Cell division: the cell cycle

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

Mitotic cell division (mitosis) leads to an increase in the number of cells and growth of the organism. This method ensures the renewal of cells when they wear out or die. It is currently known that epidermal cells live for 10-30 days, erythrocytes - up to 4-5 months. Nerve and muscle cells (fibers) live throughout a person's life.

All cells undergo changes during reproduction (division) that fit within the framework of the cell cycle. The cell cycle is the name given to the processes that occur in a cell from division to division or from division to death (death) of the cell. The cell cycle distinguishes between the preparation of the cell 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, doubles.

Replication (repetition, doubling) of nucleic acid molecules occurs. This is the process of transferring genetic information stored in the parent DNA by accurately reproducing it in 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 period of preparation for mitosis, proteins necessary for cell division are synthesized in the cell. By the end of interphase, chromatin in the nucleus is condensed.

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

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

In metaphase, the nuclear membrane is destroyed, the chromosome threads are directed to the poles, maintaining a connection with the equatorial region of the cell. The structures of the endoplasmic reticulum and the Golgi complex disintegrate into small bubbles (vesicles), which, together with the mitochondria, are distributed into both halves of the dividing cell. At the end of metaphase, each chromosome begins to split into two new daughter chromosomes by a longitudinal cleft.

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

In telophase, the chromosomes that have diverged to the poles of the cell decondense, become chromatin, and transcription (production) of RNA begins. The nuclear membrane and nucleolus are formed, and membrane structures of future daughter cells are quickly formed. On the surface of the cell, along its equator, the constriction deepens, and the cell divides into two daughter cells.

Due to mitotic division, daughter cells receive a set of chromosomes identical to the mother's. Mitosis ensures genetic stability, an increase in the number of cells and, consequently, the growth of the organism, as well as regeneration processes.

Meiosis (from the Greek meiosis - reduction) is observed in sex cells. As a result of the division of these cells, new cells with a single (haploid) set of chromosomes are formed, which is important for the transmission of genetic information. When one sex cell merges with a cell of the opposite sex (during fertilization), the set of chromosomes doubles, becomes complete, double (diploid). In the diploid (binuclear) zygote formed after the fusion of sex cells, there are two sets of identical (homologous) chromosomes. Each pair of homologous chromosomes of a diploid organism (zygote) originates from the nucleus of the egg and from the nucleus of the sperm.

As a result of meiosis of sex cells in a mature organism, each daughter cell contains only one of all pairs of homologous chromosomes of the original cells. This becomes possible because during meiosis only DNA replication and two consecutive divisions of the nuclei 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 the nucleus of the mother cell (46). As a result of meiosis, haploid sex cells have not only a halved number of chromosomes, but also a different arrangement of genes in the chromosomes. Therefore, the new organism carries not just the sum of the characteristics of its parents, but also its own (individual) features.

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