MEIOSIS

From Big Medical Encyclopedia

MEIOSIS (Greek meiosis reduction) — special type of cellular division which surely there pass sex cells during their maturing. The m is inherent in all types of the vegetable and animal organisms breeding sexually and its main patterns are universal. Biol, M.'s appointment is double. On the one hand, for preservation biol, a look it is necessary that chromosome number inherent in it (diploid number, at the person equal 46) was supported invariable among generations of organisms. On the other hand, each organism is formed at merge of two sex cells, men's and women's (see. Fertilisation ). Therefore, sex cells unlike somatic cells shall have (haploid) chromosome number reduced twice (see. Gametes ). Double reduction (reduction) of chromosome number during the maturing of gametes is also provided in the course of M. which therefore is called still meiotic division.

Fig. 1. The scheme of a recombination of linked genes in a pro-phase I of meiosis: as a result of exchange of the sites of homologous chromosomes containing allelic genes of Bb (at the left) at the end of meiosis each of four of its products has specific gene structure; And yes, B — genes of homologous chromosomes.
Fig. 2. The scheme of a recombination of not linked genes in the I meiotic division of a sex cell (at the left): as a result of accident of distribution of nonhomologous chromosomes on daughter cells four different combinations of two nonhomologous chromosomes after the I division (on the right) are possible.

Besides, thanks to M. the new combination of genes in a gamete is provided and the new organism resulting from fertilization is always unique, but is not the simple sum of lines of the parents. Such genetic recombination is carried out in M. in two ways. First, in the initial sex cell (a spermatocyte or an oocyte) still keeping diploid chromosome number there is a temporary pairing (conjugation, a synapsis) of homologous parent chromosomes and exchange between them identical chromosomal sites (a crossing-over, either an intra chromosomal recombination, or a recombination linked, i.e. being in one chromosome, genes; fig. 1). Secondly, upon completion of a crossing-over at the first division of a meiotic cell nonhomologous chromosomes disperse in daughter cells accidentally, than the recombination not linked, i.e. located in different chromosomes, genes, or interchromosomal recombination (fig. 2) is provided for each gamete.

Cytologic M. includes two consecutive cellular divisions (the first and second meiotic divisions) at one duplication of chromosomes happening in interphase only of the first division. From a stem men's sex cell (spermatogone) four gametes — a spermiya result, each of which is full-fledged. From a female sex cell (oogonium) only one of four products of meiotic divisions develops in a mature gamete (ovum), other three form so-called napravitelny (polar) little bodies which then degenerate.

Fig. 3. The diagrammatic representation of cells in stages of I and II meiotic divisions: 1 — a leptotena; 2 — a zigotena; 3 — a pachytene; 4 — diplotene; 5 — a diakinesis; 6 — metaphase I; 7 — an anaphase of I; 8 — telophase I; 9 — an interkinesis; 10 — metaphase II; 11 — an anaphase of II; 12 — telophase II. Gradual pairing of homologous chromosomes (1 — 3), exchange of sites between them in two points with education of two hiazm (4 — 5), discrepancy after exchange at first of homologous chromosomes (6 — 8), then sister chromatids (10 — 12) is visible, each of which got specific gene structure; each of four products of meiosis contains a haploid set of chromosomes.

According to two divisions of M. and with phases of each division (see. Mitosis ) allocate so-called stages to M. Profaza I the stage of a proleptotena precedes, in time the cut in a cellular kernel after the end of the period of synthesis of DNA occurs temporary condensation of chromosomes therefore they take a form of densely painted unsharply outlined educations (pro-chromosomes). Value of this phenomenon is not clear yet. Professional azu I M., unlike professional elements of a mitosis, subdivide into five stages: leptotena, zigotena, pachytene, to diplotene and diakinesis (fig. 3, 1—5).

In the leptotena all space of a kernel is filled with a ball of chromosomes which have an appearance of the fine long ends which are externally not divided into chromatids though chromatids and exist.

In the zigotena homologous chromosomes in certain, strictly homologous sites, most often in telomerny and centromere areas, begin to copulate (to conjugate), chromosomes at the same time are shortened. Pairing of chromosomes is an important condition for implementation of exchange of genetic material — a crossing-over (see. Conjugation of chromosomes ).

