Reproduction is the biological process by which organisms produce offspring of their own kind, ensuring continuity of life. It is not essential for individual survival but is essential for the survival of the species. Reproduction involves creating copies of DNA. Minor variations occur during reproduction due to DNA copying errors or sexual recombination — this variation forms the basis of evolution and adaptation. There are two main types: asexual reproduction (single parent, identical offspring) and sexual reproduction (two parents, varied offspring).

In asexual reproduction, a single parent produces offspring without fusion of gametes. Offspring are genetically identical (clones). Types: Fission — Binary fission: organism splits into two (Amoeba, Bacteria). Multiple fission: one organism splits into many (Plasmodium). Budding: outgrowth (bud) forms and detaches (Hydra, Yeast). Spore formation (sporulation): spores spread and germinate (Rhizopus, Mucor). Fragmentation: organism breaks into pieces, each piece grows into new organism (Spirogyra). Vegetative propagation: reproduction from vegetative parts — stem (potato tubers, ginger rhizomes), leaf (Bryophyllum), root (sweet potato). Tissue culture: growing plants from cells in laboratory.

Flowers are the reproductive organs of angiosperms. Parts of a flower: Stamen (male) — anther (produces pollen) + filament. Carpel/Pistil (female) — stigma (receives pollen) + style + ovary (contains ovules). Pollination: transfer of pollen from anther to stigma. Self-pollination: within same flower or plant. Cross-pollination: between different plants (by wind, insects, water, birds). Fertilisation: pollen germinates on stigma → pollen tube grows through style → male gamete fuses with female gamete in ovule (syngamy) → zygote forms. Zygote → embryo; ovule → seed; ovary → fruit. Double fertilisation: unique to angiosperms — one sperm fertilises egg (zygote), another fuses with polar nuclei (endosperm, provides nutrition).

Male reproductive system: Testes (in scrotum, below body temperature for sperm production) — produce sperm and testosterone. Vas deferens carries sperm to urethra. Seminal vesicles, prostate gland, and Cowper's gland provide fluids that form semen. Female reproductive system: Ovaries — produce eggs (ova) and hormones (oestrogen, progesterone). Fallopian tubes (oviducts) — carry egg from ovary to uterus; fertilisation occurs here. Uterus — where embryo develops; lining (endometrium) thickens each month. Cervix — lower part of uterus. Vagina — birth canal. Menstruation: if fertilisation doesn't occur, uterine lining breaks down and is shed (~28-day cycle).

Fertilisation: sperm fuses with egg in the fallopian tube to form a zygote (diploid). Zygote undergoes repeated mitotic divisions (cleavage) to form an embryo. Implantation: embryo attaches to uterine wall. Placenta: organ connecting embryo/foetus to mother's uterus — provides nutrition, oxygen, removes waste. Foetal development takes ~9 months (gestation period). Birth (parturition) occurs when the baby is ready. Identical twins: one zygote splits into two. Fraternal twins: two separate eggs fertilised by two sperm. Sexually transmitted infections (STIs): HIV/AIDS, gonorrhoea, syphilis — can be prevented using condoms.

Contraception prevents unwanted pregnancies and helps in family planning. Methods: Barrier methods — condoms, diaphragm (prevent sperm from reaching egg; also protect against STIs). Hormonal methods — oral contraceptive pills (prevent ovulation). Intrauterine devices (IUDs/Copper-T) — prevent implantation. Surgical methods — vasectomy (male), tubectomy (female) — permanent sterilisation. Emergency contraception — taken within 72 hours after unprotected intercourse. Reproductive health includes awareness about puberty, menstrual health, safe sex, and avoiding early marriage. Abortion (MTP — Medical Termination of Pregnancy) is legal in India under specific conditions.

Fission is the division of a single-celled organism into two or more daughter cells. Binary fission: the cell divides into two equal halves; the nucleus divides first (Amoeba divides in any plane; Leishmania has a whip-like flagellum at one end, so division occurs in a definite orientation). Multiple fission: a single cell divides into many daughter cells at the same time, as in Plasmodium (the malarial parasite); under unfavourable conditions Amoeba forms a protective cyst and later divides by multiple fission when conditions improve. These are asexual methods, with one parent and offspring genetically similar.

