Definition

• Mitosis is a type of cell division in which a mother cell gives rise to two daughter cells identical to each other and to the cell that gave them origin. Because mitosis retains the chromosome load, it occurs in an equational division and is represented by E!

• A diploid cell (2n) that undergoes mitosis generates two diploid daughter cells (2n). There are cases, common in plants and algae, in which a haploid cell (n) divides by mitosis, originating two haploid cells (n). Even triploid cells (3n) can undergo mitosis.

Biological roles of mitosis

• The occurrence of mitosis allows the zygote to originate all the component cells of the organism. Thus, participating in the growth and development of the individual.

• During life, we lose many cells, such as red blood cells and epidermal cells. These cells are constantly being replaced through mitosis. Sometimes living beings have tissues that have been damaged by injury or disease. These tissues have a repair tool in mitosis; this is an adaptation by the organism that allows it to stop the damage and keep the structures functioning.

• For some single-celled organisms, such as the amoeba, the mitotic division represents a mode of asexual reproduction, known as bipartition.

• Certain animals, such as sponges, have asexual reproduction by budding, where a lateral protrusion develops into a kind of miniature mother organism. Sprouting involves a high rate of mitotic divisions.

• Some cells can mutate during the process of mitotic division. When this mutation is related to cell cycle control genes, it results in an increase in the speed and frequency of cell divisions by mitosis and leads to the development of tumors, which can be benign or malignant (cancer).

Interphase

• Interphase is the stage when the cell is not dividing. But can show high metabolic activity. The cell in the interphase has an intact nucleolus. Its chromatin is decondensed and can't be distinguished from another under the microscope.

• During interphase, the replication of the genetic material takes place. And identical chromatids are formed, joined by centromeres. The period before the duplication is called G1 (gap one). The period in which DNA synthesis occurs is named the S (of synthesis) period. After the replication is complete, the cell is in the G2 period, in which it produces proteins and organelles. We will consider that the quantity of DNA in the interphase stages is 2x in G1 and 4x in G2.

• The biggest activity of protein synthesis occurs in G1, allowing for high cell metabolism and growth. Some cells usually do not divide, as is the case of neurons and myocardial cells (muscle fibers of the heart). These cells are stationary in a period before replication, called G0 (zero).

• During the S-period of the interphase of animal cells, the duplication of the centrioles takes place. The cell then enters mitosis.

The mitotic process

• The mitotic process has four phases: prophase, metaphase, anaphase, and telophase.

Prophase

• In this phase, the nucleolus is in the process of disaggregation. The ribosomal RNA of the nucleolus is distributed throughout the cell.

• The genetic material is in the process of condensation. And the chromosomes are becoming evident.

The animal cell has two pairs of centrioles. These pairs move apart, migrating to opposite poles. The cell produces filaments, known as microtubules, made up of tubulin. There are two types of microtubules: aster, around centrioles, and spindle filaments, located between pairs of centrioles.

Metaphase

• In metaphase, the two pairs of centrioles are at opposite poles. Between them, there are several spindle fibers.

• The chromosomes show maximum condensation and are attached to the spindle fibers by the centromere. At the end of this phase, the duplication of the centromeres takes place.

Anaphase

• In anaphase, there is the shortening of the spindle fibers and the total separation of the two sister chromatids, originating the sister chromosomes, which are pulled to opposite poles of the cell.

Telophase

• The chromosomes come to opposite ends and start decondensation. The microtubules of the aster and the spindle are disassembling. The nucleolus is in the process of reorganization. The cytoplasm suffers strangulation that culminates in its division.

• This strangulation, characteristic of animal cells, is due to the slipping of proteins from a ring on the cell surface.