In a pachytene of a chromosome homologs are coupled on all length (bivalents). Their length makes only 1/4 — 1/6 lengths of the same chromosomes in the leptotena, they have an accurate chromomeasured structure. Gonosomes of X and Y usually conjugate certain sites. At this stage, especially in its end, the number of bivalents corresponding to haploid number of chromosomes can be counted and according to the specific chromomeasured drawing and the size of a bivalent individual identification (the pakhitenny analysis) can be carried out them. If in chromosomal complement it is more than two homologous chromosomes (a trisomy, a tetrasomiya, etc.), their conjugation gives multivalenta (trivalenta, tetravalenta etc.). In a cell of a polyploid organism (a triploidy, a tetraploidiya etc.) all chromosomes in a pachytene can create multivalenta. At polyploid organisms when in a meyotsita there are four homologous chromosomes, their pairing among themselves can happen differently. The homologs which came from one parent can conjugate only among themselves (autosindez). Conjugation can happen only between homologs different parents (allosyndesis). In the same cell pairings of oboy type are possible (auto-allosyndesis). The chromosome which does not have a homolog (a monosomy, interspecies hybrids) will pass professional azu M., without conjugating (univalent). Autologous chromosomes (identical among themselves on gene structure and structure) which can be formed in premeiosis as a result of additional reduplication, form at conjugation of an autobivalenta.

In diplotene there is a pushing away of homologous chromosomes from each other, and in each of them sister chromatids begin to differ, as a result for the first time in a bivalent the four-filamentous structure — a tetrad (fig. 4) is shown.

In separate points on length of a bivalent homologs remain in contact, these sites have an appearance of decussations, hiazmam are called and are tsitol, manifestation of a crossing-over. The number and localization hiazm are individual for each bivalent. Distribution hiazm is influenced by topography of heterochromatinic areas in a chromosome, in such areas hiazm least of all or at all not. Different biol, types significantly differ on a hiazmoobrazovaniye. Are reckoned insects at whom hiazm does not happen in general (e.g., at males of a drosophila).

The pro-phase I M. comes to an end with a stage of a diakinesis, during a cut there is a bigger discrepancy of homologous chromosomes at further shortening of bivalents. During this process shift hiazm by the ends of a chromosome is observed (a terminalization hiazm), however bivalents remain. By the end of a diakinesis the nuclear envelope is completely dissolved, the spindle of division forms and metaphase I (fig. 3, 6) begins.

Fig. 4. The scheme of discrepancy of elements of a tetrad in meiotic divisions: And yes and — sister chromatids; AA and aa — homologous chromosomes; I \plane of meiotic division (discrepancy of homologous chromosomes); II \plane of equation division (discrepancy of sister chromatids).

In metaphase bivalents are located in the equatorial plane of a cell, at the same time centromeres of homologous chromosomes of each bivalent are oriented to different poles of a spindle. In an anaphase of I disintegration of a bivalent and discrepancy of homologous chromosomes in daughter cells comes to the end. This type of division call reducing (see fig. 4). In each of homologs sister chromatids in an anaphase of I remain connected with each other, the homolog passes into telophase I in the form of two-filamentous structure (dyad). These features distinguish meiotic division of I from mitotic therefore it is called a heterokinesis. The Interfazny state preceding meiotic division of II and called by an interkinesis, quickly as in it there is no synthesis of DNA. At separate types of organisms of an interkinesis and professional elements of II does not happen. The pro-phase II, metaphase II, an anaphase of II and telophase II follow quickly one by one (fig. 3). In the course of division of II discrepancy of sister chromatids in daughter cells is carried out. These chromatids are equivalent according to genetic contents therefore division of II is called homeotypic (see fig. 4.) At some biol, types division of I is homeotypic, and division of II — reducing. Four haploid products same an oogonium are called a tetrad; owing to abnormal M. one sexual mature cell (monad) can be a product of one goniya.