Fragmentation: a multicellular organism with simple body organisation breaks into smaller pieces on maturing, and each piece grows into a new individual (Spirogyra). Regeneration: if an organism is cut or broken into pieces, each piece grows into a complete organism (Planaria, Hydra), using specialised cells that proliferate and form tissues. Regeneration is not the same as reproduction in complex animals. Budding: a bud develops as an outgrowth due to repeated cell division at one site, becomes a small individual and detaches when mature (Hydra, yeast).

Spore formation: in Rhizopus (bread mould), thread-like hyphae bear blobs called sporangia that contain spores. Spores are covered by a thick protective wall and survive unfavourable conditions; on germination in moist, warm conditions they grow into new individuals. Vegetative propagation: new plants arise from vegetative parts such as roots, stems and leaves. Examples: bryophyllum (leaf buds), potato (eyes on tuber), sugarcane, rose and money plant (stem cutting), layering (jasmine), grafting (mango). Advantages: plants bear flowers and fruits earlier, and genetically similar plants are formed. Tissue culture grows plants from tissue in a nutrient medium.

Reproduction involves making copies of the DNA (the blueprint) in the cell nucleus along with the creation of additional cellular machinery. The DNA copying mechanism is not perfectly accurate, so slight variations arise. These variations are the basis of evolution; they may help a species survive in a changing environment (e.g. a heat-tolerant bacterium survives a heat wave). Asexual reproduction yields minor variation; sexual reproduction (combining DNA from two parents) yields much greater variation. Variation therefore ensures the survival of some individuals of a species.

A typical flower has four whorls: sepals (green, protect the bud), petals (coloured, attract insects), stamens (male: anther producing pollen grains and filament) and carpel or pistil (female: stigma which receives pollen, style, and ovary containing ovules; each ovule has an egg cell). Unisexual flowers have either stamens or carpels (papaya, watermelon); bisexual flowers have both (hibiscus, mustard). Pollen grains contain the male gametes.

Pollination is the transfer of pollen from the anther to the stigma. Self-pollination: in the same flower or on the same plant; cross-pollination: between flowers of different plants by wind, water, insects or birds. After landing on the stigma, the pollen grain grows a pollen tube through the style to the ovary, carrying the male gamete to the egg in the ovule, where fertilisation occurs to form the zygote. The zygote divides to form the embryo. After fertilisation, the ovule becomes the seed, the ovary becomes the fruit, and the other floral parts wither and fall.

Male: a pair of testes lie in the scrotum outside the abdominal cavity because sperm formation needs a lower temperature than the body; they make sperms and the hormone testosterone. Sperms pass through the vas deferens, where the secretions of the prostate gland and seminal vesicles are added, to the urethra and out via the penis. Female: a pair of ovaries produce eggs (ova) and hormones (oestrogen, progesterone); the egg travels through the fallopian tube (oviduct) where fertilisation occurs; the fertilised egg implants in the uterus; the vagina is the birth canal and the receiving passage.

From puberty, one egg is released from an ovary every month and the uterus lining thickens to receive the embryo. If fertilisation does not occur, the lining breaks and is shed through the vagina as blood and mucus: menstruation, lasting 2–8 days. If fertilisation occurs, the zygote divides to form an embryo that implants in the uterine wall. The placenta, a disc-like tissue embedded in the uterine wall, provides nutrients and oxygen from the mother's blood to the embryo and removes waste. Pregnancy lasts about nine months; birth occurs by rhythmic contractions of uterine muscles.

Methods of contraception: physical barriers (condoms, diaphragm, cervical caps) also protect against infections; chemical methods (oral pills, which alter hormonal balance; vaginal pills; copper-T/IUCD placed in the uterus); surgical methods (vasectomy in males, tubectomy in females). Sexually transmitted diseases (gonorrhoea, syphilis, warts, HIV-AIDS) spread by sexual contact. Female foeticide (sex-selective abortion) is illegal and lowers the sex ratio. Summary: asexual reproduction gives rise to genetically similar offspring; sexual reproduction increases variation and survival chances.