Reproduction, Development and Growth
Reproduction, Development and Growth covers gametogenesis, plant and animal reproduction, fertilisation, cleavage, gastrulation, growth curves, and plant growth regulators. The exam tests the sequence of events and how growth is measured and controlled.
Core rules
- Spermatogenesis produces four haploid sperm from one primary spermatocyte; oogenesis produces one ovum and polar bodies.
- Angiosperms show double fertilisation: one male gamete fuses with the egg, the other with two polar nuclei to form triploid endosperm.
- Cleavage divides the zygote without growth; gastrulation forms the three germ layers; organogenesis follows.
- Growth can be measured as increase in dry mass, length or cell number; dry mass is preferred because water content fluctuates.
- Auxin promotes cell elongation at low concentration and can inhibit at high concentration; gibberellin promotes stem elongation and seed germination.
Worked example
A root tip squash has 200 cells, of which 12 are in mitosis. Mitotic index = 12/200 × 100% = 6%. Reasoning: the index estimates the proportion of cells in the cell cycle that are dividing at that moment. A higher value suggests more rapid division, but it does not measure cell elongation; growth requires both division and expansion.
Traps
- Meiosis produces genetic variation; mitosis produces genetically identical cells.
- In oogenesis, meiosis II is completed only after fertilisation in many animals.
- Dry mass, not fresh mass, avoids false growth due to water uptake.