From Big Medical Encyclopedia

CHROMOSOMES (Greek chroma color, coloring + soma a body) — the main structurally functional elements of a cellular kernel keeping the genes and the providing storages, reproduction of genetic information located in a linear order, and also the initial stages of its implementation in signs; change the line structure in a cellular cycle. The term «chromosomes» is offered by Valdeyer (W. Walde-уег) in 1888 because of a rod form and intensive coloring of these elements the main dyes during cell division.

The term «chromosome» in its full value is applicable to the relevant nuclear structures of cells of multicellular eukariotny organisms (see). In a kernel of such cells of chromosomes always a little, they make chromosomal complement (see). Somatic cells of a chromosome of a parna since occur from two parent (a diploid set of chromosomes), in mature sex cells contain an unary (haploid) set of chromosomes. Everyone biol. the look is characterized by constant number, the sizes and other morphological features of chromosomes (see. To are io type). At heterosexual organisms chromosomal complement includes two chromosomes bearing the genes defining a sex of an individual (see the Gene, Paule), to-rye call sexual, or gonosomam, contrary to all to other, called by autosomes. At the person of steam of gonosomes it is made: at women from two X-chromosomes (the XX set), and at men — from X and Y-chromosomes (XY set). Therefore mature sex cells — gametes at women contain only X-chromosome whereas at men a half of spermatozoa contains X-chromosome, and another — a Y-chromosome.

History. The first observations of chromosomes in a kernel of a cell executed in the 70th 19 century by I. D. Chistyakov, O. Gertvig, E. Strasburger laid the foundation for the cytologic direction in studying of chromosomes. Prior to the beginning of 20 century this direction was the only thing. Use of a light microscope allowed to receive data

on behavior of chromosomes in mitotic and meiotic divisions (see Meiosis, the Mitosis), the facts about constancy of chromosome number at this look, special types of chromosomes. In 20 — the 40th 20 century gained preferential development comparative morfol. studying of chromosomes at different types of organisms, including the person, for the purpose of clarification of the general principles of their organization, features of individual chromosomes and their changes in the course of evolution. In studying of this problem the special contribution was made by domestic scientists S. G. Navashin, G. A. Levitsky, L. N. Delon, P. I. Zhivago,

A. G. Andres, M. S. Navashin,

A. A. P rokofjeva-Beljgovsky, and also foreign — E. Heitz, Darlington (S. of D. Darlington), etc. From 50th for a research of chromosomes the supermicroscope began to be used. Began

Iza Tschenije morfol. changes of chromosomes in the course of their genetic functioning. In 1956 H. J. Tjio and A. Levan finalized chis

lo chromosomes at the person, equal 46, described their morphological features in metaphase of a mitosis. Significant progress in studying of chromosomes was made in the 70th after development of various methods them coloring which allowed to reveal heterogeneity of structure of chromosomes on length in a meta phase of cell fission.

Comparison of behavior of chromosomes in meiotic division with patterns of inheritance of characters (see Mendel laws) laid the foundation for cytogenetic researches. At the end of 19 — the beginning of 20 centuries W. Sutton, Boveri (Th. Boveri), Wilson (E. V. of Wilson) laid the foundation of the chromosomal theory of heredity (see), according to a cut genes are localized in chromosomes and the behavior of the last during the maturing of gametes and their merge at the time of fertilization explains laws of transfer of signs in generations. The theory received final justification in the cytogenetic experiments made on a drosophila (see) T. Morgan and his pupils, to-rye proved that each chromosome is group of genes, it is linked inherited and located in a linear order that in meiosis the recombination of genes (see the Recombination) homologous (identical) chromosomes is carried out.

The studying of the biochemical nature of chromosomes begun in 30 — is the 40th 20 century, originally was based on cytochemical qualitative and quantitative test of content of DNA, RNA and proteins in a kernel. From 50th began to apply to these purposes photo and spectrometry (see. With a pektrofotometriya), the X-ray crystallographic analysis (see) and other physical. - chemical methods.

