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Cell reproduction is achieved by cell division, which is a cell cycle from the beginning of one division to the end of the next. There are four types of cell division, of which there are three types of eukaryotic cells: mitosis, amitosis, meiosis, the first two are somatic cell division, and prokaryotic cell division is two division. Ji'nan cell engineering
From the end of one division to the beginning of the next division, the cell cycle is mostly in an interphase, accounting for about 90%-95% of the cell cycle, during which the cell completes the replication of DNA molecules and the synthesis of related proteins. After the end of the interphase, it enters the division phase. The splitting period is a continuous process, which is divided into four stages for the convenience of research: early, middle, late and late.（Living cells）
1. early stage; the beginning of cell division. The cell shape is generally round, the centrosome of the centrosome is separated and moves toward the poles of the cell. The emission filament appeared around. The nuclei are enlarged, deoxyribonucleic acid increases, nuclear staining deepens, and irregular chromatin forms filamentous chromosomes, shortening and coarsening. The nucleolus and nuclear membrane disappear and the cytoplasm is mixed with nucleolus.
2. metaphase; two centrosome near two poles, they are linked by wires, spindle shaped, called spindles. The chromosomes moved to the central equator of the cell, arranged in a star shape, and then the chromosome split two.
3. Later, chromosomes that have been split lengthwise are divided into two groups, moving from the equator toward the center of the polar body, and the organelles are evenly distributed. The cell bodies tend to narrow at the equator.
4. At the end of the chromosome cycle, the chromosome moves to the center of the two poles, reunites, transforms into chromatin filaments, and the nuclear envelope, nucleoli, and reappears. The cell bodies are more narrow in the equator, and the mitosis of plant cells is similar to that of animal cells. However, there is no centrosome in the cells of higher plants, and spindle threads are emitted by two levels of cells. At the end of division, the two cells are not separated from each other by an inward depression of the cell membrane, but a cell plate is formed at the central equator of the cell.
Direct division is one of the earliest methods of cell division, which was seen in the blood cells of chicken embryos as early as 1841. Because this division is the direct division of the nucleus and cytoplasm, it is called direct division. It is also called mitosis because there is no spindle wire when splitting. Only some cells in some animals can undergo mitosis, such as the red blood cells of frogs.
At the early stage of direct division, the nuclei and nucleoli of the globular nucleus elongated. Then the nucleus elongated to form dumbbell shaped and the central part was narrow and thin. Finally, the nucleus divides, and the cytoplasm divides as well, forming a cell membrane with the participation of the smooth endoplasmic reticulum. In direct division, the nuclear envelope and nucleolus do not disappear, there is no chromosome, of course, can not see the regular changes in chromosome replication. However, this does not mean that there is no profound change in chromatin. In fact, chromatin is duplicated and cells are enlarged. When the nuclear volume doubles, the nucleus divides and the genetic material in the nucleus is distributed into daughter cells. The distribution of DNA in the nucleus remains to be further studied.
For a long time, there are different views on the issue of direct splitting. Some people think that direct division is not a normal cell proliferation mode, but an abnormal phenomenon of division; others advocate that direct division is one of the normal cell proliferation mode, mainly seen in highly differentiated cells, such as liver cells, renal tubular epithelial cells, adrenal cortical cells.
This form of cell division occurs with gametogenesis, and meiosis occurs in all plants and animals that reproduce sexually. The difference between meiosis and normal mitosis is that the cells undergo two consecutive mitotic divisions during meiosis, with the chromosome dividing only once, resulting in a reduction in the number of chromosomes by half.
Meiosis occurs at a fixed time for each group of organisms, but can be different between different groups of organisms. It can be roughly divided into three types: first, zygotic meiosis, or primordial meiosis, occurs when the fertilized egg begins to cleave, resulting in the formation of an organism with half the number of chromosomes. This meiotic form occurs only in a small number of lower organisms. The second is spore meiosis, or intermediate meiosis, which occurs during sporogenesis between sporophyte and gametophyte generations. This is the characteristic of higher plants. The third is gamete meiosis, or terminal meiosis, which is a common feature of animals, including all metazoans, humans and some protozoans. This meiosis occurs during gametogenesis, the last two divisions of maturity during gametogenesis, resulting in the formation of sperm and eggs.
The specific process of meiosis is very complicated. It includes 2 cell divisions. The prophase of the first division is usually divided into fine-line, even-line, pachytene, double-line, and diakinesis stages. After the prophase I (representing the prophase of the first division), it is metaphase I, anaphase I, and anaphase I; after the meiotic interval (very short or invisible), it enters prophase II, metaphase II, and anaphase I. Phase II, late stage II, and some do not pass through the interval.
During meiosis, the cell divides twice, but the chromosome divides only once, resulting in a reduction in the number of chromosomes by half. Generally speaking, the first division is the division of homologous chromosomes, the number of chromosomes reduced by half, is meiosis. The second division is sister chromatid division, and the number of chromosomes does not decrease. But strictly speaking, this is a general rule. If we analyze it from heredity, it is not so simple.Simple, because it involves the exchange and recombination of chromosomes.
The way of asexual reproduction of most bacteria, such as bacteria. In the process of division, the length of the cell first extends about twice. The middle part of the cell is separated by the cytoplasm and cell wall. The chromosome divides and distributes to the two cells. Finally, two daughter cells of the same shape and size are formed. The proliferation rate increases in geometric series. After division, the daughter cells may be separated from each other or linked together to form pairs, chains, clumps or superimposed. Yeast also propagated  by two division method.
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