How do Organisms Reproduce?

REPRODUCTION is essentially the creation of a DNA COPY. DNA in the chromosomes carries the information for building the body. Reproduction is NOT needed to keep an individual alive — it is needed for the species to continue.

DNA copying is never perfectly accurate, so VARIATIONS arise. If a population is uniform and the environment changes drastically (a temperature rise, a new disease), the whole population may be wiped out. Variation means a few individuals may survive — so variation is the BASIS OF EVOLUTION.

ASEXUAL reproduction = ONE parent, no germ cells (gametes), offspring are genetically almost identical (clones). SEXUAL reproduction = TWO parents, DNA from both is combined, so there is much MORE variation.

WHY THE SEXUAL MODE WORKS (NCERT 7.3.1 — heavily examined). If BOTH parents passed on a FULL set of DNA, the amount of DNA would DOUBLE in every generation. The solution: GERM CELLS (gametes), made in specialised reproductive organs, carry only HALF the number of chromosomes and HALF the DNA of an ordinary body cell. When the male and female gametes FUSE, the normal number is RESTORED in the zygote. This is why 'gametes have half the number of chromosomes' — a standard 1–2 mark question.

THE TWO GAMETES ARE NOT ALIKE — a division of labour. The MALE gamete is SMALL and MOTILE (it must travel). The FEMALE gamete is LARGER and STORES FOOD for the developing embryo. This is true both in flowering plants (pollen vs ovule) and in humans (sperm vs egg).

FISSION (single-celled organisms). BINARY FISSION — the parent splits into TWO. In AMOEBA the plane of division can be anywhere (no fixed shape); in LEISHMANIA (causes kala-azar) it happens in a DEFINITE plane with respect to its whip-like structure. MULTIPLE FISSION — the parent splits into MANY daughter cells at once, as in PLASMODIUM (the malarial parasite).

FRAGMENTATION — a simple multicellular organism breaks into pieces and each piece grows into a new individual. Example: SPIROGYRA. This works only in organisms that are simple, loose masses of cells.

REGENERATION — an organism cut into pieces regrows each piece into a complete organism (PLANARIA, HYDRA). Specialised cells divide, make a mass of cells, and that mass then develops into different tissues and organs. IMPORTANT: regeneration is NOT the same as reproduction, because most such organisms would not normally depend on being cut up to multiply.

BUDDING — a BUD develops as an outgrowth on the parent, grows, and then detaches to live independently. Examples: HYDRA and YEAST.

SPORE FORMATION — RHIZOPUS (bread mould) grows thread-like hyphae bearing knob-like SPORANGIA, which contain SPORES. Each spore has a THICK WALL that protects it until it lands in a moist place, where it germinates.

VEGETATIVE PROPAGATION — new plants grow from the ROOTS, STEMS or LEAVES of the parent. BRYOPHYLLUM grows buds in the NOTCHES of its leaf margin. Sugarcane, rose, grapes and jasmine are grown this way, by LAYERING, GRAFTING and TISSUE CULTURE.

THREE ADVANTAGES of vegetative propagation: (1) plants that have LOST the ability to produce viable seeds can still be grown; (2) the new plants reach FLOWERING and FRUITING EARLIER than seed-grown plants; (3) every plant produced is GENETICALLY IDENTICAL to the parent — a CLONE.

TISSUE CULTURE — new plants are grown from the growing tip (or a few cells) of a plant on a nutrient medium; the mass of cells formed is called a CALLUS, which is then moved on to a hormone medium to form plantlets. Used for ornamental plants.

PARTS OF A FLOWER, from outside in: SEPALS (green, protect the bud) → PETALS (usually coloured, attract insects) → STAMENS (male) → PISTIL/CARPEL (female).

STAMEN = the MALE reproductive part = ANTHER (produces the POLLEN GRAINS, which contain the male germ cells) + FILAMENT (the stalk).

