experimentally, according to a cut of DNA consists from two complementary (i.e. complementing) the polynucleotide chains twirled in a spiral around the general axis. Chains are kept against each other by hydrogen bindings which are formed only between strictly certain bases (in each couple one basis there has to be purine, and another — pirimidinovy): i.e. between adenine (A) and thymine (T) — couple of AT, and tsitoziny (C) and guanine (G) — couple of HZ. Strict compliance each other of the bases of two chains of molecule DNA is a starting point for the matrix mechanism of reproduction of genetic material in the course of cell division and, finally, transfer to descendants of features of a structure of the site of DNA concluded in this. G.'s synthesis happening within larger structures — chromosomes, is possible only in the presence of other G. which serves as a matrix. Process of reproduction of genetic material is very exact, and a mistake (mutation) in it occur extremely seldom. This process is called autocatalytic function G., or an autoreproduktion, and ability of to self-reproduction — replication (see. Reproduction of chromosomes , Replication ).

Function

Function G. consists in formation of a specific character. However it is possible to find such G. only when it meets in the alternative allelic forms exerting various impact on the same sign. G.'s removal or its change leads respectively to loss of this sign or to its change. Any sign of an organism is result of G.'s interaction with surrounding and internal environment. The environment is a set of the external factors influencing development of an individual in these conditions of her existence; the internal, genotypic millieu is defined by influence at each other of all G. of set. Preservation of characteristic manifestation of G. is caused by preservation of conditions internal and the environment. G.'s action can be shown by hl. obr. concerning any one sign (monotropy), or the same G. can take part in formation of several signs (see. Pleiotropia ). In turn can take part in formation of one sign several G. (polymerism). Polymeric G. can work is independent or interconnected, supplementing action of each other. Owing to this fact externally simple distinction in a phenotype of two individuals does not mean only distinction in their genotypes, and can be result of participation of many genes (the phenomenon of a polygeny).

G.'s action can be reduced by the next moments: a) direct action when G. controls synthesis of an end product completely (e.g., G. of antigenic specificity, G. of self-sterility); b) complex interaction with other G., when one G. otvetstven for synthesis of a starting material necessary for other G.'s functioning; c) cooperative interaction when two or more G.' products directly interact, synthesizing an end product; d) the competitive relations when G. compete for a product necessary for them; e) duplitsirovanny (parallel) interaction when two (or more) G. provide synthesis of an identical end product.

G.'s expression is defined by extent of manifestation at an individual of the sign controlled by this G. (so-called expressivity of a gene). Even within related group of the individuals who are in similar living conditions, the same G.'s manifestation can be unequal on the frequency or probability. The percent of individuals of related group of organisms at whom the sign determined by this G. is expressed characterizes manifestation, or penetrance of a gene (see).

Is not separate, absolutely independent in the manifestation from each other units. G.'s action can be changed if there was a restructuring which pulled out G. from its usual environment in a chromosome.

Interaction of two G. belonging to different factor pairs at Krom a dominant allele of one of couples suppresses manifestation of a dominant allele of other couple, carries the name «epistasis». So, the gene And can epistazirovat over a gene of V. which is hypostatic in relation to

A. V gene of 1925 of A. H. Sturtevant during the studying of a gene of Bar (narrow eyes) found the phenomenon in drosophilas, a cut the name gained a position effect of a gene. In 1934 N. P. Dubinin and B. N. Sidorov, studying a gene of +ci (a normal structure of a wing) at drosophilas, found out that transfer of an allele + ci in other chromosome and consequently, change of a gene environment, leads to loss of property of dominance at it. Thus, such important property as dominance developed in the course of evolution can be changed owing to a position effect of. Change of manifestation of G. as a result of a position effect of G. is reversible, i.e. the position effect of G. at structural changes of chromosomes is not permanent structural change of G. like mutations.

