Reproduction and Developmental Biology
Reproduction and Developmental Biology covers gamete formation, pollination and fertilisation in plants, human reproduction and the menstrual cycle, and the early stages of development.
Plant reproduction
- A stamen has an anther and filament; a carpel has stigma, style and ovary. The microspore mother cell (2n) undergoes meiosis to give microspores (n), each forming a pollen grain with a generative and a vegetative cell.
- The megaspore mother cell gives a functional megaspore, which forms an 8-nucleate embryo sac: 3 antipodals, 2 polar nuclei, an egg and 2 synergids.
- Double fertilisation: one male gamete fuses with the egg to form the diploid zygote; the other fuses with two polar nuclei to form the triploid primary endosperm nucleus.
Human reproduction
- Spermatogenesis occurs in seminiferous tubules; oogenesis begins before birth, with meiosis I arrested in prophase I until puberty.
- The menstrual cycle averages 28 days: FSH and LH stimulate follicle growth, oestrogen thickens the endometrium, ovulation occurs around day 14, and the corpus luteum secretes progesterone.
- Fertilisation occurs in the ampulla; the acrosome reaction precedes fusion, and the cortical reaction blocks polyspermy. The zygote cleaves to a morula, then a blastocyst, which implants around day 7. hCG maintains the corpus luteum.
Development
Cleavage, blastula, gastrula and organogenesis follow; the three germ layers are ectoderm, mesoderm and endoderm.
Worked example
Classify ploidy: nucellus 2n, microspore mother cell 2n, pollen grain n, egg n, synergid n, endosperm 3n, embryo 2n. The reasoning is that meiosis halves the chromosome number, fertilisation restores 2n, and triple fusion adds a second polar nucleus to give 3n.
Traps
Endosperm is triploid, not diploid; there are two polar nuclei; ovulation is counted from the start of menstruation; hCG supports the corpus luteum; one oocyte yields one ovum and three polar bodies.