Cell Cycle, Mitosis & Meiosis

The cell cycle is the ordered sequence of events by which a cell duplicates its contents and divides. It consists of interphase (G₁, S, and G₂ phases) and the M phase (mitosis and cytokinesis). Interphase is the longest period, occupying about 95% of the total cycle duration, during which the cell prepares for division. G₁ phase is characterised by active RNA and protein synthesis, enabling cell growth. S phase (synthesis phase) is when DNA replication occurs — the DNA content doubles from 2C to 4C, though the chromosome number remains 2n because sister chromatids stay joined at the centromere. G₂ phase involves final preparations including synthesis of tubulin for the spindle apparatus. Some cells exit the cycle and enter G₀ phase (quiescent phase), a resting state — neurons and mature red blood cells are classic examples of cells that do not divide further.

Mitosis (equational division) maintains chromosome number and produces two genetically identical daughter cells. It proceeds through four stages: Prophase — chromatin condenses into visible chromosomes, each consisting of two sister chromatids joined at the centromere; the nucleolus disappears and the mitotic spindle begins to form. Metaphase — chromosomes align at the metaphase plate (equatorial plane) attached to spindle fibres at their kinetochores. Anaphase — centromeres split, and sister chromatids (now individual chromosomes) are pulled to opposite poles by shortening spindle fibres. Telophase — chromosomes decondense, the nuclear envelope reforms around each set, and the nucleolus reappears. Cytokinesis follows: in animal cells, a cleavage furrow constricts centripetally (outside-in); in plant cells, a cell plate forms centrifugally (inside-out) from Golgi-derived vesicles.

Meiosis involves two successive divisions — meiosis I (reductional) and meiosis II (equational) — producing four haploid cells from one diploid cell. Meiosis I is unique because homologous chromosomes pair and separate. Prophase I is the longest and most complex stage, subdivided into five substages: Leptotene — chromosomes begin condensation. Zygotene — homologous chromosomes undergo synapsis (pairing), forming bivalents (tetrads). Pachytene — crossing over occurs between non-sister chromatids of homologous chromosomes, forming chiasmata (visible exchange points). Diplotene — the synaptonemal complex dissolves and chiasmata become clearly visible; homologues begin to separate but remain connected at chiasmata. Diakinesis — terminalization of chiasmata (they move towards chromosome ends), the nuclear membrane breaks down, and chromosomes are fully condensed. In Metaphase I, bivalents align at the equator. In Anaphase I, homologous chromosomes (not sister chromatids) separate and move to opposite poles — this is the reductional step that halves the chromosome number. Meiosis II resembles mitosis: sister chromatids separate during anaphase II.

Mitosis functions in growth, repair, and asexual reproduction. Meiosis is essential for gamete formation and generates genetic variation through crossing over and independent assortment of chromosomes.

The key testable concept is the distinction between events of meiosis I (reductional — homologous chromosomes separate) and meiosis II (equational — sister chromatids separate), and the substages of prophase I where crossing over occurs in pachytene and chiasmata become visible in diplotene.