Physical and chemical nature of chromosomes. Physical. - the chemical nature of chromosomes depends on complexity of the organization biol. look. The chromosome an eukaryote consists of a molecule of deoxyribonucleic acid (see), gistono-vy and negistonovy proteins (see Histones), and also RNA (see). The main chemical component of a chromosome concluding genetic information in structure of the molecule is DNA. Under natural conditions in certain sites of a chromosome of DNA can be free from structural proteins, however generally it exists in the form of a complex with histones, and as well as in interphase, and in metaphase the weight relation the DNA/histone makes unit. Content of acid proteins in chromosomes varies depending on their activity and extent of condensation in a cellular cycle. In chromatin (see) interfazny kernels and at any stage of mitotic condensation of DNA exists in a complex with histones, and interaction of these molecules creates elementary structural particles of chromatin — a nucleosoma. In a nucleosoma its central part is made by 8 molecules of histones of four types (on 2 molecules from each type). These are the histones of N2A, N2V, NZ and N4 interacting among themselves, apparently, S-trailer sites of molecules. N-trailer sites of gisto-new molecules interact with molecule DNA in such a way that the last is wound on a gistonovy skeleton, doing two rounds on one its party and one on another. It is the share of one nucleosoma apprx. 140 couples of the bases of DNA. Between the next nucleosoma there is a piece of DNA (10 — 70 couples of the bases) varying on length. When it is straightened, DNA takes a form of thread with beads. If the piece is in the put state, nucleosoma closely adjoin to each other, creating fibrilla with a diameter of 10 nanometers. The structure from nucleosoma-nykh of particles is the principle of the organization of chromatin (see) both in interfazny, and in a metaphase chromosome.

In a genome the eukaryote (see the Genome) is allocated by several classes DNA on number of the repeating sequences of nucleotides, structure of the sequences, their sizes. At the person of DNA DNA with moderately repeating sequences (apprx. 12,3%), DNA with their low repeatability (13,4%), and also DNA consisting of the unique sequences can be subdivided into DNA with repeatedly repeating sequences of nucleotides, including satellite DNA (apprx. 10,3%) (apprx. 64%). At the person four main types of satellite DNA are localized in the majority of chromosomes, but unequally distributed on types and quantity. DNA with repeatedly repeating sequences contains preferential in heterochromatin (strongly a spiral-zovannykh and intensively painted regions of a chromosome). One molecule DNA is the share of the diameter of a chromosome at its maximum dekondensation. In a metaphase chromosome the molecule DNA making it shall be truncated by 104 times in comparison with it of a state, free from proteins. Interaction of DNA with histones during the formation of nucleosoma and thread with a diameter of 10 nanometers provides shortening of the DNA initial thread approximately by 6,5 — 7 times and increase in diameter from 3 nanometers to 10 nanometers. In native chromatin threads of the second order with a diameter of 20 — 30 nanometers prevail, in fibrilla of this level the general shortening of DNA is approximately 40-fold.

DNA with moderate number of repetitions is found hl. obr. in the G-painted segments. With on -

power of the flyuorokhrom (see) which are differently communicating about adenine-thymine and a guanine-tsitozin in couples of bases of DNA showed distinction of sites of a metaphase chromosome on structure of the bases. Specificity of DNA in different sites of chromosomes probably determines their distinction by genetic activity.

Structurally functional organization of chromosomes. Functions of chromosomes are: storage ge

it is new — carriers of the genetic information concluded in molecular structure of DNA (see the Gene, Deoxyribonucleic acid); self-reproduction genetiches

, which information (see Replication, the Reproduction of chromosomes); transfer of genetic information for implementation in a sign (see RNA, the Transcription); the recombination of linked genes between homologous chromosomes in a gametogenesis providing a recombination of signs of parents in posterity (see Meiosis, the Recombination, chromosomes); reversible change of structures of chromosomes (condensation — a dekondensation), necessary for differential activity of genes and the correct distribution of chromosomes in daughter cells during division (see the Gene, the Mitosis); change of number of groups of linked genes and order of their coupling as important factor of variability biol. types in their evolution (see the Mutation). Functioning of chromosomes is closely connected with transformations of their structure. Interaction of structure and function has the features at the different levels of the organization of chromosomes.

At the svetooptichesky microscopic level the morphology of chromosomes is various at the separate moments of their transformations, to-rye are a part of a cellular cycle and hl consist. obr. in condensation of chromosomes on the way to a mitosis or meiosis and a dekondensation upon transition to interphase.

In interphase of a chromosome as much as possible dekondensirovana, are individually indiscernible and occupy all volume of a kernel, forming so-called chromatin (see). Density of chromatin in different sites of a kernel is usually not identical — the sites which are poorly painted by the main dyes alternate with intensively painted. Comparison of differently painted sites of interfazny chromatin to morphology of individual chromosomes at their mitotic condensation and a dekondensation allowed to allocate two types of chromatin — euchromatin and heterochromatin. The topography of heterochromatinic segments in an interfazny kernel testifies in favor of orderliness of an arrangement in it of chromosomes, their bonds with a nuclear membra-

ache. The morphology of chromosomes is connected with their reproduction and therefore differs in different phases of a cellular cycle. It is possible to judge it with the help of induction of condensation of chromosomes of a kernel of a cell in interphase. In a Gj-phase interfazny chromosomes of odes-nonitchaty (odnokhromatidna). In G2 phase when the reproduction is complete, all chromosomes consist of two chromatids throughout.

