From Big Medical Encyclopedia

CONJUGATION OF CHROMOSOMES (Latin conjugatio connection; chromosomes; synonym: synapsis of chromosomes, pairing of chromosomes) — close connection of chromosomes with each other at all organisms, including the person, possessing the created cellular kernel.

Fig. 1. Conjugation of polytene (huge) chromosomes in cells of sialadens of a drosophila: 1 — repeatedly bred chromomeres (disks) which are in a condition of conjugation; 2 — sites of homologous chromosomes in which conjugation is absent because of a mutation in the corresponding locus of one of chromosomes; x 4000.

Distinguish conjugation of homologous and nonhomologous chromosomes. Conjugation of homologous chromosomes is an obligatory stage meiosis (see), and also occurs in some somatic cells, napr, during the formation of polytene (huge) chromosomes (see) in cells of sialadens of larvae of flies, mosquitoes and other dipterous insects. This type K. x. differs in the fact that homologous chromosomes due to specific mutual «recognition» and an attraction of a homologous genes densely adjoin one to another on all length so that the chromomeres containing a homologous (allelic) genes are precisely against each other. If in one of the conjugating chromosomes occurred translocation (see) or inversion (see), the site of a chromosome with the changed sequence of an arrangement of genes is not capable to conjugate with the opposite site of the second, homologous chromosome (fig. 1). However if reorganization happened on rather long piece, then partner chromosomes, forming a loop or a cross, are capable to provide comparison of homologous loci and by that to carry out conjugation.

In polytene chromosomes conjugation of homologs has irreversible character, homologous chromosomes remain connected until the end of existence of sialadens. Biol. value such To. x. remains not clear. Conjugation of homologous chromosomes in meiosis has reversible character and is the cornerstone of exact division of diploid set of chromosomes into two haploid sets which disperse in different cells (a reduction of chromosome number). This phenomenon creates conditions for sexual process and a genetic recombination at diploid organisms. Process of conjugation of homologous chromosomes happens In a pro-phase I of meiotic division and begins at a stage of a zigotena. At the same time at first there is a rapprochement of homologous chromosomes from distance of several micrometers to the distance about 0,2 microns providing contact of chromosomes. Then there is actually «recognition» and a specific attraction of homologous chromomeres (loci of genes). As a result two conjugating homologous chromosomes lay down in parallel one another so that homologous chromomeres form couples. Two connected homologous chromosomes carry the name of a bivalent, and a stage of meiosis, on a cut connection (conjugation) came to the end on all length of chromosomes, is called a pachytene. During a pachytene there is a crossing-over (decussation) — exchange of sites of homologous chromosomes (see. Recombination ). At a stage of a pachytene each chromosome consists of two longitudinal half — chromatids (4 chromatids in a bivalent). In this place of a bivalent the crossing-over comes only between two not sister chromatids from four. At the following stage — in diplotene — homologous chromosomes make a start one from another in all points of a bivalent, except where there were decussations. As a result of the place of decussations (hiazma) become visible in a microscope. At the following stages of meiosis — in diplotene, a diakinesis and metaphase I of division under the influence of condensation and shortening of chromosomes of a hiazma move by the ends of bivalents. There is a so-called terminalization hiazm. At the same time hiazma continue to hold couples of former partners in conjugation. In metaphase I each chromosome in a bivalent connects thread of a spindle only to one pole of cellular division. Thanks to it in an anaphase of the I meiotic division homologous chromosomes disperse to opposite poles and to each pole gets on one chromosome from each bivalent. Thus, To. x. and hiazma provide the correct reduction of chromosome number. If in karyotype (see) there are unpaired chromosomes, napr, the single gonosome is normal at males of some species of insects or a single gonosome at the person at a syndrome Turner, such chromosomes do not enter conjugation because of the absence of the partner, remain univalent in a pro-phase I and in a random way go to this or that pole in an anaphase of I. The opposite pole has a set deprived of one chromosome. Interspecies hybrids, napr, at a mule, have no pair chromosome since a half of chromosomes is received from chromosomal complement of a horse, and another — a donkey. In a result in to a pro-phase I K. x. in general is absent, and all chromosomes remain univalent. In an anaphase of the I division they chaotically disperse to poles, and daughter cells, and also gametes forming from them receive unbalanced both on species composition, and on number sets of chromosomes. It results in frailty of gametes or zygotes. Infertility of hybrids like a mule is caused by absence at them To. x. in meiosis.

