Reproduction

Asexual versus sexual strategies

Asexual reproduction (binary fission, budding, fragmentation, parthenogenesis) preserves the parental genotype and is rapid in stable environments. Sexual reproduction shuffles alleles via meiosis and fertilisation, generating variation that fuels adaptation — Punjab Textbook Board Biology XII Chapter 18 and Campbell 12e Chapter 46 frame this as the Red Queen rationale for sex despite its twofold cost. Plants commonly combine both: vegetative propagation through runners, rhizomes, or tubers, plus seed reproduction.

Human gametogenesis

Spermatogenesis takes about 64 days in seminiferous tubule walls: a spermatogonium (2n) divides to a primary spermatocyte (2n, 4c) → meiosis I → two secondary spermatocytes (n, 2c) → meiosis II → four spermatids → Sertoli-cell-mediated spermiogenesis to produce mature spermatozoa. A normal ejaculate contains 40–300 million sperm per mL (WHO 2010 reference). Oogenesis begins in foetal life: by week 20 the female foetus has ~6–7 million oogonia, falling to ~1 million at birth and ~400 000 at puberty, of which only ~400 ovulate. A primary oocyte arrests in prophase I from foetal life until ovulation, when it completes meiosis I producing one secondary oocyte and a polar body; meiosis II finishes only on fertilisation.

Menstrual cycle and hormones

The 28-day cycle has follicular (days 1–14), ovulatory (~day 14), and luteal (days 15–28) phases. GnRH from the hypothalamus pulses every 60–90 min, driving anterior-pituitary FSH and LH. FSH stimulates follicular growth; granulosa cells aromatise androstenedione to oestradiol (peaks ~200–400 pg/mL pre-ovulation). High oestrogen flips negative feedback to positive at the pituitary, triggering an LH surge that ruptures the dominant follicle (~24 hours later). The corpus luteum secretes progesterone (peaks ~25 ng/mL mid-luteal), preparing the secretory endometrium. Without hCG (~25 mIU/mL by day 28 of conception cycle), the corpus luteum regresses and menses follows. Combined oral contraceptives suppress this axis with ethinyl-estradiol and a progestin.

Fertilisation and early development

Fertilisation occurs in the ampulla of the fallopian tube. Capacitated sperm undergo the acrosome reaction, releasing hyaluronidase and acrosin to penetrate the zona pellucida; sperm-egg fusion triggers cortical-granule release and the zona block to polyspermy. Ca²⁺ waves activate the egg, completing meiosis II. Cleavage divisions reach the morula by day 3 and blastocyst by day 5; implantation occurs around day 7. Trophoblast secretes hCG, the basis of pregnancy tests. Gastrulation (week 3) establishes ectoderm, mesoderm, and endoderm. Twenty weeks of organogenesis follow, with the fetus reaching ≈3.4 kg average birth weight at full term (40 weeks).

Reproduction in flowering plants

The angiosperm flower bears stamens (anther + filament) producing pollen, and a carpel (stigma, style, ovary) housing ovules. Microsporogenesis: microspore mother cell undergoes meiosis to four microspores, each maturing into a two-celled (or three-celled in grasses) pollen grain. Megasporogenesis: megaspore mother cell undergoes meiosis to four megaspores; usually three degenerate, the surviving one forms a 7-celled, 8-nucleate embryo sac (Polygonum type). Double fertilisation, discovered by Sergei Nawaschin (1898): one sperm + egg → zygote (2n), second sperm + two polar nuclei → triploid endosperm (3n) that nourishes the embryo. Pollination is wind-, insect-, bird-, or self-mediated; self-incompatibility systems (gametophytic SI in Solanaceae) prevent inbreeding.