Meiosis process
1. In the interval before cell division, DNA and chromosomes are copied, the number of chromosomes remains unchanged, and the number of DNA becomes twice that of the original cell.
2. Subtract the prophase homologous chromosome association to form a tetrad.
3. Mid-subtraction. The centromeres of homologous chromosomes are symmetrically arranged on the equatorial plate (or homologous chromosomes are arranged at both ends of the equatorial plate). (almost the same as mitosis in animal cells)
4. In the later stage of subtraction, homologous chromosomes are separated, and non-homologous chromosomes freely combine and move to the two poles of the cell.
5. At the end of subtraction, the cell divides into two, forming secondary spermatocytes or polar bodies and secondary oocytes.
6. The chromosomes in the secondary spermatocytes gather again before the second meiosis and form the spindle again.
7. Centromeres of negative metaphase chromosomes are arranged on the equatorial plate.
8. At metaphase, the centromere separates and the chromosome moves to the poles.
9. At the end of the second meiosis, the cell divides into two parts: spermatogonia forms spermatocytes, and oogonia forms egg cells and polar bodies.
Meiotic first division
earlier stage
According to the morphology of chromosome, it can be divided into five stages:
In the "thin line stage", slender linear chromosomes appeared in the nucleus, and the volume of nucleus and nucleolus increased. Each chromosome contains two sister chromatids.
Even line periods are also called matching periods. Homologous chromosomes in cells are closely paired on both sides, which is called synapse. Because there are four chromatids in a pair of homologous chromosomes, it is called tetrad.
In pachytene, chromosomes become shorter and thicker, and DNA fragments are exchanged between non-sister chromatids in tetrads, which leads to gene exchange and gene recombination of parents, but each chromatid still has exactly the same gene.
[Double-line phase] The crossed chromatids begin to separate. Because exchanges often occur at more than one site, chromosomes take on various shapes, such as V, X, 8, O and so on.
[Terminal stage] (also called concentration stage) Chromosomes become closely clustered and close to the periphery of the nucleus. After that, the nuclear membrane and nucleoli disappeared, and finally a spindle was formed.
middle period
Pairs of homologous chromosomes move to the equatorial plate in the center of the cell, and centromeres are arranged in pairs on both sides of the equatorial plate to form spindles in the cytoplasm.
later stage
Pairs of homologous chromosomes are separated and moved to the poles under the traction of spindle filaments.
final stage
Homologous chromosomes that have reached the two poles gather again, and the nuclear membrane and nucleoli reappear, and then the cell divides into two daughter cells. The number of chromosomes in these two daughter cells is only half of the original. The regenerated cells then undergo a second division.
note:
1. Chromosome replication takes place during the first division. Once replication is completed, spermatogonia are called primary spermatocytes.
2. A primary spermatocyte undergoes the first meiosis to become two secondary spermatocytes, and a primary oocyte undergoes the first meiosis to become a secondary oocyte and polar body.
3. The purpose of the first meiosis is to separate homologous chromosomes and halve the number of chromosomes. The number of DNA molecules is halved. (as opposed to after copying)
Meiotic second division
The second meiosis is followed by the first meiosis, and there may be a short pause. Chromosomes no longer replicate. The centromere of each chromosome divides, and the sister chromatids separate and move to the cell poles, sometimes accompanied by cell deformation.
earlier stage
Chromosomes are first dispersed in cells. Then they gather again, and the nuclear membrane and nucleolus disappear again, forming a spindle again.
middle period
Centromeres of chromosomes are arranged on the central equatorial plate of cells. Note that there are no homologous chromosomes at this time.
later stage
When the centromere of each chromosome is separated, two sister chromatids are also separated into two chromosomes. Under the traction of spindle filament, these two chromosomes move to the two poles of the cell respectively.
final stage
Reproduce the nuclear membrane and nucleoli, reach the chromosomes at the two poles, and enter the two daughter cells respectively. Compared with the primary spermatocyte, the chromosome number of the two daughter cells is reduced by half. At this point, the second division ended. Note: 1. The purpose of the second meiosis is centromere division and chromatid separation. The result of division is that the number of chromosomes remains unchanged and the number of DNA molecules is halved. 2. After the second meiosis, two secondary spermatocytes become four sperm cells, and the sperm cells can only become sperm after a series of complex morphological changes. The result is that a spermatogonia undergoes meiosis and a series of morphological development, and finally becomes four sperm. 3. The secondary oocytes undergo the second meiosis to become egg cells and polar bodies; One polar body produced by the first division is subdivided into two polar bodies. Soon, all three polar bodies will degenerate and disappear. The result is that the oogonium eventually becomes an egg cell after meiosis.