How do Organisms Reproduce? Notes

Reproduction is not needed to keep an individual alive, but it is essential for a species to continue. At the heart of reproduction is the copying of DNA, which carries the information for building the body.

DNA copying is never perfectly accurate, so the copies are similar but not identical. These small differences are variations. Variations that let some individuals survive better in a changed environment allow the species to persist — for example, if heat-tolerant bacteria exist in a population, a rise in temperature will not wipe out the whole population.

Consistency of DNA copying keeps the body plan of a species stable across generations, while the small inaccuracies provide the raw material for evolution.

In asexual reproduction a single parent produces offspring that are genetically almost identical to it. Fission: Amoeba divides into two (binary fission); Plasmodium splits into many cells at once (multiple fission). Budding: in Hydra and yeast, a bud grows on the parent body and detaches as a new individual.

Fragmentation: Spirogyra breaks into pieces, each growing into a new filament. Regeneration: Planaria cut into pieces can each regrow a whole body — this is carried out by specialised cells. Spore formation: Rhizopus (bread mould) produces hundreds of spores in a sporangium; each spore can germinate into a new mould in moist conditions.

Vegetative propagation: new plants grow from roots, stems, or leaves of the parent — potato eyes, ginger rhizome, Bryophyllum leaf buds. Growers also use layering and grafting, and tissue culture, to produce many identical plants quickly, including seedless varieties and plants that flower earlier.

The flower is the reproductive part. The stamen (anther + filament) is the male part and produces pollen grains. The carpel/pistil (stigma, style, ovary) is the female part; the ovary contains ovules, each with an egg cell.

Pollination is the transfer of pollen from anther to stigma — self-pollination within the same flower, or cross-pollination between flowers, carried by wind, water, or insects. After pollination a pollen tube grows down the style to an ovule, and a male gamete fuses with the egg — this is fertilisation.

The fertilised egg (zygote) develops into an embryo inside a seed; the ovule becomes the seed and the ovary becomes the fruit. Under suitable conditions the seed germinates, the embryo grows using stored food, and a new plant is established.

Sexual maturation begins at puberty, marked by changes such as growth of hair in new places, changes in body shape, and voice changes. These changes are brought about by sex hormones — testosterone in boys and oestrogen in girls.

Male system: the testes (held in the scrotum, outside the body cavity because sperm formation needs a slightly lower temperature) make sperms and testosterone. Sperms travel through the vas deferens, receive fluid from the seminal vesicles and prostate to form semen, and leave through the urethra.

Female system: the ovaries produce egg cells (one matures roughly every 28 days) and hormones. The egg travels through the fallopian tube (oviduct), where fertilisation can occur. The fertilised egg implants in the lining of the uterus and develops, exchanging materials with the mother through the placenta. If no fertilisation occurs, the thickened uterine lining breaks down and is shed as menstruation.

Contraception is used to prevent pregnancy and to space or limit children. Methods include barrier methods (condoms, which also prevent sexually transmitted infections), hormonal methods (oral pills that change the hormone balance), intra-uterine devices (copper-T), and surgical methods (vasectomy in males, tubectomy in females).

Sexually transmitted diseases such as gonorrhoea, syphilis (bacterial) and HIV-AIDS, warts (viral) spread during sexual contact. Using condoms greatly reduces the risk.

Sex-selective abortion (female foeticide) is illegal in India and has caused a fall in the child sex ratio in some regions. Prenatal sex determination is banned by law.