Cell cycle
Cell division is a process by which a cell divides into two or four daughter cells.
A cell divides when it has grown to a certain maximum size. The daughter cells formed after the division of a mother cell may grow in size and divide again. This constituents the cell cycle.
The period between two successive divisions is called generation times . A series of changes or sequential events, which occur in the life of a dividing cell is called cell cycle.
The cell cycle includes the period from the beginning of next cell division. The cell cycle consists of two main phases i.e. interphase and M (mitotic or meiotic) phase.
Interphase
It was formerly called resting phase. Actually, the cell is highly active and prepares itself for the ensuing division during this phase. Therefore, it is now called preparatory phase. It involves many metabolic activities. The interphase can be further divided into phases - G1 phase. S phase and G2 phase.
G1 phase
It is the first growth phase, also called the post-mitotic gap phase. It involves synthesis of proteins (enzymes) and energy molecules required for DNA replication. Proliferation of cell organelles takes place or cell may withdraw from cell cycle and enter in the G0 or quiescent stage. The cell in G0 state may re-enter the G1 phase or may get differentiated and become permanent cell.
S-phase
It is also called phase of synthesis or synthetic phase. Replication of DNA takes place during this stage. Entire genetic material (DNA) duplicates. Nucleus becomes double in size. Histone proteins, required for organisation of chromosomes are also produced.
G2 phase
It is second growth phase and also called pre mitotic gap or growth phase. In involves the synthesis of the tubulin, the spindle division protein and RNA. There is duplication of cell organelles, such as mitochondria and chloroplast. The centrioles duplicate and two centrosomes are formed. Thus the cell prepares to enter mitotic phase.
Types of cell division
Though all the cells divide, the division is 'atypical' (Binary fission) in prokaryotes and 'typical' in eukaryotes. In eukaryotes cell division may be of following types- Amitosis, Mitosis and Meiosis.
1. Amitosis
2. Mitosis
3. Meiosis.