Actively dividing eukaryote cells pass through a series of stages known as “the cell cycle”. The most basic function of the cell cycle is to accurately duplicate the chromosomal DNA and then segregate the copies into two genetically identical daughter cells. It consists of G1 and G2 (Gap) phases, a S (Synthesis) phase, in which the genetic material is duplicated; and an M phase, in which mitosis partitions the genetic material and the cell divides (Figure 3).
The basic organization of the cycle and its control system are essentially the same in all eukaryotic cells.
During interphase, the cell is engaged in metabolic activity and preparing for the mitosis. Chromosomes are not clear in the nucleus, although a dark spot called the nucleolus and centrioles may become visible. Interphase comprises G1, G2, and S phases of the cell cycle.
- G1 phase. . It is a very active phase biochemically during which cell mass increases. At the restriction point, the cell is committed to division.
- S phase. DNA synthesis occurs after the G1 phase. Each chromosome forms two sister chromatids, which are firmly attached to the centromeric region. The centrosome is duplicated during the S phase. These two centrosomes will give rise to the mitotic spindle, the apparatus that regulates the movement of chromosomes during mitosis. Centrioles present in centrosomes help organize cell division. Centrioles are not found in plants and most fungi.
- G2 phase. In the G2 phase, the cell replenishes its energy stores and synthesizes proteins necessary for chromosome cell division. Cell gets prepared metabolically and physically before it enters first stage of mitosis.
- M phase. A nuclear division (mitosis) followed by a cell division (cytokinesis).
Mitosis is the process of cellular division that produces two identical daughter cells from one parent cell. Chromosomes replicated during the S phase are divided in such a way that each daughter cell receives a copy of a chromosome. In actively dividing animal cells, the whole process takes about one hour.