One of the main functions of chromosomes — reading of genetic information — is also carried out in interphase. Features of morphology of chromosomes at this moment are unavailable to a research on interfazny kernels of diploid cells, but them it was succeeded to investigate on polytene chromosomes (Greek poly much + tainia a tape, a strip) — interfazny X., the found hl. obr. in cells of sialadens of larvae of nek-ry types of group of dipterous insects and the initial chromatids closely adjacent consisting from repeatedly reduplitsirovanny and not dispersed to each other. In a light microscope they look in the form of tapes, cross ischerchenny because of alternation on all length of intensively painted sites (disks) and light (interdisco-vykh) spaces (fig. 1, a). The disk represents the site of densely put chromatinic thread (chromomere). For each chromosome given biol. a look the number, the sizes and topography of disks are strictly certain. The chromomere of a polytene chromosome contains one or more genes in an inactive state. Alternate swelling and a loosening of disks — formation of so-called padded stools (fig. 1,6) is observed. Huge padded stools of nek-ry specific disks are called Balbiani's rings. Process of formation of padded stools represents a dekondensation of the chromatinic threads packed in a disk

of Fig. 1. The site of a polytene chromosome with disk structure (a) and formation of a padded stool. The scheme illustrates emergence of a padded stool by a dekondensation of four chromatinic threads laid in the chromomere (v).

Fig. 2. The scheme of a chromosome in a meta phase of cell division: 1 — the satellite; 2 —

a secondary sputnichny banner; 3 — primary (centromere) banner; 4 — a secondary nesputnich-ny banner; 5 — sister chromatids.

(fig. 1, c) also is reversible. In cytogenetics emergence of padded stools is considered as morphological expression of transkriptsionny activity of genes (see

the Transcription).

Individually distinguishable chromosomes form by the time of cellular division, a mitosis or meiosis, as a result of progressively accruing condensation of chromosomes. In a pro-phase of mitotic division of a chromosome are visible in a light microscope in the form of the long and bound threads therefore individual chromosomes throughout are indiscernible. In a pro-phase of the first meiotic division of a chromosome undergo difficult specific morfol. the transformations connected by hl. obr. with conjugation of homologous chromosomes (see Conjugation of chromosomes) and a genetic recombination (exchange of sites) between them. In a pachytene (when conjugation comes to an end) alternation the chromomere on length of chromosomes is especially indicative, and the chromomeasured drawing is specific to each chromosome and changes in process of condensation. Many chromosomes in an oogenesis and a Y-chromosome in a spermatogenesis have high transkriptsionny activity. At nek-ry types of organisms such chromosomes received the name of «lamp brushes». They consist of the axis constructed from the chromomere and the interchromomere-nykh of sites, and numerous side loops — dekondensirovan-ny the chromomere, being in a condition of genetic functioning (transcription).

In metaphase of cell division of a chromosome have the smallest length and it is easy to investigate them therefore the description of individual chromosomes, as well as all their set in a cell, give in relation to their state in this phase. The sizes of metaphase chromosomes at the same type of organisms strongly differ: chromosomes the sizes in shares of micron have a dot appearance, with a length more than 1 micron they look as rhabdoid bodies. Usually it is the educations doubled on length consisting of two sister chromatids (fig. 2, 3) as in metaphase of a chromosome of a reduplitsirovana.

Fig. 3. Chromosomal complement of the person in metaphase of cell division: / — akrotsent-

a richesky chromosome with the satellite in a short shoulder; 2 — a metacentric chromosome; 3 — a submetacentric chromosome with a secondary banner in the okolotsentro-dimensional region of a long shoulder.

Individual chromosomes of set differ among themselves on length and others morfol. to signs. The methods applied till 70th provided uniform coloring of a chromosome on its length. Nevertheless such chromosome as an obligatory building block has primary banner — the site where both chromatids are narrowed, probably without separating one from another, and are badly painted. This region of a chromosome is called tsentromery, he supports specialized structure — a kinetochore, to-ry participates in formation of threads of a spindle of division of chromosomes. Based on the ratio of the sizes of the chromosomal limbs lying on both sides from primary banner are subdivided into three types: metatsentri-chesky (with it is median the located banner), submetacentric

(the banner is displaced from the middle), akrotsentrichesky (a centromere it is located close to the end of a chromosome, fig. 3). The person has all three types of chromosomes. The ends of chromosomes are called telomeres. On length of chromosomes to some constancy so-called secondary banners can meet not having relations to a centromere. If they are located close to a telomere, the distal site of a chromosome separated by a banner call the satellite, and a banner — sput-nichny (fig. 2). At the person ten with a secondary banner of chromosomes, all of them are akrotsentricheski-m, satellites are localized in a short shoulder. Nek-ry secondary banners contain ribosomal genes and are called yadryshkoobrazuyushchy as thanks to their functioning in products of RNA in an interfazny kernel the kernel forms (see). Other secondary banners are formed by heterochromatinic regions of chromosomes; at the person

from such banners okolotsentromerny banners in the 1, 9 and 16 chromosomes are most expressed.

