Meiosis is a division mode in which the number of chromosomes in biological cells is halved.During sex cell division, chromosomes are duplicated only once, cells divide twice in succession, and the number of chromosomes is halved.Meiosis is not only a mechanism to ensure the stability of the number of chromosomes in a species, but also a mechanism for species to adapt to environmental changes and continue to evolve.
Meiosis refers to a special mode of division that occurs during the formation of germ cells in individuals who have sexual reproduction.
Difference from mitosis
Unlike mitosis andAmitosisMeiosis occurs only at a certain stage of the life cycle. It is a special mode of division that occurs during the maturation of biological mother cells for sexual reproduction and the formation of gametes.During meiosis, chromosomes are duplicated only once. Cells divide twice in successionHomologous chromosomeBoth the chromosome and sister chromosome are allocated to daughter cells, so that the chromosome in the final gamete is only half of the sex mother cell.During fertilization, male and female gametes combine to restore the parental chromosome number, thus maintaining the constant chromosome number of the species.In the process of meiosis, homologous chromosomes are exchanged, which makes gamete genetic diversity and increases the adaptability of offspring. Therefore, meiosis is not only a mechanism to ensure the stability of the number of chromosomes of biological species, but also a mechanism for species to adapt to environmental changes and continue to evolve.Meiosis is not only a means to maintain the stable transmission of genetic material of species;During meiosis, through the cross exchange of homologous chromosomes,Nonhomologous chromosomeThe free combination of and the exchange of some segments of non sister chromosomes in the tetrad increase the variety of gene variation and the populationGenetic diversity, providing more raw materials for natural selection
Meiotic second division
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The second meiotic division is immediately followed by the first meiotic division, and a short pause may also occur.Chromosomes no longer replicate.The centromere of each chromosome splits, and sister chromatids separate and move to the two poles of the cell, sometimes accompanied by cell deformation.
early stage
Chromosomes are first scattered in cells.Then it gathers again, and the nuclear membrane and nucleolus disappear again, forming the spindle again.
metaphase
The centromeres of chromosomes are arranged on the central equatorial plate of the cell.Note that there is no homologous chromosome at this time.
later stage
The centromere of each chromosome separates, and the two sister chromatids also separate, becoming two chromosomes.Under the traction of the spindle, these two chromosomes move to the two poles of the cell respectively.
late
Reproduce the nuclear membrane and nucleolus, reach the two poles of chromosomes, and enter the two daughter cells respectively.Chromosome number of two daughter cells andPrimary spermatocyteIt is reduced by half.At this point, the second division ended.
Note
1. The purpose of the second meiosis isCentromere splittingTo realize chromatid separation.The result of division is that the number of chromosomes remains unchanged,DNA moleculeThe number is halved.
2. Two secondary spermatocytes become four spermatocytes after the second meiosis, and the spermatocytes must undergo a series of complex morphological changes to become sperm.The result is aSpermatogoniaAfter meiosis and a series of morphological development, they finally become four sperm.
3. OneSecondary oocyteAfter the second meiosis, it becomes an egg cell and a polar body of the second polar body;One polar body generated by the first division is further divided into two polar bodies.Soon, all three polar bodies will degenerate and disappear.The result is that an oocyte only becomes an egg cell after meiosis.
