Cell division: cell cycle

The growth of the body occurs due to the increase in the number of cells by division. The main methods of cell division in the human body are mitosis and meiosis. The processes occurring in these methods of cell division proceed in the same way, but lead to different results.

Mitotic division of cells (mitosis) leads to an increase in the number of cells, the growth of the body. In this way, the cells are renewed when they wear out or die. At present, it is known that epidermal cells live 10-30 days, red blood cells - up to 4-5 months. Nerve and muscle cells (fibers) live throughout the life of a person.

In all cells, during reproduction (division), changes are observed that fit within the framework of the cell cycle. A cell cycle is the process that takes place in a cell from division to division or from division to death (death) of the cell. In the cell cycle, prepare the cells for division (interphase) and mitosis (the process of cell division).

In the interphase, which lasts approximately 20-30 hours, the rate of biosynthetic processes increases, the number of organelles increases. At this time, the mass of the cell and all its structural components, including centrioles, is doubled.

There is a replication (repetition, duplication) of nucleic acid molecules. This process of transferring genetic information stored in the parental DNA, by accurately reproducing it in the daughter cells. The parent DNA chain serves as a template for the synthesis of daughter DNA. As a result of replication, each of the two daughter DNA molecules consists of one old and one new chain. During the preparation for mitosis, proteins are synthesized in the cell, which are necessary for cell division. By the end of the interphase, chromatin in the nucleus is condensed.

Mitosis (mitosis, from the Greek mitos - thread) is a period when the mother cell is divided into two daughter cells. Mitotic division of cells ensures a uniform distribution of the structure of the cell, its nuclear substance - chromatin - between two daughter cells. The duration of mitosis is from 30 minutes to 3 hours. Mitosis is subdivided into prophase, metaphase, anaphase, telophase.

In the prophase the nucleolus gradually disintegrates, the centrioles diverge to the poles of the cell. The microtubules of the centrioles are directed toward the equator, and overlap each other in the equatorial region.

In metaphase, the nuclear envelope is destroyed, the chromosomal filaments are sent to the poles, keeping in touch with the equatorial region of the cell. The structures of the endoplasmic reticulum and the Golgi complex disintegrate into small vesicles (vesicles) that, together with the mitochondria, are distributed into both halves of the dividing cell. At the end of the metaphase, each chromosome begins to split by a longitudinal slit into two new daughter chromosomes.

In anaphase, the chromosomes separate from each other and diverge to the poles of the cell at a rate of up to 0.5 μm / min. At the end of the anaphase, the plasma membrane is invaginated along the equator of the cell perpendicular to its longitudinal axis, forming a fissure furrow.

In the telophase, the chromosomes that have dispersed to the poles of the cell are decondensed, transferred to chromatin and transcription (production) of RNA begins. A nuclear envelope, a nucleolus are formed, the membrane structures of future daughter cells are rapidly formed. On the surface of the cell, along its equator, the constriction deepens, the cell divides into two daughter cells.

Due to the mitotic division, the daughter cells receive a set of chromosomes identical to the parent one. Mitosis provides genetic stability, an increase in the number of cells and, consequently, body height, as well as regeneration processes.

Meiosis (from the Greek meiosis - decrease) is observed in sex cells. As a result of the division of these cells, new cells are formed with a single (haploid) set of chromosomes, which is important for the transfer of genetic information. When a single sex cell merges with a cell of the opposite sex (when fertilized), the set of chromosomes doubles, becomes full, double (diploid). In the diploid (binuclear) zygote formed after the confluence of the sex cells, there are two sets of identical (homologous) chromosomes. Each pair of homologous chromosomes of the diploid organism (zygote) is derived from the nucleus of the egg and from the nucleus of the spermatozoon.

As a result of the meiosis of the germ cells in the mature body, only one of all pairs of homologous chromosomes of the original cells appears in each daughter cell. This becomes possible because with meiosis only DNA replication and two consecutive nuclear fission occur. As a result, two haploid cells are formed from one diploid cell. Each of these daughter cells contains half as many chromosomes (23) as in the nucleus of the mother cell (46). As a result of meiosis, haploid germ cells have not only a halved number of chromosomes, but a different arrangement of genes in chromosomes. Therefore, the new organism carries not only the sum of the signs of its parents, but also its own (individual) features.

[1], [2], [3], [4], [5]