Cell division

Division is not only a pledge of restoring the integrity of tissues, but also the reproduction of organisms. By itself, cell division involves 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 transfer of information between generations.

Cell division. Mitosis and meiosis

There are two main types of division - mitosis and meiosis. The first implies the formation of two daughter cells from one parent cell. The number of chromosomes in both cells remains the same as in the parental cell, that is, the cells are the same. An increase in the number of chromosomes occurs before division. Thus, the cell turns from diploid into tetraploid. At the end two diploid cells are formed.

Meiosis is the formation of four cells. At the same time, each of them has chromosomes half that of the mother chromosomes. Meiosis often occurs immediately after the first division. As a result, one diploid produces four haploid ones.

Between the divisions the cell must grow and develop, preparing for a new division. During this time, many proteins accumulate in it and, moreover, the most important organoids are doubled, and together with them the chromosomes.

It is customary to distinguish four phases of mitosis, which replace one another and last up to two hours. Mitotic division of the cell involves the following stages.

At the first stage the centrioles move to the poles of the cell. After this, spindle fission is formed. Chromosomes become more prominent. At the same time, the nuclear envelope dissolves, and then the nucleolus disappears.

The second stage of division is that the chromosomes must be located along the equator of the cell and attach to the fission spindles.

In the third stage, the daughter chromosomes or chromatids begin the process of divergence to the poles of the cell due to the spindles.

At the next stage, the fission spindles disappear, nuclear shells begin to form around the already dispersed chromosomes. After that, the cytoplasm and daughter cells are divided. This is how the cell divides.

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 fission processes, however, chromosome doubling occurs only before the first division. This is why the cells contain half the set of chromosomes. The process of meiosis contains two main stages - before division and before division, each of which is divided into separate stages.

At the first stage before division, the chromosomes are quite clearly visible. In this case, homologous chromosomes begin to form pairs, densely adjoining each other, and then twisting along their entire length. After that, the homologous regions of the chromosomes exchange areas between themselves and separate. Further, homologous chromosomes should line up along the equator. At the next stage, the chromosomes, which consist of two chromatids, begin to diverge to the poles. In this case, the divergence of each pair does not depend on the divergence of the other pairs of chromosomes. Thus, daughter cells are formed with a half chromosome set.

The second division of the cell. The first stage involves the destruction of the nuclear envelope and the disappearance of the nucleolus. After that, the spindle of division appears. Then the chromosomes should line up along the equator, attaching the spindles. In the third stage, the chromatids of both daughter cells gradually diverge to the poles. After this, cells with a single set of chromosomes appear. Next, the sex cells are formed from them. After the confluence of the sex cells, the number of chromosomes in the new cell is restored.