The growth and reproduction of all organisms depend on the division and enlargement of cells. The mechanisms of division and multiplication of cells together constitute cell reproduction.
Cell Cycle
It is the life period of a cell during which a cell synthesizes DNA (replication), grows, and divides into two daughter cells.
Cell growth (cytoplasmic increase) is a continuous process, but DNA synthesis occurs only at a specific stage.
Duration of the cell cycle varies by organism and cell type. For example, a typical eukaryotic cell cycle (e.g., human cell) lasts about 24 hours, while in yeasts, it is 90 minutes.
Phases of Cell Cycle
The cell cycle includes two basic phases: Interphase and M Phase.
1. Interphase (Resting Phase)
It is the phase between two successive M phases, lasting more than 95% of the cell cycle duration. It includes cell growth and DNA synthesis.
Interphase has three sub-phases:
a. G1 Phase (Gap 1 or Antephase):
- First growth phase.
- Interval between mitosis and DNA replication.
- Main events:
- Continuous cell growth.
- Cell becomes metabolically active.
- Prepares machinery for DNA replication.
- Synthesizes RNA and proteins.
b. S Phase (Synthetic):
- DNA replication occurs.
- Amount of DNA per cell doubles, but chromosome number remains unchanged.
- In animal cells, replication begins in the nucleus, and the centriole duplicates in the cytoplasm.
c. G2 Phase (Gap 2):
- Second growth phase.
- Cell growth continues.
- Synthesis of RNA and proteins continues.
- Cell prepares for mitosis.
2. M Phase (Mitosis Phase)
It represents the actual cell division (mitosis), lasting about one hour in the human cell cycle.
M Phase includes karyokinesis (nuclear division) and cytokinesis (division of cytoplasm).
Some cells, like heart cells, do not divide. Others divide occasionally to replace damaged or dead cells.
Cells that do not divide further exit G1 phase and enter an inactive stage called the quiescent stage (G0). These cells remain metabolically active but do not proliferate.