Reproduction
Reproduction in the Tamil Nadu HSE Second Year Biology syllabus covers asexual and sexual reproduction in plants and animals, the human reproductive system, and reproductive health. Questions test ploidy changes, the sequence of reproductive events, hormone timing, and the purpose of each assisted reproductive technique.
Core rules
- Asexual reproduction uses vegetative propagules such as potato eyes, ginger buds, and Bryophyllum leaf buds. It produces clones; variation arises only by mutation.
- In angiosperms, the microspore mother cell undergoes meiosis to form pollen, while the megaspore mother cell forms an eight-nucleate embryo sac. Double fertilisation produces a diploid zygote and a triploid primary endosperm nucleus.
- Spermatogenesis begins at puberty and is continuous. Oogenesis begins in the fetus; the primary oocyte is arrested in prophase I, and meiosis II is completed only after sperm entry.
- Around ovulation, an LH surge ruptures the mature follicle. The corpus luteum secretes progesterone, which maintains the endometrium.
- Reproductive health includes barrier, hormonal, and intrauterine contraception, plus ART: IVF, ZIFT, GIFT, and ICSI.
Worked example
A diploid plant has 2n = 24. Find the chromosome number in the endosperm, an antipodal cell, a synergid, and the pollen tube nucleus. Reasoning: endosperm forms by triple fusion, so it is 3n = 36. Antipodal cells and synergids are part of the haploid embryo sac, so each is n = 12. The pollen tube nucleus is also haploid, n = 12.
Traps
Endosperm is not always triploid; in Pinus it is haploid. Meiosis II in the secondary oocyte is not completed at ovulation. Vegetative propagation does not generate genetic variation. Condoms reduce HIV transmission, but not all contraceptive methods protect against STDs.