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During meiosis (reduction division), special kind of cell division, a number of chromosomes halves from diploid (2n) into haploid (n). Cells divide in this way during gametogenesis and it results in a formation of gametes, i.e. the gametal cells that contain halved number of chromosomes. This is a crucial step of sexual reproduction. Indeed as gametes formed during meiosis are haploid then after fertilization, i.e. fusion of two gametal cells, the formed zygote contains regular (diploid) number of chromosomes. Actually meiosis is a kind of mitosis as the same mechanisms of chromosomes duplication, movement and cell division are involved in both processes. Meiosis consists of two consecutive cell divisions, although chromosomes duplicate prior to the first division the only. Therefore after the secondary division a number of chromosomes in the gamete formed is halves. Another important property of meiosis is an exchange of parts chromosomes (crossing over) between homologous ones during prophase I. Male sex cells formed are identical by size. In contrary during oocyte meiosis a cytoplasm is distributed between dividing cells irregularly, i.e. one cell remains to be large while three others are much smaller and contain just nuclei the only. These small cells serves as a stores of abundant genetic information and do not participate in a subsequent development. After the division of the gametal cells finished the male and female gametes fuse and produce the zygote. During fertilization a haploid number of chromosomes fuses and as a result the zygote contains the diploid number of chromosomes - one number from one parent and one from another one (the number of chromosomes is settled for each type of organism). Random difference between chromosomes and exchange of genetic information between homologous chromosomes results in production of new gene combinations thus it increases genetic diversity. Each formed zygote develops into individual organism. The animation is operated by the "Start"/"Stop" and the "Reset" buttons.
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