GENOME

From Big Medical Encyclopedia

GENOME (English genome, from grech, genos an origin, a sort) — the main (haploid) set of chromosomes with the genes localized in them, i.e. set of the chromosomal hereditary factors transferred from a parent individual to affiliated. The term is offered by Vinkler (N. to Winkler) in 1920.

All live organisms depending on the structurally functional organization of a genome divide on eukaryotes and prokariot (protokariot). Eukaryotes call organisms in which cells there is morphologically isolated kernel containing chromosomes (see) and separated from cytoplasm by a nuclear membrane. Carry all animals and plants to them, except blue-green seaweed. Prokariotami is called by organisms which cells are not differentiated on a kernel and cytoplasm and do not contain morfol, analogs of chromosomes of eukaryotes. Carry bacteria and blue-green seaweed, and also the most primitive life forms to them — viruses and bacteriophages. The general for eukaryotes and prokariot is what in both cases the carrier of hereditary information is nucleinic to - they are DNA or at the RNA some viruses (see. Deoxyribonucleic acid , RNA ).

Existence in G.'s structure more than one chromosome is characteristic of eukaryotes, each of which represents a certain linkage group of genes. The number, a form and the sizes of chromosomes, and also structure of linkage groups are species-specific. In a life cycle of the majority of eukaryotes diplophase prevails, and haplophase is presented only by mature sex cells, i.e. only kernels of mature sex cells (gametes) contain according to one G., and kernels of other cells — on two. At fertilization sex cells and their kernels merge, and their G. combine, but each chromosome at the same time keeps the identity. In the subsequent cellular cycles of a chromosome of both G. double (see. Replication ) and in process mitosis (see) strictly are equally distributed between affiliated kernels, providing constancy of a diploid state in somatic cells. At the same time formation of sex cells is carried out in the way meiosis (see) which can schematically be presented as two consecutive nuclear fissions (mitosis) at one round of replication of chromosomes therefore four haploid kernels containing on one genome are formed. Thus, the mitosis provides constancy of a diploid state in somatic cells, and meiosis and the subsequent fertilization — constancy of species-specific set of chromosomes in alternation of generations. Chromosome replication in a mitosis and meiosis and consequently replication of all genes localized in it, provides in such a way one of the major functions G. — storage and transfer from generation to generation of bulk of hereditary information.

Other major function G. — management of coordinate processes of biosynthesis in cells. At the same time each separate gene as a part of G. performs elementary, strictly certain function (see. Transcription , Broadcasting ). Coordination of biosynthetic processes in ontogenesis is reached at the expense of differential gene activation in different cells of different fabrics and at different times. In turn, such differential gene activation is carried out by means of the special regulator genes controlling time of inclusion and termination of activity of these or those structural genes coding structure of the corresponding proteins.

At diploid organisms two homologous G. of somatic cells function as uniform system of the interacting genes. It is confirmed with such phenomena as dominance or recession (see. Dominance ), existence of different pair combinations of alleles of one gene, and also the phenomena of an epistasis, a complementarity, polymerism (see the Gene) and in operation modifiers at phenotypical manifestation of nonallelic genes. Thus, the final phenotype of an individual (see the Genotype) is a product of implementation in certain conditions of the environment of all information both ’.

In addition to functions of storage, a reproduction and implementation of hereditary information, G. acts also as unit of hereditary variability, unit mutations (see). Refer change of chromosome number without change of an arrangement of genes in chromosomes to genomic mutations and without mutations of genes. Genomic mutations divide into two classes: euploidiya (polyploidy) and aneuploidy (heteroploidy). At euploidny mutations there is a multiple increase in number G. so there are polyploid individuals (triploid, tetraploid, pentaploidny etc.). Euploidiya is widespread preferential in flora where many polyploid series of types are known, and some polyploid forms of cultivated plants are of economic value. The polyploidy is widespread in fauna much less often since the majority of such forms or is impractical, or is fruitless. Refer surplus or lack of separate chromosomes as a part of to aneuploid mutations. Merge in the course of fertilization of such gamete to the sex cell containing normal G. leads to emergence of a zygote with hyper - or a gipoploidny set of chromosomes.

E.g., if of an ovum any chromosome is presented to G. twice, then at fertilization the zygote will arise a normal spermatozoon, this chromosome is presented to a cut three times (trisomy). Different combinations at fertilization of euploidny and aneuploid sex cells can bring, except a trisomy as well to a nullosomiya (lack of couple of homologous chromosomes in diploid set), monosomies, tetrasomiya etc. Genomic mutations like aneuploidy are known both in vegetable, and in fauna, including the person. The majority of possible types of an aneuploidy at the person is incompatible with normal pre-natal development and leads to spontaneous abortions. The lethality of children with an aneuploidy once again emphasizes functional unity of G.: aneuploids have all genes which are available for euploid, and only the genic balance is broken. Separate types of an aneuploidy at the person, especially on small chromosomes, are compatible to life, but cause development, severe forms of pathology — so-called. chromosomal diseases (see), the most known and widespread of which are a Down syndrome (a trisomy on a chromosome 21) and Turner and Klaynfelter's syndromes (aneuploidies in system of gonosomes).

Of prokariot differs from G. of eukaryotes in a number of essential features: the absent kernel with a set of chromosomes at these organisms is replaced with one, morphologically not isolated, huge molecule DNA (at the RNA some viruses), edges and represents set of linearly located genes. Sometimes, to emphasize structural difference of G. of prokariot from G. of eukaryotes, use the term «gynophore» (i.e. the carrier of genes) though it is quite often possible to meet also the term «bacterial chromosome», or «a chromosome of a phage».

Both at prokariot, and at eukaryotes along with G. the insignificant volume of genetic information contains in some cytoplasmatic structures. Set of such not chromosomal «genes» is designated the term «plasmon». Here the hl belongs capable to replication and a transcription of DNA of mitochondrions at all organisms, and at plants also DNA of plastids. obr. chlorolayers. Are characteristic of phenotypical manifestation of a plasmon a maternal mode of inheritance (the corresponding signs can be inherited only from mother) and a variety of the numerical relations of phenotypes among descendants (unlike certain Mendelian relations at conditionality of signs by chromosomal genes). The plasmon is not completely autonomous genetic system; on the contrary, many lines of the organization and function of carriers of a plasmon (mitochondrions, plastids) are controlled G.

Nakonets, in addition to G. and a plasmon, as optional elements of hereditary information at bacteria it is possible to allocate episomes (see) — the DNA-containing structures which can or join in G. of bacteria and be replicated together with it, or be in an autonomous state in cytoplasm and to be replicated irrespective of G. Primerami of classical episomes hereditary material of moderate phages, a sexual factor of bacteria (F-factor), a transfer factor of resistance, etc.

See also can be cytoplasmic inheritance , Chromosomal complement .


Bibliography: Lobashev M. E. Genetics, L., 1967, bibliogr.; It is Page. Genetic mechanisms of progressive evolution, the lane with English, M., 1973; G. S Stent. Molecular genetics, the lane with English, M., 1974; VogelF. Genotype and phenotype in human chromosome aberrations and in the minute mutants of Drosophila melano-gaster, Hum. genet., v. 19, p. 41, 1973.

V. I. Ivanov.

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