In 1941 Haldane (J. Century of S. Haldane), studying a position effect of pseudoalleles, by analogy with chemistry entered into genetics a concept about cis-and trans-provisions. E.g., heterozygotes on genes and and b can have or cis-, or trans-configuration, i.e. in the first case of a+/+b, and in the second ++/ab. The analysis of functional relationship of G. in conditions cis-and trans-provisions showed dependence in manifestation of some functionally connected next G.

The central moment in all problem of action of G. is programming of synthesis of protein by it. G. is not directly involved in synthesis of a polypeptide chain which is carried out in special subcellular structures — ribosomes. One of two chains of DNA of a gene complementary each other serves as a matrix for synthesis of information (matrix) RNA (IRNK) which is carried out with the participation of DNA-dependent enzyme of a RNA polymerase. Thus, the sequence of the bases of DNA (is more true than one its thread) is copied by the sequence of the bases of IRNK, and then RNA works as a genetic matrix, managing process of compound of amino acids in polypeptide chains. The DNA matrix function is called heterocatalytic function of G. or heterosynthesis. Process of reproduction of the sequence of nucleotides of DNA in molecule IRNK is called transcription (see). A. Dounce in 1952 and G. Gamow in 1954 independently from each other introduced the idea that the order of an arrangement of nucleotides in DNA defines an order of inclusion of amino acids in polypeptide. Later it was proved that inclusion of one amino-acid rest is controlled by three consistently located nucleotides (triplet). It was shown also that between alternation of nucleotides in molecule DNA and the sequence of the amino-acid remains in polypeptide under construction strict compliance — a so-called collinearity a gene — protein is observed. Process of reproduction of the sequence of nucleotides of IRNK in the sequence of amino acids of a polypeptide chain under construction is called broadcasting (see).

The triplet of consistently located nucleotides is the coding unit of. It defines inclusion of one amino acid in a polypeptide chain and is designated by the term «codon».

The principles of a genetic code established for DNA in experiences with bacteriophages, bacteria and animals are quite applicable also to viruses which material basis of heredity represents ribonucleic to - that. Inclusion of amino acids in protein of a capsid of a virus is coded by the same triplets inherent to DNA, with the only difference, a cut consists in replacement of thymine by uracil. The sequence of nucleotides in the codons coding inclusion of all amino acids in a polypeptide chain is established (see. Genetic code ). The linear size G. is connected with length of the polypeptide chain which is under construction under its control.

In 1969 J. R. Beckwith with sotr., using ability of some bacteriophages to transfer fragments of a chromosome of the bacterial cell infected by them to other bacterial cells, so-called. transductions (see), allocated purely individual G. of colibacillus, precisely determined its sizes and photographed by means of a supermicroscope. In 1967 — 1970 of X. The Koran with sotr. carried out chemical synthesis of an individual gene.

It is established that G. on average contains 1000 — 1500 nucleotides that corresponds to 0,0003 — 0,0005 mm.

In certain cases primary action of G. is limited only to process of a transcription (i.e. synthesis of RNA on DNA). Synthesis of molecules of acceptor RNA (TRNK) and ribosomalny RNA (RRNK) belongs to such cases.

On localization distinguish G. autosomal and linked to a floor. Autosomal G. are localized in all chromosomes (autosomes), except for sexual. Sexual X-chromosome of the person consists of two parts: one of these parts is specific to X-chromosome, and another is homologous to the respective site of a sexual Y-chromosome. In G.'s X-chromosome it can be localized in a segment, nonhomologous to a Y-chromosome (absolute X-coupling), and in a Y-chromosome — nonhomologous to X-chromosome (absolute Y-coupling), or G. can be located in homologous X-segments y Y-chromosomes (an incomplete sex linkage).