Emergence in diploid chromosomal complement (see) an excess chromosome (a trisomy on this chromosome) leads to disturbance To. x. because of the competition between three partners in conjugation. The research of such cases led to opening of the rule, according to Krom in each point (locus) of a chromosome conjugation only of two partners is possible. However in other locus there can be a change of the partner, and trivalenta as a result appear. It also breaks discrepancy of chromosomes in an anaphase of I. Various cases of disturbance To. x. lead to emergence of hypohaploid or hyper haploid gametes (a lack of chromosomes or their surplus). If such gametes survive and create a zygote, then there are aneuploid germs (disturbance of strict diploidy), at the same time chances of a monosomy (lack of one of chromosomes), trisomies (emergence of an excess chromosome), etc. At the person mono - both a trisomy on gonosomes and the nek-eye to autosomes brings to to chromosomal diseases (see), and an aneuploidy on large autosomes — to death of a fruit in the pre-natal period.

Sometimes at hybrids of close types, in particular at vegetable hybrids, or at the organisms bearing large reorganizations of chromosomes the partial homology of chromosomes is observed. Then ability of chromosomes to conjugation judge on number of bivalents in a cell at stages diplotenes — metaphases I and «force» of this conjugation — on number hiazm on a bivalent. However cases of an akhiazmatiya are known (total absence hiazm) at normally proceeding conjugation of chromosomes in the zigotena and a pachytene. Premature separation of homologs (disintegration of bivalents) because of absence hiazm is called desinapsisy. Desinapsis leads to the same disturbance of discrepancy of chromosomes in an anaphase I what is observed at a mule.

Fig. 2. The electronic microphoto of a sinaptonemalny complex in spermatocytes of an ascarid at a stage of a zigotena of meiosis: 1 — the central element; 2 — side elements; 3 — chromatin; 4 — a nuclear membrane; 5 — cytoplasm; X 50 000.

It is established that process of conjugation of homologous chromosomes is in meiosis under control of many genes operating only during meiosis. Such genes are found in a drosophila, mushrooms and a number of the higher plants, but their action is identical at the most different organisms. Genetic control To. x. demonstrates that To. x. it is provided with the specific proteins synthesized during meiosis. Rapprochement of chromosomes from a long distance is probably carried out at the expense of some factors in a nuclear membrane: the ends of homologous chromosomes attached to a nuclear membrane «slide» on it towards each other, providing thereby rapprochement of chromosomes. It is possible that rapprochement of chromosomes from a long distance happens due to nonspecific interaction of DNA localized in pritsentromerny sites of chromosomes at the expense of intercalary heterochromatin (i.e. the heterochromatin which is between two disks of euchromatin), capable to so-called ectopic pairing — temporary conjugation of nonhomologous sites of chromosomes. Molecular mechanisms of interaction of chromosomes in short distances are better known. Participation in this process of the unique sequences of nucleotides of DNA, whether all chromosomes located on locally on length and which are replicated the day before or in time To is proved. x. at a stage of a zigotena (zetdnk). It is established that To. x. at a stage of a zigotena is followed by formation of the so-called sinaptonemalny complex (SC). It forms in process To. x. in meiosis at all eukaryotic organisms (from infusorians and yeast to the person) also represents submicroscopic structure, edges consists from three ribonukleoproteidny tyazhy, the homologous chromosomes going along each couple in a bivalent (fig. 2). Two outside tyazh are called the SK side elements, internal — central. Side elements arise in each chromosome before their conjugation and approach at the time of conjugation to distance of 150 — 200 nanometers. At this time between them the central element forms. It is supposed that the central element serves as the place of formation of hetero-duplexes zetdnk (hybrid molecules DNA), in a cut polynucleotide threads of a double helix belong to different chromosomes — partners in a bivalent. It is supposed that SK interferes with irreversible connection of homologous chromosomes, holding them at strictly certain distance because it is torn away from chromosomes after the termination of their conjugation at a stage diplotenes, collapses and brought out of a kernel quite often in the form of the polycomplexes packed into a three-dimensional stack of pieces of side and central elements. SK is absent at irreversible To. x. in polytene chromosomes. It is proved that formation of SK is result of activation in meiosis of specific genes, in particular a normal allele of a gene with (3) G at a drosophila. Formation of SK provides the high frequency of a crossing-over, but is not an indispensable condition for its implementation; for lack of SK the crossing-over can go, but with the lowered frequency.

Conjugation of nonhomologous chromosomes (conjugation of homologous sites in nonhomologous chromosomes) is observed in meiosis at haploid plants, in time mitosis (see) in somatic cells of many plants and animals (ectopic pairing). On the basis of nonrandom discrepancy of nonhomologous chromosomes in meiosis at drosophilas with the reconstructed chromosomal complement it is possible to draw a conclusion that in a pro-phase I of meiosis there was a conjugation of nonhomologous chromosomes. Experimentally the hypothesis of a polylocal arrangement in chromosomes of a drosophila of the DNA certain fraction consisting of so-called moderate repetitions of nucleotides is confirmed. They are capable to provide mutual «recognition» of identical sites in the same and other chromosomes of a cell and to cause, thus, conjugation of nonhomologous chromosomes.

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Yu. F. Bogdanov.