The sequence of events by which a cell duplicates its genome, synthesizes the other constituents of the cell and eventually divides into two daughter cells is termed cell cycle.
PHASES OF A CELL CYCLE
There are two basic phases of a cell cycle:
1. Interphase: divided into G1 phase, S phase and G2 phase
2. M Phase: divided into Prophase, metaphase, anaphase and telophase
- G1 (Growth 1) phase is the period when the cell grows and carries out normal metabolism. Most of the organelle duplication also occurs during this phase.
- S (Synthesis) phase marks the phase of DNA replication and chromosome duplication.
- G2 (Growth 2) phase is the period of cytoplasmic growth
- Prophase: The chromosome condensation occurs during prophase. Simultaneously, the centrioles move to the opposite poles. The nuclear envelope and the nucleolus disappear and the spindle fibres start appearing.
- Metaphase: This phase is marked by the alignment of chromosomes at the equatorial plate.
- Anaphase: During anaphase, the centromeres divide and the chromatids start moving toward the opposite poles.
- Telophase: Once the chromatids reach the two poles, the chromosomal elongation starts, nucleolus and the nuclear membrane reappear. This stage is called the telophase.
Nuclear division is then followed by the cytoplasmic division and is called cytokinesis. Mitosis is the equational division in which the chromosome number of the parent is conserved in the daughter cell.
Meiosis occurs in the diploid cells, which are destined to form gametes. It is called reduction division since it reduces the chromosome number by half while making the gametes. It is divided into two phases: meiosis 1 and meiosis 2. It the first meiotic division, the homologous chromosomes pair and form bivalents, and undergo crossing over. Meiosis 1 has a long prophase, which is further divided into five phases: leptotene, zygotene, pachytene, diplotene and diakinesis. Meiosis 2 is similar to mitosis. At the end of meiosis, four haploid cells are formed.