The most important party of the modern doctrine about G. is the question of ways of regulation of function. The mechanism of regulation is fullestly studied at bacteria. In 1961 fr. geneticists F. Jacob and Mono (J. Моnod) came to conclusion that there are two groups G.: structural and regulatory. Structural G. define the sequence of amino acids in polypeptide. Regulatory G. exercise control of activity of structural G. which consists in programming of synthesis of specific substances of the proteinaceous nature, so-called repressor. Repressora were purely allocated in 1967 an amer. the scientist Ptashne (M. Ptashne) with sotr. (repressor of a phage of X and lac-operon of E. coli). Repressor specifically contacts the area located at the very beginning of a series of structural G. This DNA small area received the name «operator». The operator does not code synthesis of any product, and is the site which interacts with a repressor therefore the whole series of structural G. can be switched off. In system of regulation one more element which received the name «promoter» — the site is allocated, the RNA polymerase joins Krom. Quite often the operator and promoter regulate activity of several structural G. which are located in a chromosome with a row connected by a community consecutive biochemical, reactions (the enzymes catalyzing a chain of consecutive reactions). Set of structural genes (a) with the operator and promoter is called operon (see). Mark out special category G. — the so-called integrating G. which provide integration of proteins into cellular structures. Refer G. providing timely inclusion of functions G. in the course of ontogenesis that allows to regulate a differentiation and at the same time unity of developments to category G. of a general meaning.

In addition to G. which are localized in chromosomes the complex of extra chromosomal G. which submit to Mendelian laws of inheritance (see is found. Mendel laws ). These G. are connected with cytoplasmatic structures of a cell (plastids, mitochondrions, etc.). The complex of the extra chromosomal G. defining set of genetic properties of cytoplasm at this look call a plasmon, and chromosomal G.' complex — genome (see), Extra chromosomal G. can be in structure episomes (see) of which ability to be integrated with a chromosome is characteristic., the opportunities which are in cytoplasm and deprived to be integrated, received the name plasmids (see).

The problem of extrachromosomal inheritance receives the increasing practical value because formation of medicinal stability, toxicogenic and immunogene properties of microbes is connected with it.

Development of the theory of G. took place in close connection with development of the doctrine about material carriers of heredity and the doctrine about mutations.

The main problem of the modern doctrine about the molecular nature of G. is development of the structural and biological principles. Represents not only a piece of molecule DNA, but, besides, the comprehended, historically created, difficult microsystem. The structure and functioning of this system has adaptive character, providing cell activity and an organism in general. The problem of process control of variability of G., a problem of the directed mutagenesis shall be solved on the basis of knowledge of the nature of G. as molecular and biological system. G.'s theory makes a basis of a recent trend — genetic engineering, a cut is exclusively perspective for the solution of a number of problems of medicine (see. Genetic engineering ). The genetic (gene) engineering is an applied molecular and cellular genetics, i.e. the operations with simple biol, systems (molecules of biopolymers and a cell) imitating in vitro natural processes of heredity. An ultimate goal of genetic engineering is creation by means of laboratory receptions of organisms with new hereditary properties. Manipulations of genetic engineering come down to receiving fragments of DNA, their hybridization and introduction to a retsipiyentny cell, molecular cloning and reproduction of these molecules. It is possible to expect that normal G.' introduction to a sick organism it will be possible to treat genetically caused diseases — endocrine and mental diseases, enzymopathies, etc. (see. Gene therapy ).


Bibliography: Gershkovich I. Genetics, the lane with English, M., 1968; Dubinin N. P. General genetics, M., 1976, bibliogr.; Ichas M. A biological code, the lane with English, M., 1971, bibliogr.; Ratner V. A. Principles of the organization and mechanisms of molecular and genetic processes, Novosibirsk, 1972, bibliogr.; Stent of. Molecular biology of viruses of bacteria, the lane with English, M., 1965; it, Molecular genetics, the lane with English, M., 1974; Watson J. D. Molecular biology of a gene, the lane with English, M., 1967; Finchem Dzh. Genetic complementation, the lane with English, M., 1968, bibliogr.; Harris G. Fundamentals of biochemical genetics of the person, the lane with English, M., 1973, bibliogr.; Carlson E. A. The gene, a critical history, Philadelphia — L., 1966, bibliogr.; Gardner E. J. Principles of genetics, N. Y. — L.t 1972, bibliogr.

H. P. Dubinin.

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