Reproduction, Development and Growth
Reproduction, Development and Growth
This domain tests gamete formation, fertilisation, embryonic development, hormonal control of reproduction, and patterns of growth.
Core rules
- Meiosis produces four haploid cells from one diploid cell. Meiosis I separates homologous chromosomes; meiosis II separates sister chromatids.
- Crossing over in prophase I and independent assortment in metaphase I generate genetic variation.
- In humans, spermatogenesis produces four sperm; oogenesis produces one ovum and polar bodies, conserving cytoplasm for the embryo.
- Fertilisation restores the diploid number. Cleavage increases cell number without growth, forming a blastula; gastrulation forms the three germ layers.
- Growth can be determinate, stopping at a set size, or indeterminate, continuing as in many plants; growth curves often show exponential, linear, or sigmoidal phases.
Worked example
A diploid cell with 2n = 8 enters meiosis. After meiosis I, each secondary cell has n = 4 chromosomes, but each chromosome still has two chromatids. After meiosis II, four cells each have 4 chromosomes with one chromatid. Reasoning: track chromosome number and DNA content separately; DNA replicates before meiosis I but not before meiosis II.
Traps
- Meiosis II is not preceded by DNA replication.
- Mitosis produces genetically identical diploid cells; meiosis produces genetically varied haploid cells.
- In humans, the egg contributes mitochondria; sperm contribute a haploid nucleus and centriole.
- Placental hormones such as hCG maintain the corpus luteum early in pregnancy; do not confuse hCG with progesterone.