Meiosis

Meiosis (synonym reduction division) - a special type of mitotic divisions of the kernel in germ cells in the process of maturation, during which the normal diploid or double) number of chromosomes as characteristic of somatic and immature germ cells, reduced to half (haploid). Meiosis is carried out by two successive divisions of the kernel. During the first of them homologous (or appropriate) chromosome kongugiruut (connect) in pairs and exchange genetic material. It is at this stage meiosis possible redistribution of hereditary material, ensuring the emergence of new combinations of heritable properties. In the first division of the nucleus of the cell formed two daughter cells with haploid number of chromosomes, which are immediately or after a short delay, pass the second stage of fission - already by type of mitosis. Eventually formed four Mature sex cells, each containing haploid set of chromosomes. The male body from all four cells are developing sperm, in the female - only one turns into the egg, and the remaining cells are abortive, or polar, Taurus, which will quickly die. When fertilization occurs Association haploid number of chromosomes maternal and paternal organisms, resulting in a diploid set is restored. Thus, meiosis, ensures the preservation of intergenerational the normal number of chromosomes.
Cm. also, Genetics, Mitosis, Fertilization.

Meiosis (from the Greek. meiosis - reduction, decline; obsolete synonym: reduction division, equazione division) - a special type of mitosis observed in the developing germ cells of all living beings, in which normal (diploid) the number of chromosomes is reduced to half (tabloidnova). Meiosis, ensures the preservation of intergenerational diploid set of chromosomes, so when fertilization (see) connect two sex cells with a haploid set of chromosomes. Meiosis is primarily comprised of two quickly following each other divisions kernel, during which the chromosomes are split only one time, which leads to the emergence of four cells with a haploid set of chromosomes. At the first stages of meiosis (prophase of the 1st division of maturation) interact homologous, i.e. appropriate maternal and paternal chromosomes; at this moment it is possible free redistribution of hereditary material, that provides new, often more favorable for the body to a combination of hereditary properties.
In prophase M in the nucleus (Fig. 1,1) are detected thin, curved spiral going in all directions filaments of chromatin (lapotentova stage, Fig. 1,3).

the scheme conjugation of chromosomes and division ripening
Fig. 1. The scheme conjugation of chromosomes and division of maturation.

It is preceded prolactinoma stage (Fig. 1,2). Next (sitenovo - Fig. 1,4) stage filaments of chromatin shifted in one part of the kernel and are in the form of paired, divergent fan-shaped formations (stage bouquet - Fig. 1,5). Paired strands consist of homologous chromosomes (the so-called bivalents). These chromosomes are converging (Pohitonova stage, Fig. 1,6), wrapped around one another (streptoderma stage, Fig. 1,7) and crossed (the"crossover" English authors), forming a while as if the whole (chromosome pairing). It is at this stage may involve the exchange of hereditary beginnings and formation within each chromosome new combination of genes. Next (diplominiai - Fig. 1,8) stage there is a longitudinal splitting each bivalent, in result of that group (the number of which is less than half the original number of chromosomes), consisting of four connected together chromatid. The latter are shortened and compressed, forming four-groups - of Fallot, usually located under the shell and kernel-shaped rings, ovals, and so on, Then comes the metaphase 1st division puberty, during which begins chromosomes (Fig. 1,9); of Fallot are divided into two groups of two chromosomes in each (dyad - Fig. 1,10)that without interphase pass in metaphase, the 2nd division of maturity (Fig. 1,11). When it ends, each daughter cell contains haploid number of chromosomes (Fig. 1,12). According to the old views, in most cases, the 1st division of maturation is reducing (diverge whole homologous chromosomes), i.e. reduced by half the number of chromosomes; 2nd division maturation believed equazioni (equal), because at this time diverge chromatids - half of one chromosome. Currently, this terminology is obsolete, since the opening of the crossing-over does not allow to speak about purely paternal or maternal chromosomes because of what is happening on stage conjugation (Fig. 2) exchange of plots of homologous chromosomes. In addition, each tetrad Autonomous in relation to differences of its constituent of the chromosomes, and the alternation of discrepancies homologous chromosome and chromatid can be in each tetrad different.

the scheme conjugation of chromosomes
Fig. 2. The scheme conjugation of chromosomes: I - the scheme of division of chromosomes (1) the length of the mitotic cycle; formed chromosomes (2) is not qualitatively different from one another; II - the scheme of formation of a pair of qualitatively different from one another chromosome (3) after conjugation (1 and 2) two identical chromosomes - male and female.

Fundamentally meiosis occurs in both sexes equally, but localization is different: in the male sex cells spermatogonia - he is in the center and gives four of a kind and full cell - Spermatid becoming sperm. In the primary female sex cells oogonia - meiosis occurs on the periphery, and in the result of this process appear four unequal cell - ripened egg and three abortive dialects or polar, Taurus, which will quickly die.
On the inability of all or some alien chromosomes to conjugation at the monastery founded often seen in hybrids phenomena infertility (mules).
Apparently, for the beginning of meiosis need exposure to a particular chemical substances produced, for example in plants, in the male genital organs (Antero). M should be considered as mitosis, which changed the ratio over time conjugation and longitudinal splitting of chromosomes: at the monastery comes early profesa and therefore homologous chromosomes kongugiruut before they doubled (Fig. 2). In meiosis is a redistribution of genes and random and independent of their recombination.
Cm. also Mitosis.