Division is not only the key to restoring the integrity of tissues, but also the reproduction of organisms. Cell division itself implies the formation of two or more daughter cells from one parent. Chromosomes occupy a very important place in the process of cell division. They are responsible for the transmission of information between generations.
Cell division. Mitosis and meiosis
There are two main types of division - mitosis and meiosis. The first involves the formation of two daughter cells from one parent cell. The number of chromosomes in both cells remains the same as in the parent cell, that is, the cells are the same. An increase in the number of chromosomes occurs before division. Thus, the cell is transformed from diploid to tetraploid. At the end, two diploid cells are formed.
Meiosis refers to the formation of four cells. Moreover, each of them has half the chromosomes than the mother. Meiosis often occurs immediately after the first division. As a result, four haploids are obtained from one diploid.
Between divisions, the cell must grow and develop, preparing for a new division. During this time, a lot of proteins accumulate in it and, moreover, the most important organoids double, and with them the chromosomes.
It is customary to distinguish four phases of mitosis, which succeed each other and last up to two hours. Mitotic cell division involves the following stages.
At the first stage, centrioles move to the poles of the cell. After this, spindles of division are formed. Chromosomes become more visible. In this case, the nuclear membrane dissolves, and then the nucleolus disappears.
The second stage of division consists in the fact that the chromosomes should be located along the equator of the cell and attach to the division spindles.
At the third stage, daughter chromosomes or chromatids begin the process of divergence to the poles of the cell due to spindles.
At the next stage, fission spindles disappear, around the already separated chromosomes nuclear shells begin to form. After this, the cytoplasm is divided and daughter cells are formed. This is how cell division occurs.
Meiosis
Meiosis is most often associated with sexual reproduction of cells, as well as the formation of germ cells in fungi, plants and animals. Cell division consists of two successive division processes, however, chromosome duplication occurs only before the first division. That is why the formed cells contain a half set of chromosomes. The meiosis process contains two main stages - before division and before division, each of which is divided into separate stages.
In the first stage, chromosomes are quite clearly visible before dividing. In this case, homologous chromosomes begin to form pairs, fitting tightly to each other, and then twisting along its entire length. After that, homologous chromosome regions exchange regions between themselves and separate. Next, homologous chromosomes should line up at the equator. At the next stage, chromosomes, which consist of two chromatids, begin to diverge towards the poles. Moreover, the divergence of each pair does not depend on the divergence of other pairs of chromosomes. So daughter cells with a half set of chromosomes are formed.
The second division of the cell. The first stage involves the destruction of the nuclear membrane and the disappearance of the nucleolus. After this, the spindles of division appear. Then the chromosomes should line up at the equator, attaching spindles. In the third stage, the chromatids of both daughter cells gradually diverge towards the poles. After this, cells with a single set of chromosomes appear. Further, germ cells form from them. After the fusion of germ cells, the number of chromosomes in the new cell is restored.