The m at the person completely corresponds to the described general scheme. Between male and female M. the person has essential distinctions on time of its course in ontogenesis and duration of stages. At men meiotic divisions begin at pubertal age and continue continuously during puberal life. Transformation of a spermatogone in mature spermiya (see. Spermatogenesis ) borrows apprx. 8 — 9 weeks. Various stages of M. at men differ on duration, and different frequency is connected with it, about a cut are found on drugs of biopsy material of testicles of a meyotsita of different stages. Rather seldom meet a zigoten and diplotene, is more often — a leptotena, a pachytene and a diakinesis. At the woman transformation the oogonium begins in the early embryonal period, and by 7th month of an antenatal life all an oogonium pass into primary oocytes, reaching a stage diplotenes. At this stage there is a dekondensation of bivalents, and oocytes remain in steady-state condition to puberal age (a stage of a diktiotena). They are exposed to meiotic division of II from the beginning of the puberal period of life of the woman, ripening in ova in turn in each ovulyatsionny cycle. Division of the II oocyte continues up to fertilization, and with an ovum there are in communication three polar little bodies (see. Oogenesis ).

Stages of a leptotena, zigotena and pachytene are available to studying at persons of both floors. At both floors to a pachytene individual bivalents do not manage to be allocated. In a pachytene, especially late, some individual bivalents it is possible to distinguish on length, situation centromeres, to number and the sizes the chromomere. Bivalents of acrocentric chromosomes it is easier to identify others on their communication with a kernel. The integrated pakhitenny analysis is complicated by the fact that it is difficult to receive separately lying bivalents on drugs. Gonosomes at men, unlike gonosomes of women, conjugate the ends; besides, they are condensed and form a so-called sexual bubble therefore it is easy to find them in a pachytene.

A chromosomal research of stages a diakinesis — metaphase I was succeeded to conduct only at men. At usual coloring from 23 bivalents only XY a bivalent and sometimes a bivalent of IX with confidence can be allocated, and also division of all bivalents into groups A — G similarly group identifications of chromosomes in metaphase of a mitosis can be carried out (see. Karyotype ). By means of differential colourings by a technique of Q and S which allow to reveal individual morphology of shoulders of chromosomes and situation centromeres in a diakinesis — metaphase I 23 bivalents are identified all and the number and localization hiazm is studied. On average it is the share of one cell apprx. 50 hiazm at dispersion of numbers from 33 to 66. Hiazma are found in all autosomal bivalents; the number hiazm, falling on one bivalent, fluctuates from 1 to 5, increasing in proportion to length of a chromosome. X and Y chromosomes conjugate the ends of short shoulders, and hiazm is not found in this bivalent. The bivalent is sharply condensed by XY and is intensively painted by the main dyes (the phenomenon of a so-called heteropycnosis). Sometimes gonosomes are visible as univalents. In normal chromosomal complement autosomal univalents are extremely rare, multivalenta are also not found though polyploid meyotsita of I regularly meet.

Studying of chromosomes in the medicogenetic purposes in the basic is carried out at a stage a diakinesis — metaphase I; it has diagnostic and predictive value at chromosomal diseases (see. Chromosomal diseases ) and male infertility. As a rule, the chromosomal diseases of the person caused by change of chromosome number arise owing to the broken discrepancy of chromosomes in meiotic divisions. It is established that in gametogenesis (see) there is a partial elimination of gametes, unbalanced on chromosome number. Therefore M.'s studying gives the answer to a question of the frequency of not discrepancy of different chromosomes in I and II divisions and frequency of elimination of gametes, abnormal on these or those chromosomes. As chromosomes in M. conjugate strictly homologous sites, studying of a configuration of the coupled chromosomes in a diakinesis — metaphase I gives additional material for judgment of an origin of excess chromosomes and of disturbance of their structure. So, at trisomies the excess chromosome forms with two homologs trivalent or the bivalent and a univalent are formed. Translocations of acrocentric chromosomes like centric fusion look as trivalenta, and Reciprocal translocations — as quadrivalents of a various configuration. Sometimes this or that chromosomal anomaly, including autosomal is the reason of male infertility, and studying of behavior of chromosomes helps with M. to diagnosis.

