How do Organisms Reproduce?

No individual organism is immortal, yet species continue for millions of years. This is possible only because organisms reproduce — they make new individuals of the same kind. The real chemical event inside a cell is the copying of DNA, the blueprint that carries information for body design. When a cell divides, it first makes a copy of its DNA so that each new cell gets a complete set.

DNA copying is never perfectly accurate. Small differences, called variations, creep in. If the environment changes drastically (heat, water shortage, disease), some variants may survive better and the species is not wiped out. So variation is useful for the survival of a species over time, even if a particular variation is not immediately useful to an individual.

In asexual reproduction, a single parent produces offspring that are genetically almost identical to it. Its main modes are:

• Fission: a unicellular organism splits into two (binary fission, e.g. Amoeba, Leishmania which divides along a definite plane, bacteria) or into many (multiple fission, e.g. Plasmodium).
• Budding: a small outgrowth or bud develops on the parent, grows, and detaches, e.g. Hydra and yeast.
• Fragmentation: a simple multicellular body breaks into fragments, each growing into a new individual, e.g. Spirogyra.
• Regeneration: fully differentiated organisms can grow lost body parts or a whole body from a piece, e.g. Planaria, Hydra. This is not the same as normal reproduction.
• Spore formation: the parent body produces hundreds of tiny spores in a sporangium; each spore can germinate into a new organism, e.g. Rhizopus (bread mould).
• Vegetative propagation: new plants arise from vegetative parts — roots, stems or leaves — e.g. buds on potato eyes, leaves of Bryophyllum, runners, and layering/grafting/tissue culture used in agriculture.

A concrete example: the leaf of Bryophyllum that falls on moist soil grows tiny plantlets from notches on its margin — one leaf can give many identical plants without any seed.

Sexual reproduction involves two parents and the fusion of male and female gametes. Because DNA from two individuals combines, offspring show greater variation than in asexual reproduction. In flowering plants the reproductive organ is the flower.

A typical flower has four whorls attached to the receptacle:

• Sepals (calyx) — usually green, protect the bud.
• Petals (corolla) — often coloured, attract insects for pollination.
• Stamens — the male part; each has an anther (makes pollen grains) on a filament.
• Carpel/Pistil — the female part; made of stigma (receives pollen), style (a tube), and ovary (contains ovules; each ovule has an egg cell).

A flower with only stamens or only carpels is unisexual (e.g. papaya, watermelon); one having both is bisexual (e.g. Hibiscus, mustard).

Pollination is the transfer of pollen from an anther to a stigma, carried out by wind, water or insects. If pollen lands on the stigma of the same flower it is self-pollination; if on a different flower of the same kind it is cross-pollination.

Fertilisation: after pollination the pollen grain produces a pollen tube that grows down through the style into the ovary and reaches the ovule. The male gamete travels down this tube and fuses with the egg (female gamete) in the ovule. This fusion of gametes forms the zygote.

After fertilisation: the zygote divides to form the embryo inside the ovule; the ovule develops into the seed, and the ovary grows into the fruit. Sepals, petals, stamens and stigma usually dry and fall off. When a seed gets suitable water, oxygen and temperature, it germinates — the embryo grows into a new seedling.

Humans reproduce sexually and become capable of reproduction at puberty, when hormones bring about changes in the body. In males the hormone testosterone (secreted by the testes) drives changes such as growth of facial hair, deepening of voice and enlargement of reproductive organs.

The main parts of the male reproductive system and their functions are:

• Testes (a pair): produce the male gametes called sperms and secrete testosterone. They lie in a small muscular pouch, the scrotum, outside the abdominal cavity. This keeps them about 1-3 degrees Celsius cooler than the body, a temperature needed for sperm formation.
• Vas deferens (sperm duct): carries sperms from the testes towards the urethra.
• Associated glands — the seminal vesicles and prostate gland — add their secretions. The fluid provides nutrition to the sperms and makes their transport easier. Sperms plus this fluid together form semen.
• Urethra: a common passage inside the penis for both semen and urine.
• Penis: delivers semen into the female body.

A sperm is a tiny cell with a genetic-material-carrying head, a middle piece for energy, and a long tail that helps it swim towards the egg. Its main job is to carry the male's DNA to the egg.

In females the reproductive hormones oestrogen and progesterone control changes at puberty and the reproductive cycle. The main organs are:

• Ovaries (a pair): present in the abdomen; they contain thousands of immature eggs (ova) from birth and also secrete female hormones. On reaching puberty, usually one egg matures and is released roughly every 28 days (ovulation).
• Oviducts / Fallopian tubes: carry the egg from the ovary to the uterus. Fertilisation normally occurs here.
• Uterus (womb): a bag-like muscular organ where the baby develops.
• Vagina: receives sperms and forms the birth canal.

Fertilisation: during mating, sperms deposited in the vagina swim up and one sperm fuses with the egg in the oviduct to form a single-celled zygote. The zygote divides repeatedly to form a ball of cells (embryo) that moves to the uterus and gets embedded in its thick lining — this is implantation.

Development: the embryo gets nutrition from the mother's blood through a special tissue, the placenta, which is embedded in the uterine wall. It contains villi that give a large surface area; glucose, oxygen and nutrients pass from mother to embryo, and wastes pass back. The developing child (now called a foetus) grows for about nine months and is then born by the mother's muscles pushing it out.

Menstruation: if the egg is not fertilised, it lives about a day. Meanwhile the uterus lining had thickened (with blood and tissue) to receive an embryo. When no fertilisation occurs, this lining breaks down and is discharged through the vagina as blood and mucus. This monthly bleeding is menstruation, lasting about 2-8 days, with a cycle of roughly 28 days.

Reproductive health is the state of complete physical, mental and social well-being in all matters relating to the reproductive system. It includes avoiding sexually transmitted infections (STIs), planning families, and preventing unwanted pregnancies. Some STIs are bacterial (e.g. syphilis, gonorrhoea) and some are viral (e.g. warts, HIV-AIDS).

Contraception means deliberately preventing pregnancy. The main methods are:

• Barrier methods: physically stop sperm from reaching the egg, e.g. condoms and diaphragms. Condoms also help prevent the spread of STIs including HIV.
• Chemical / hormonal methods: oral contraceptive pills change the hormonal balance so that eggs are not released; but they may cause side effects.
• Intra-Uterine Contraceptive Devices (IUCD): loop or Copper-T placed in the uterus by a doctor to prevent pregnancy.
• Surgical methods: in males the vas deferens is blocked (vasectomy) to stop sperm transfer; in females the fallopian tubes are blocked (tubectomy) to stop the egg reaching the uterus. These are more permanent.

Contraception is also linked to keeping the population in check so that resources are shared for good health of all. A concrete example: using a condom during intercourse both prevents fertilisation and reduces the risk of transmitting HIV — one method with two benefits.

Sex determination: a note on trust and equality — the sex of a child is decided by the father's chromosomes, not the mother's. All eggs carry an X chromosome; sperms carry either X or Y. A child that gets XX is a girl, and one that gets XY is a boy. Therefore blaming a woman for the child's sex is scientifically wrong, and prenatal sex determination is banned by law to prevent female foeticide and to maintain a healthy sex ratio.