🔄 Cell Cycle and Cell Division

1. The Cell Cycle

The sequence of events by which a cell duplicates its genome, synthesises other constituents, and eventually divides into two daughter cells.

Interphase: Resting phase (95% of duration). Divided into G1, S (DNA replication), and G2 phases.
M Phase (Mitosis): Phase of actual division.

Note: Quiescent stage (G0) - cells exit G1 to enter an inactive stage where they do not divide unless called upon to do so.

2. Mitosis (Equational Division)

Occurs in somatic cells. Results in two genetically identical diploid daughter cells. Used for growth and repair.

Stages of Mitosis

Prophase: Condensation of chromatin into chromosomes. Spindle formation begins.

Metaphase: Chromosomes align at the equator (metaphase plate).

Anaphase: Centromeres split, sister chromatids move to opposite poles.

Telophase: Chromosomes decondense, nuclear envelope reforms.

Cytokinesis: Division of cytoplasm (Cell furrow in animals, Cell plate in plants).

3. Meiosis (Reductional Division)

Occurs in specialized germ cells to produce gametes. Reduces chromosome number by half. Involves two sequential cycles (Meiosis I and II) but only one cycle of DNA replication.

Prophase I (Crucial Stage)

Divided into Leptotene, Zygotene (synapsis of homologous chromosomes), Pachytene (Crossing over/recombination occurs), Diplotene (chiasmata visible), Diakinesis.

Significance: Maintains specific chromosome number across generations and increases genetic variability.