A cell cycle starts from the time a new cell is formed and it ends when it completes its own division. Then the cell cycle starts again for each new daughter cell formed.
A cell cycle may be defined as the “events in a cell by which there is an increase in the mass and cytoplasmic components of the cell; duplication of DNA; and then division of nucleus and cytoplasm of the cell. Thus, a cell cycle extends from the time a cell is formed till the time it completes division.”
The cell cycle starts with the interphase during which a cell prepares itself for cell division. The interphase is the longest phase of cell cycle. It is metabolically the most active phase of the cell cycle. It has three sub-phases:
1. G1 or First Growth Phase:
This is the first Gap’ (interval) phase of cell growth and functioning before DNA replication. During this phase:
i. RNA and proteins are synthesized and volume of cytoplasm increases to almost double.
ii. Mitochondria (in plant and animal cells) and chloroplasts (in plant cells) divide.
iii. Now the cell either enters the resting phase (R phase) or a synthetic phase (S phase).
2. S or Synthesis Phase:
This is the phase of DNA replication. The DNA is synthesized and chromosomes are duplicated during this phase.
3. G2 or Second Growth Phase:
It is the second ‘Gap’ phase after DNA replication. It is a shorter phase in which RNA and proteins necessary for cell division continue to be synthesized. Now the cell becomes ready for next cell division, i.e. mitosis.
It has nuclear division (prophase, metaphase, anaphase and telophase) usually followed by cytoplasmic division.
The cell cycle does not go endlessly. It stops permanently at some point of time. Its duration differs from one cell type to another cell type. For example, nerve cells (neurons) in our brain once formed in embryo do not divide further. All blood cells form and replace the worn-out cells at an average rate of 2-3 million each second. Surface skin cells are continuously replaced by underlying cells. Liver cells divide once every 1—2 years to replace damaged cells.