Every cell that is capable of undergoing division passes through a cyclic sequence of events involving growth and division. It is called Cell Cycle. It encompasses the entire sequence of events that occur in a cell from the time it is formed from its parent cell till the time of its own division into daughter cells.
Cell cycle has three main stages namely:
Interphase
This is a period of intense synthesis and growth in the cell. The cell produces many materials required for its own growth and activities. The genetic material DNA replicates during interphase.
Karyokinesis
It is the process of nuclear division, which involves separation of chromatids and their redistribution as chromosomes into daughter cells.
Cytokinesis
It is the process of division of the cytoplasm to result in the formation of daughter cells.
fig. 17.1 - The Cell Cycle
| Phase | Events within cell |
|---|---|
| G1 | Intensive cellular synthesis, mitochondria, chloroplasts (in plants), ER, lysosomes, golgi complex, vacuoles and vesicles produced. Nucleus produces rRNA, mRNA and tRNA and ribosomes are synthesised. Cell produces structural and functional proteins. Cell metabolic rate high and controlled by enzymes. Cell growth occurs. Substances produced to inhibit or stimulate onset of next phase. |
| S | DNA replication occurs. Protein molecules called histones are synthesised and cover each DNA strand, Each chromosome has become two chromatids. |
| G2 | Intensive cellular synthesis. Mitochondria and chloroplasts divide. Energy stores increase. Mitotic spindle begins to form. |
| Mitosis | Nuclear division occurs in four phases |
| C | Equal distribution of organelles and cytoplasm into each daughter cells |
The length of the cycle depends on the nature of cell and various external factors like temperature food and oxygen availability. Bacterial cells may divide every 20 minutes, epithelial cells living the small intestine divide once in 8 to 10 hours, onion root tip cells take about 20 hours to divide. Some specialised cells like the nerve cells never divide.
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