describe cell cycle briefly.(long question carring 7marks)
- The sequence of events by which a cell duplicates its genome and synthesizes all other cell contents and eventually divides into two daughter cells is called the cell cycle.
- A typical eukaryotic somatic cell has two main stages—along undividing state called the interphase also known as the intermitosis or I phase and a shorter phase of nuclear division called the mitotic or M phase. These main phases are followed by a still shorter phase of cytoplasmic division known
- Interphase involves a series of changes which take place in a newly formed cell and its nucleus before it gets ready for division again. It is also called intermitosis.
- During this phase, DNA replication, synthesis of nuclear histones, division of centrioles to form a new pair of centrioles, synthesis of energy-rich compounds and synthesis of RNA and proteins take place.
- Interphase is further divided into three periods—first gap or G1 phase, synthetic or S phase and second gap or G2 phase. Duration of these phases is different in different organisms.
- G1 Phase (Post-mitotic Gap Phase):
- The G1 phase is the interval phase between mitosis and the initiation of DNA replication.
- The cell grows to its maximum size to prepare for DNA replication.
- The duration of the G1 phase is highly variable. The cells which divide frequently have a shorter G1 phase, while the cells which divide less frequently have a longer G1 phase.
- During the S phase, synthesis or replication of DNA occurs on the template of the existing DNA strand.
- The amount of DNA per cell doubles, but there is no change in the chromosome number of the cell.
- The S phase lasts for 6–8 hours.
- The G2 phase is a stage of further growth of the cell and preparation for its division.
- Synthesis of RNA and proteins continues.
- Spindle protein synthesis and aster formation take place.
- The G2 phase lasts for 2–5 hours.
- The mitotic or M phase follows interphase. It starts with nuclear division, corresponding to the separation of daughter.
- The M phase lasts for only about an hour.
- After completion of mitosis, the daughter cells may enter the G1 phase of the next cell cycle or the
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