Cell Division and Plant Growth
Cell cycle and division
The exam tests the phases of mitosis and meiosis, chromosome behaviour, crossing over, and the significance of reduction division. It also tests growth phases, growth curves and plant growth regulators.
Core rules
- Interphase has G1, S and G2; DNA replicates in S phase.
- Mitosis preserves chromosome number; prophase, metaphase, anaphase and telophase are followed by cell plate formation in plants.
- Meiosis has two divisions; synapsis occurs in zygotene, crossing over in pachytene, and chiasmata become visible in diplotene.
- Meiosis produces four haploid cells and generates variation.
- Growth is sigmoid: lag, log, stationary and senescence phases; primary growth is apical, secondary growth is lateral.
- Auxin causes apical dominance and rooting; gibberellin causes stem elongation and bolting; cytokinin promotes cell division and delays senescence; ABA closes stomata and induces dormancy; ethylene ripens fruit and causes abscission.
Worked example
A shoot bends towards light. Reasoning: auxin moves to the shaded side, cells there elongate more, and unequal growth produces curvature. The response is phototropism, and the auxin redistribution is the cause, not light destroying auxin on the lit side.
Traps
Meiosis does not produce two identical diploid cells. Mitosis does not create genetic variation by crossing over. Ethylene is gaseous, unlike the other four classical regulators. High auxin can inhibit root elongation even while promoting root initiation; concentration matters.