The initial method of use of dye of Gimza and other chromosomal dyes gave uniform coloring on all length of a chromosome. Since the beginning of the 70th a number of methods of coloring and obrab is developedotka of metaphase chromosomes, to-rye allowed to find differentiation (division into ghost and dark lines) of a line structure of each chromosome on all its length: The Q-coloring (Q — from English

quinacrine quinacrine) received by means of quinacrine, quinacrine-ip-rita and other flyuorokhrom; The G-coloring (G — from a surname of Giemsa) received by means of Gimza's dye (see Romanovsky — Gimza a method) after an incubation of drugs of chromosomes in special conditions; R-coloring (R — from English reverse the return; chromosomes are painted back to G-coloring). The body of a chromosome is subdivided into segments of different intensity of coloring or fluorescence. The number, situation and the size of such segments are specific to each chromosome therefore any chromosomal complement can be identified. Other methods allow to paint certain specific regions of chromosomes differentsialno. Perhaps selective coloring by Gimza's dye of heterochromatinic regions of a chromosome (S-coloring; With — from centromere of a centromere), located near tsentromery — S-segments (fig. 4). S-segments are found in the person in the okolotsentromerny region of all autosomes and a long shoulder of Y - chromosomes. Heterochromatinic areas vary in size at different individuals, causing polymorphism of chromosomes (see. Chromosomal polymorphism). Specific colourings allow to reveal the yadryshkoobrazuyushchy areas, and also kinetochores functioning in interphase in metaphase chromosomes.

Fig. 4. A chromosome of 1 person at different ways of coloring: and — continuous coloring; — Q-coloring; in — G-coloring; — R-coloring; d — the drawing of the sequence of DNA replication revealed by means of a 5-bromdezoksiuridin; e — S-coloring; — the scheme of differentiation of a chromosome on length.

Fig. 5. The diffraction pattern of the isolated metaphase chromosome in physiological conditions;

X 1 0000.

At the elektronnomikroskopichesky level the main to an ultrastr to t at r - ache unit in-those rfazny chromatin at the translucent submicroscopy (see) thread with a diameter of 20 — 30 nanometers is. The package density of threads is various in sites of dense and diffusion chromatin.

The metaphase chromosome on a cut in the translucent supermicroscope is represented evenly filled with fibrilla of 20 — 30 nanometers in the diameter, to-rye depending on the plane of section have an appearance of the roundish, oval or extended educations. In a pro-phase and telophase in a chromosome it is possible to find thicker threads (to 300 nanometers). At a submicroscopy the surface of a metaphase chromosome is presented by chaotically laid numerous fibrilla of different diameter seen, as a rule, on a short piece (fig. 5). Threads with a diameter of 30 — 60 nanometers prevail.

Variability of chromosomes in ontogenesis and evolution. Constancy of chromosome number in chromosomal complement and structures of each chromosome — an indispensable condition of normal development in ontogenesis (see) and preservations biol. look. During life of an organism there can be changes of number of separate chromosomes and even their haploid sets (genomic mutations) or structures of chromosomes (chromosome mutations). The unusual options of chromosomes causing uniqueness of chromosomal complement of an individual are applied as genetic markers (marker chromosomes). Genomic and chromosome mutations play an important role in evolution biol. types. The data obtained during the studying of chromosomes make a big contribution to a systematics of types (kariosistema-tic). At animals one of the main mechanisms of evolutionary variability is change of number and structure of separate chromosomes. Also change of content of heterochromatin in separate or several chromosomes is important. Comparative study of chromosomes of the person and modern subhuman primates allowed on the basis of similarity and distinction of individual chromosomes to establish degree of phylogenetic

relationship of these types and to simulate a karyotype of their general closest ancestor.

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I am N of of U. Tsitogenetik, the lane with English, M., 1969; Cell biology, A comprehensive treatise, ed. by L. Goldstein a. D. M. Prescott, p. 267, N. Y. a. o., 1979; S e u y n e z H. N, The phylogeny of human chromosomes, v. 2, B. a. o.\of 1979; S h a r m an A. K. a. S h a r-m an A. Chromosome techniques, L. a. o., 1980; ThermanE. Human chromosomes, N. Y. a. o., 1980. A. F. Zakharov.