Different cells have different duration of cell cycle. Some cell actively divides like liver cells. But some cell never divides again like nerve cell. GI phase is a key period in the control of cell cycle and cell division. A crucial checkpoint occurs la 1– in the GI before DNA synthesis (the S phase). This is called restriction p. int. If all the internal and external factors are favorable, the cells copy its DNA. T en it divides.
T e cell may exit the cell cycle at the restriction point. It may switch to non-dividing state. This state is called Go phase. The conversion of the G1 to Go is c trolled by following factors.
1 Environmental factors
M st cells of the body are actually in the Go phase. The most specialized cells never divide again, e.g. nerve and muscle cells. Some other cells like liver cells c n be stimulated by certain environmental factors. These environmental factors m y be nutritional status, cell population, and the developmental status of the cells
- Growth factors
The growth factor also controls the cell cycle. For example, the fibroblast requires platelet-derived growth factor (PDGF) PDGF is released during injury.
- Size of the cells
Some cells actively divide in favorable conditions. Here cell size is the most important condition for passing the restriction point. The cell must grow to a certain size during GI before DNA synthesis start. The ratio of cytoplasmic volume to genome size is most important indicator. The call grows by adding cytoplasm. But the amount of DNA in the nucleus remains constant. Once the cell’s volume-to-genome ratio reaches a certain threshold value, the cell passes the restriction point and copy DNA.
- Control by MPF (maturation promoting factor)
Researchers have identified signals. These signals are in the form of complex of proteins called MPF. It stimulates the cell to enter into mitosis from interphase (G2). The amount of MPF in the cell rises and falls in the cell cycle. MPF appears in late interphase and reaches its highest concentration during mitosis. It disappears at the end of mitosis. When MPF reaches a certain threshold co centration in a G2 cell, prophase starts.
- The high concentration of MPF during mitosis is required for starting of mitosis.
- The decline of MPF signals end the mitosis and the G1 phase of next cell Cycle start. M F induces mitosis by acting as an enzyme. MPF belongs to a family of
enzymes called protein kinases. A protein kinase is an enzyme that activate: ot ler proteins and enzymes. It catalyzes the transfer of a phosphate group from A P to each target protein. MPF has two parts:
( ) Cdc2: It is a protein kinase enzyme. It is responsible for transition from interphase to mitosis. It remains inactive until attached by cyclin. The concentration of cdc2 remains constant through out the cell cycle.
( ) Cyclin: It is attached with the cdc2 and activates it. Its concentration varies
according to the different stage of cell cycle. It is continuously produced during interphase. Its concentration is increased at G2. Thus, it is attached with the cdc2 and makes the MPF active. So cell division start. Without cyclin the cell cycle enters into G0 phase. Cyclin is destroyed at the end of cell cycle.