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 constitutes the cell cycle.
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| Cell Cycle |
The period between two successive divisions is called generation time. A series of changes or sequential events, which occurs in the life of a dividing cell is called cell cycle. The cell cycle includes the period from the beginning of one cell division to 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 three 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 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 organisms of chromosomes are also produced.
G2 phase :
It is second growth phase and also called pre mitotic gap or growth phase. It involves the synthesis of the tubulin, the spindle 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.
Amitosis:
Meganucleus of Paramoecium, foetal cells, endosperm cells, etc. exhibit this type of division. Here the nuclear membrane is retained throughout the division. The nucleus enlarges, elongates and divides directly. It is followed by cytokinesis leading to the formation of two daughter cells. As amitosis does not distribute equally in the daughter cells, it may lead to structural and functional irregularities.