How NEET Tests 'Cell Cycle' - High-Yield Traps
The hard questions here are which-event-in-which-phase, mitosis-vs-meiosis and DNA/chromosome-number problems. Master these rank-splitters:
- DNA replication happens ONLY in the S phase. DNA content doubles (2C→4C) but chromosome number does not change in S.
- Order: G1 → S → G2 → (Prophase → Metaphase → Anaphase → Telophase) → cytokinesis. G0 = quiescent exit from G1.
- The centromere splits in ANAPHASE (mitosis) and ANAPHASE II (meiosis) - NOT in anaphase I. In anaphase I the homologous chromosomes separate (sister chromatids stay together).
- Prophase-I sub-stages (L-Z-P-D-D): synapsis = zygotene; crossing over = pachytene (recombination nodules); chiasmata seen = diplotene; terminalisation + envelope breakdown = diakinesis.
- Chromosomes are most condensed at metaphase; nuclear envelope disappears in prophase, reappears in telophase.
- Cytokinesis: plant = cell plate (centre → outward); animal = cleavage furrow. Sometimes karyokinesis occurs without cytokinesis → a syncytium/coenocyte (e.g. liquid endosperm).
- Meiosis = one DNA replication + two divisions → four haploid cells. Meiosis I is reductional, Meiosis II is equational (mitosis-like).
- Variation comes from crossing over (pachytene) + independent assortment (metaphase I).
- DNA amount vs ploidy: after S the cell is still diploid (2n) but has 4C DNA; after meiosis I each cell is n but 2C; after meiosis II each is n and 1C.
Below are challenging assertion-reason, match-the-column, multi-statement and sequence questions built on exactly these distinctions.