Cell cycle is the sequence of events carried out between the formation of a cell and its division into daughter cells. The period of division phase is very small in comparison to total period at which cell grows and prepares for division.
Growth and preparatory phase is known as interphase. The division phase is divided broadly into nuclear division (karyokinesis) and cytoplasmic division (cytokinesis). Interphase is divided into three phases viz: G, S and G2 on the basis of biochemical events.
G, phase or Gap 1 phase is the presynthetic phase. Cell growth along with active synthesis of RNA and proteins are marked in this phase. The nuclear pool, amino acid and protein pools are also increased to meet the demand of the next phase.
S phase is the synthetic phase which follows the Gi phase. Synthesis of DNA occurs in this phase, which facilitates the duplication of chromosomes. The DNA content is doubled and so are the genes. The duplicated genes are distributed equally between the two sister chromatids of the chromosome. The chromatids are held together at the centromere.
After S phase the cell enters into the G2 phase or Gap 2 phase in which RNA and protein synthesis continues.
Division phase follows the G7 phase. Two types of divisions are generally essential for continuity of life. One type of division is marked in vegetative or somatic cells, where the daughter cell produced, is similar to the mother cell with regard to the chromosome number.
The type of division is known as mitosis. However, the type of division marked in reproductive or germ cells produces daughter cells each of which contains half the chromosome number of the mother cell.
Such a division is known as meiosis. The cells of higher plants and animals contain tv-. Sets of chromosomes, one set being replica of the other. Such cells are known as diploids.
The DNA content of a diploid cell, if considered as 2C, it becomes 4C in the S phase. In mit c divisions the 4C content is halved to 2C in daughter cells. But in meiotic division the daughter cell contains only 1 C DNA.