Mechanisms M. in many respects remain not clear. Morphological, biochemical and genetic researches of mechanisms of transition from a mitosis to M., conjugation and a crossing-over, other stages of a differentiation of meyotsit in gametes are intensively conducted. Differences from a mitosis begin in premeiotic interphase when lengthening of the period of synthesis of DNA, incompleteness of synthesis of DNA and histones by the time of approach professional elements is found. In the zigotena synthesis of the DNA additional fraction is observed, in a pachytene there is a reparative synthesis of DNA. Synthesis of histones continues to a stage of a pachytene. At the same stages structural and enzymatic proteins, specific to M., are found. Professional elements of I — conjugation and a crossing-over connect specific processes with these biosinteza. At the submicroscopic level in the conjugating chromosomes structures, characteristic of M. — sinaptonemalny complexes are found. Formation of sipaptonemalny complexes begins in the leptotena. They have an appearance of the axial tyazh which is consisting generally of proteins and passing on all length of a chromosome. In the zigotena homologous chromosomes conjugate these axial tyazha (side elements of sinaptonemalny complexes). Consider that these complexes provide temporary conjugation of chromosomes on strictly homologous loci. The nature of forces which are carrying out rapprochement of homologous chromosomes before their conjugation remains unsolved. Opinions of rather molecular mechanisms of conjugation per se are contradictory; there is a representation that it is defined by interaction not of molecules DNA, and proteinaceous components of sinaptonemalny complexes. Also development of molecular model of a crossing-over is not finished; many data say that the recombination of molecules DNA in a crossing-over occurs by a rupture of both DNA threads and their reunion in each of homologous chromatids and is carried out by means of various enzymes. In the course of M. also other chromosomal transformations connected with a differentiation of meyotsit in mature gametes are carried out. In the leptotena begins and in diplotene transkriptsionny activity of many chromosomes and their sites reaches the maximum expression that occurs through a dekondensation of the functioning chromomeres and formation of side loops (chromosomes like «lamp brushes»). At animals and the person this process happens as in spermatocytes, and especially — in oocytes. The transcribed DNA provides the sinteza of diverse proteins proceeding during the formation of gametes. High activity during this period in an oogenesis is shown by ribosomal genes. The big need for ribosomal RNA becomes covered by repeated multiplication (amplification) of ribosomal genes. Tsitol, expression of it is increase in number and the sizes of kernels. In an oogenesis of the person it finds reflection in emergence in diplotene of many additional kernels which are not connected with the main a kernel the forming chromosomes, to-rymi five acrocentric chromosomes are.

Meiotic process is under genetic control, being though a high-specific, but special case of genetically adjustable cellular differentiation. Genetic conditionality of M. in general and its stages in particular is shown by receiving on plants and the animal mutations changing M. Opisana's current of a mutation at which M. completely is absent or there is no first or second division. The numerous mutations influencing conjugation of chromosomes, frequency of a crossing-over (hiazmoobrazovaniye), a terminalization hiazm are received; the mutations breaking discrepancy of chromosomes in an anaphase of I or II. Studying of meiotic mutants allows to dismember all process of M. on simple events and through them morfol., biochemical and other characteristics it is deeper to get closer to understanding of mechanisms of this process.



Bibliography: Fundamentals of cytogenetics of the person, under the editorship of A. A. Prokofieva-Belgovskaya, page 144, M., 1969; Sokolov I. I. Cytologic bases of a syngenesis of metazoans, the Management on tsitol., under the editorship of A.S. Troshin, etc., t. 2, page 390, M. — L., 1966, bibliogr.; Cytology and genetics of meiosis, under the editorship of V. V. Hvostov and Yu. F. Bogdanov, M., 1975, bibliogr.; HultenM. Lindsten J. Cytogenetic aspects of human male meiosis, Advanc, hum. Genet., v. 4, p. 327, 1973, bibliogr.; J o h n B. a. L e w i s K. R. The meiotic system, Wien — N. Y., 1965, bibliogr.; Rhoades M. M. Meiosis, in book: The cell, ed. by J. Brachet a. A. E. Mirsky, v. 3, p. 1, N. Y. — L., 1961, bibliogr.


A. F. Zakharov.

Яндекс.Метрика