How do Organisms Reproduce?
Reproduction is the biological process by which an organism produces new individuals of its own kind. It is not essential for the survival of an individual, but it is essential for the continuity of a species. Reproduction involves the copying of DNA (the blueprint of body design) and the creation of new cells.
Importance of Variation
DNA copying is never perfectly accurate, so the offspring differ slightly from the parents. These differences are called variations. Variations allow a species to survive changes in the environment (niche). If a population is wiped out by a drastic change (like a rise in temperature), only the variants able to tolerate the change survive. Thus variation is the basis of evolution and survival of species. Sexual reproduction produces greater variation than asexual reproduction.
Asexual Reproduction
A single parent produces offspring without the formation or fusion of gametes. Offspring are genetically identical to the parent (clones).
- Fission: In unicellular organisms. Binary fission — parent splits into two (e.g. Amoeba, Leishmania divides in a definite plane). Multiple fission — many daughter cells form at once (e.g. Plasmodium).
- Budding: A small bud develops as an outgrowth, grows, and detaches to become a new individual (e.g. Hydra, Yeast).
- Fragmentation: The body breaks into pieces, each growing into a new organism (e.g. Spirogyra).
- Regeneration: A fully differentiated organism regrows lost body parts; specialised cells proliferate and differentiate (e.g. Planaria, Hydra). It is not the same as reproduction.
- Spore formation: Erect hyphae (sporangia) bear spores that are dispersed and germinate under favourable conditions (e.g. Rhizopus/bread mould). Spores have thick walls for protection.
- Vegetative propagation: New plants grow from vegetative parts (root, stem, leaf). Examples: roots of Dahlia, stems of potato (eyes/buds), leaves of Bryophyllum (buds on margins), runners, layering, grafting. Advantages: plants bear flowers/fruits earlier, identical to parent, allows growing of seedless plants (banana, orange, rose).
- Tissue culture: Cells from the growing tip (meristem) of a plant are grown in a nutrient medium to form a callus, which is transferred to a hormone medium to form plantlets. Used to grow disease-free plants and ornamental plants (micropropagation).
Sexual Reproduction in Flowering Plants
The flower is the reproductive organ. Parts of a flower:
- Sepals (green, protect bud), Petals (coloured, attract insects).
- Stamen — male part: anther (makes pollen grains) and filament.
- Carpel/Pistil — female part: stigma (receives pollen), style, and ovary (contains ovules with egg cells).
A flower with both stamens and carpels is bisexual (e.g. hibiscus); with only one, it is unisexual (e.g. papaya, watermelon).
Pollination: Transfer of pollen from anther to stigma. Self-pollination (same/similar flower) and cross-pollination (different flower), carried by wind, water or insects.
Fertilisation: After pollination, the pollen grain produces a pollen tube that grows through the style into the ovule. The male germ cell fuses with the egg (female gamete) to form a zygote. The zygote divides to form the embryo.
Formation of seed & fruit: The ovule develops into a seed (containing the embryo). The ovary develops into a fruit. Under suitable conditions the seed germinates into a new plant.
Human Reproductive System
Humans reproduce sexually. Sexual maturation begins at puberty, marked by secondary sexual characters (hair growth, voice change, breast development, etc.).
Male Reproductive System
- Testes: Produce sperms and the hormone testosterone; located in the scrotum outside the body cavity to keep a lower temperature suited for sperm formation.
- Vas deferens carries sperms to the urethra.
- Glands — seminal vesicles and prostate gland — add fluids; sperms + fluid = semen.
- Sperm is a single cell with a head, middle piece and a tail for movement.
Female Reproductive System
- Ovaries: Produce egg cells (ova) and hormones oestrogen and progesterone. A girl is born with all eggs already present.
- Fallopian tube (oviduct): Carries the egg to the uterus; fertilisation occurs here.
- Uterus (womb): Where the embryo develops.
- Cervix opens into the vagina.
Menstruation
Each month the uterine lining thickens with blood vessels to receive a fertilised egg. If fertilisation does not occur, the lining breaks down and is released through the vagina as blood and mucus. This monthly cycle is menstruation and lasts about 2–8 days; the cycle is roughly 28 days.
Fertilisation and Development in Humans
Sperms deposited in the vagina travel to the fallopian tube where one fuses with the egg to form a zygote. The zygote divides and gets embedded in the uterus lining (implantation). The placenta — a disc-like tissue — provides the embryo with glucose and oxygen from the mother's blood and removes wastes. The development inside the mother takes about nine months (gestation) and ends with the birth of the baby.
Reproductive Health
Reproductive health means physical and mental well-being in matters of reproduction.
Contraception (Methods to prevent pregnancy)
- Barrier methods: Condoms and diaphragms physically prevent sperm from reaching the egg; also protect against STDs.
- Chemical/Hormonal methods: Oral contraceptive pills change hormonal balance to prevent egg release.
- Intra-uterine devices (IUD/Copper-T): Placed in the uterus to prevent pregnancy.
- Surgical methods: Vasectomy (vas deferens cut/tied in males) and tubectomy (fallopian tubes blocked in females).
Contraception also helps regulate population and, in some cases, protects health. Female foeticide — determining and aborting on the basis of the sex of the unborn child — is illegal and has skewed the sex ratio.
Sexually Transmitted Diseases (STDs)
Diseases spread through sexual contact. Bacterial — gonorrhoea, syphilis; Viral — warts, HIV-AIDS. Barrier methods like condoms help prevent their transmission.