Sec 4 Biology Reproduction notes

Asexual reproduction needs one parent and produces offspring that are genetically identical to it — clones. The engine behind it is mitosis: a cell division that produces two daughter cells with exactly the same genes and the same chromosome number as the parent cell. Mitosis also serves growth (a baby’s cells multiplying), repair (skin and blood cells replaced) and the asexual reproduction of organisms from potatoes’ buds to amoebas.

The cloning guarantee is the point: every daughter cell inherits the full, unchanged set of chromosomes. Starting from one cell, four rounds of mitosis yield 2⁴ = 16 cells — identical copies, ideal for building tissue, but producing no new gene combinations.

Sexual reproduction fuses the nuclei of two gametes to form a zygote — so offspring carry a mix of genes from two parents and are genetically dissimilar, both from their parents and from each other. Body cells are diploid: human body cells hold 46 chromosomes, arranged as 23 homologous pairs — matching pairs, one of each pair from each parent. Gametes are haploid: sperm and egg carry 23 chromosomes, one of each pair.

Gametes are made by meiosis, the reduction division: it separates the homologous pairs, halving the chromosome number while shuffling which member of each pair goes into which gamete. Without meiosis, fertilisation would stack 46 + 46 = 92 chromosomes into every child and double again each generation — the halving must happen first, so the zygote’s 23 + 23 restores the diploid 46.

An insect-pollinated dicotyledonous flower carries its machinery in four whorls: sepals (protect the bud), petals (bright, scented — advertise to insects), stamens, the male parts (an anther that makes pollen on a filament stalk), and the carpels, the female parts (a sticky stigma to catch pollen, a style leading down to the ovary, which contains the ovules). Wind-pollinated flowers invert the advertising: small, dull petals, large dangling anthers releasing light, smooth pollen, and feathery stigmas hanging out to catch it.

Pollination is the transfer of pollen from anther to stigma. Self-pollination lands it on the same flower or plant; cross-pollination carries it to a different plant of the same species — with an insect, wind or other courier between. Cross-pollination matters genetically: mixing genes from two plants gives the offspring more variety, while self-pollination preserves a type but inbreeds it. After landing on a compatible stigma, the pollen grain grows a pollen tube down the style into the ovule; its nucleus travels down the tube and fuses with the ovule’s nucleus — fertilisation, forming the zygote that becomes the seed’s embryo.

The male system: testes in the scrotum (held outside the body, cooler for sperm production) make sperm and testosterone; sperm travel up the sperm ducts, receiving fluids from the prostate gland along the way, and leave through the urethra inside the penis. The female system: ovaries release eggs and female hormones; the oviducts carry eggs toward the uterus — and are where fertilisation usually happens; the uterus hosts the developing foetus; the cervix narrows into the vagina, which receives sperm and is the birth canal.

Route of the egg: ovary → oviduct → uterus. Route of the sperm: vagina → cervix → uterus → oviduct. The meeting point — the oviduct — is where a sperm nucleus fuses with an egg nucleus: fertilisation. The zygote then divides by mitosis into a ball of cells that travels down and sinks into the uterus lining: implantation.

Day 1 is the first day of menstruation: the thickened uterus lining breaks down and is shed over several days. The hormone oestrogen, rising from the ovary, rebuilds and thickens the lining. Around the middle of the cycle — about day 14 in a 28-day cycle — an egg is released from an ovary: ovulation. After ovulation, progesterone keeps the lining thick and ready for a possible embryo. If fertilisation does not happen, progesterone falls, the lining breaks down, and the cycle restarts.

Cycles vary naturally — from about 23 to 35 days between women and month to month. The egg survives only about a day after ovulation and sperm can wait a few days, so the fertile phase clusters around ovulation: for a 28-day cycle roughly days 10–17; for a 26-day cycle, ovulation falls around day 26 − 14 = 12 (the post-ovulation phase is the steadier half); other days are comparatively infertile phases.

Once implanted, the embryo develops inside the amniotic sac, bathed in amniotic fluid — a shock-absorbing cushion that also keeps a steady temperature and weightless freedom of movement. The placenta grows into the uterus lining and links to the foetus by the umbilical cord: across the placenta’s large, thin surface, dissolved nutrients and oxygen pass from mother to foetus, and carbon dioxide and urea pass back for disposal. Crucially, the two blood supplies stay separate — exchange happens by diffusion across the placenta, not by blood mixing, so pressures and blood types never clash.

HIV — the virus that causes AIDS — travels in body fluids: unprotected sexual contact, shared needles, transfusions of unscreened infected blood, and from mother to child during birth or breastfeeding. It does not spread by insects, handshakes, shared cups or casual contact. Its spread is reduced by using condoms, screening donated blood, never sharing needles, and treating infected mothers — blocks placed on every transmission route.