How do Organisms Reproduce? — Notes

Reproduction is not needed for an individual to stay alive, but it is essential for the survival of the species.

The DNA in the nucleus carries the information for making proteins, which build the body design. The basic event in reproduction is the creation of a DNA copy, together with additional cellular apparatus; the cell then divides so that each new cell has a copy.

DNA copying is very accurate but not perfect, so offspring are similar but not identical: this produces variation. Consistency in DNA copying keeps the body design suited to the organism's niche. Variation is useful for the species: if the niche changes drastically (for example, water temperature rises), a few individuals with a suitable variation may survive and reproduce, so the species is not wiped out.

Asexual reproduction needs a single parent and no germ cells; the offspring are nearly identical to the parent.

• Binary fission: one cell splits into two equal halves. Amoeba can divide in any plane; Leishmania (kala-azar) divides in a definite plane related to the whip-like structure at one end.
• Multiple fission: one cell divides into many daughter cells at once, often inside a protective cyst — Plasmodium (malaria parasite).
• Fragmentation: a simple multicellular organism such as Spirogyra breaks into pieces on maturing; each piece grows into a new individual.
• Regeneration: specialised cells of Planaria and Hydra can rebuild a whole organism from a cut piece. It is not the usual way of reproducing, because most organisms do not depend on being cut up.
• Budding: an outgrowth (bud) forms at one site by repeated cell division, grows into a small individual and detaches — Hydra, yeast.

Spore formation: bread mould (Rhizopus) has thread-like hyphae and round sporangia on stalks. Sporangia contain spores with thick protective walls; when a spore lands on a moist surface with food, it germinates into a new mould.

Vegetative propagation: new plants develop from roots, stems or leaves. Bryophyllum produces buds in the notches of its leaf margin; a potato tuber (a stem) sprouts from its eyes. People use cuttings (sugarcane, rose), layering (jasmine) and grafting (mango, grapes).

Advantages: plants flower and fruit earlier than plants grown from seed; plants that have lost the capacity to produce seeds, such as banana, rose and jasmine, can be multiplied; all the new plants are genetically similar to the parent, so good qualities are kept.

Sexual reproduction combines DNA from two individuals, so it produces more variation than asexual reproduction. Germ cells (gametes) carry half the number of chromosomes and half the DNA of body cells, so that when a male and a female germ cell fuse, the zygote has the normal amount and the number stays constant from generation to generation.

The flower is the reproductive part of a flowering plant: sepals, petals, stamens and a pistil.

• Stamen (male): anther on a filament; the anther makes pollen grains.
• Pistil (female): stigma (sticky top), style (long middle) and ovary (swollen base) containing ovules; each ovule has an egg cell.
• Bisexual flowers have both stamens and pistil (hibiscus, mustard); unisexual flowers have only one of them (papaya, watermelon).

Pollination is the transfer of pollen from an anther to a stigma: self-pollination (same flower or plant) or cross-pollination (another plant), by wind, water, insects or birds.

Fertilisation: a pollen grain on the stigma grows a pollen tube through the style to the ovule; the male germ cell passes down it and fuses with the female germ cell (egg), forming a zygote.

After fertilisation: the zygote divides many times to form an embryo within the ovule; the ovule develops a tough coat and becomes the seed; the ovary grows and ripens into the fruit; petals, sepals, stamens, style and stigma usually shrivel and fall off.

Germination: under suitable conditions the embryo grows into a seedling: the plumule forms the shoot, the radicle forms the root, and the cotyledons supply stored food.

Puberty: the reproductive organs mature and germ cells begin to be made. Changes in both sexes include thick hair in the armpits and genital area, thinner hair on the face, arms and legs, and oilier skin. Girls: breasts develop, the skin of the nipples darkens and menstruation begins. Boys: facial hair grows and the voice cracks and deepens.

Male system: the testes make sperms and testosterone; they lie in the scrotum, outside the abdomen, because sperm formation needs a lower temperature than body temperature. Sperms pass along the vas deferens, which joins the urethra from the urinary bladder; the urethra is a common passage for urine and sperms. The seminal vesicles and prostate gland add fluid, so sperms travel in semen. A sperm is mostly genetic material with a long tail.

Female system: the ovaries contain thousands of immature eggs from birth; after puberty one egg is usually released each month by one ovary. They also make oestrogen. The oviduct (fallopian tube) carries the egg towards the uterus, an elastic, bag-like organ, which opens into the vagina through the cervix.

Sperms deposited in the vagina travel through the cervix and uterus into the oviduct, where fertilisation usually happens. The zygote divides as it moves to the uterus and implants in the thick, blood-rich lining.

The placenta is a disc embedded in the uterine wall. On the embryo’s side it has villi; on the mother’s side are blood spaces around the villi. This gives a large surface area for glucose and oxygen to pass from mother to embryo and for wastes to pass from embryo to mother. The embryo is joined to the placenta by the umbilical cord. Development takes about nine months; the baby is born through rhythmic contractions of the uterus muscles.

If the egg is not fertilised (it lives about one day), the thickened lining is not needed; it breaks down and passes out through the vagina as blood and mucus: menstruation, roughly every month, lasting about two to eight days.

Sexually transmitted diseases (STDs): bacterial — gonorrhoea, syphilis; viral — warts, HIV-AIDS. Condoms reduce their spread; HIV also spreads through infected blood, shared needles and from mother to baby.

Contraception:

Surgery can also end a pregnancy; misusing it after illegal prenatal sex determination leads to female foeticide, which is a crime in India and lowers the child sex ratio. Spacing and limiting births protects the health of the mother and helps families care for their children.