Meiosis is the process of dividing nuclei of germ cells during their transformation in the gametes.Meiosis involves two cell division, called meiosis I, respectively, and meiosis II.Each of these divisions formally consists of the same stages as mitosis: prophase, metaphase, anaphase and telophase.
Meiosis I is also known as reduction division, as a result of the division of the number of chromosomes in the newly formed cells is decreased in 2 times.In prophase I chromosomes are already included in the separated chromatids, which are connected to the centromere.It is at this stage of meiosis is extremely important event from the point of view of the creation of genetic diversity - the exchange of homologous regions nesestrinskih chromatids, ie, chromatid belonging to different pairs of homologous chromosomes...This exchange is called crossing-over, or recombination.Prophase I continues long enough, it can be divided into 5 stages: leptotene, zygotene, pachytene, and diplotene diakinesis.In step leptot
Each chromosome is now visible longitudinal slot - chromosome is divided into chromatids.Bivalent represented 4 chromatids arranged side by side to each other: nesestrinskie bivalent chromatids joined together at certain points, forming a so-called chiasm.Chiasmus are a manifestation of exchange of genetic material between chromosomes.These exchanges in formal genetics is called crossing-over.Each corresponds to one chiasm crossover event.Crossing-over is carried out with very high accuracy, so none of the chromatids do not lose or acquire genes.In diplotene homologous chromosomes begin to diverge, keeping only those points where the optic chiasm observed.Chiasmus is produced more in the large chromosomes, just as one gamete accounts for about 40 crossing over.In diakinesis chromosomes condense as much as possible of their differences, homologous chromosomes remain suppressed chiasma.
In metaphase I nuclear envelope disappears, and the chromosomes are distributed in the equatorial plane of the cell.To attach the thread centromeres spindle in mitosis, and to start their delay cell poles.In anaphase I completed terminalization chiasmus, t. E., They are moved to the ends of chromosomes and disappear.Homologous chromosomes move to opposite poles by reducing the spindle threads.As a result, I have telophase cells collected poles haploid sets of chromosomes and germinal cell, completing the division gives rise to two new daughter cells, which are referred to as secondary spermatocytes spermatogenesis, oogenesis and - oocytes.
Meiosis II mechanism is similar to the conventional mitosis, but mitotically divided doubled haploid set of chromosomes.As a result of the second meiotic division formed in the male gametogenesis 2 spermatids and in the female germ line - the egg, as the second daughter cell, a so-called body of the guide.
meiosis genetic explains many phenomena, including the rules of Mendelian inheritance.Firstly, as a result of meiotic germ cells are produced comprising haploid set of chromosomes, so the child receives from each of his parents half sets of chromosomes, and the contribution of each child's parent in the same genotype.Secondly, in meiosis 1 bivalents diverge independently to different poles of the cell, which explains the inheritance independent features if the genes are on different chromosomes.The probability that two individuals will gametes contain the same set of chromosomes, is very low.Third, as a result of crossing over each chromatid contains DNA which comes from both parental chromosomes.As a result, the probability of genetic identity of gametes is practically reduced to zero, and this is the basis of genetic individuality at the chromosomal level.