PISTIL (carpel) = the FEMALE reproductive part, with THREE regions: STIGMA at the top (receives the pollen), STYLE in the middle (a tube), and OVARY at the base, which contains the OVULES. Each ovule contains the female germ cell (the egg).

BISEXUAL flower = has BOTH stamens and pistil (hibiscus, mustard). UNISEXUAL flower = has only ONE of them (papaya, watermelon).

POLLINATION = transfer of pollen from an anther to a stigma. SELF-pollination — onto the same flower, or another flower of the SAME plant. CROSS-pollination — onto a flower of a DIFFERENT plant of the same kind, carried by AGENTS: wind, water, insects and birds.

FERTILISATION in a flower: the pollen grain on the stigma grows a POLLEN TUBE down through the style to an ovule in the ovary. The MALE germ cell travels down the tube and FUSES with the FEMALE germ cell (egg) → a ZYGOTE is formed.

AFTER FERTILISATION (learn as a set): the ZYGOTE develops into the EMBRYO; the OVULE becomes the SEED; the OVARY becomes the FRUIT; and the petals, sepals, stamens and style usually dry up and fall off. GERMINATION follows when the seed gets water, air and a suitable temperature.

PUBERTY is when the reproductive system becomes functional. Common changes: growth of hair in new places (armpits, genital area), thinner hair on arms and legs, oilier skin and pimples. In boys the voice deepens and the chest broadens; in girls the breasts develop and menstruation begins.

MALE REPRODUCTIVE SYSTEM: TESTES (in the scrotum) → VAS DEFERENS → joins the URETHRA → penis. The TESTES make (a) the SPERMS and (b) the male hormone TESTOSTERONE, which controls the changes at puberty.

WHY THE TESTES LIE OUTSIDE THE BODY: they are held in the SCROTUM, a pouch outside the abdominal cavity, because sperm formation needs a temperature LOWER than the normal body temperature. (A very common 1-mark question.)

The SEMINAL VESICLES and the PROSTATE GLAND add their secretions to the sperms as they pass through the vas deferens. This fluid NOURISHES the sperms and makes their transport easier. Sperms are small, mostly a genetic-material head with a long tail that helps them swim.

FEMALE REPRODUCTIVE SYSTEM: a pair of OVARIES → FALLOPIAN TUBES (oviducts) → UTERUS (womb) → CERVIX → VAGINA. The OVARIES produce the EGG CELLS and the female hormones OESTROGEN and PROGESTERONE.

A girl is BORN with all the egg cells she will ever have — thousands of immature eggs are already present at birth. From puberty, ONE egg matures roughly every MONTH, released by one of the ovaries.

FERTILISATION IN HUMANS takes place in the FALLOPIAN TUBE (oviduct), not in the uterus. The zygote then STARTS DIVIDING as it travels down, forming a ball of cells — the EMBRYO. It is the EMBRYO (not the zygote) that embeds itself in the lining of the uterus, and that embedding is called IMPLANTATION.

The PLACENTA is the special tissue that connects the embryo to the mother's uterus wall. It is a disc embedded in the uterine wall; it has VILLI on the embryo's side, which give a LARGE SURFACE AREA. Through it GLUCOSE and OXYGEN pass from mother to embryo, and the embryo's WASTES pass back into the mother's blood. Development takes about NINE MONTHS.

MENSTRUATION: if the egg is NOT fertilised, the thick, spongy uterus lining (prepared to receive an embryo) is no longer needed, so it breaks down and is shed through the vagina as blood and mucus. It lasts about 2–8 days and repeats in a cycle of roughly ONE MONTH.

SEXUALLY TRANSMITTED DISEASES (STDs) spread through sexual contact. BACTERIAL: gonorrhoea, syphilis. VIRAL: warts, HIV-AIDS. Using a CONDOM prevents pregnancy AND checks the spread of these infections to a large extent (NCERT's own wording — barrier methods do not block skin-contact infections such as warts completely).

CONTRACEPTION — FOUR types. (1) BARRIER: condoms — physically prevent the sperm reaching the egg. (2) HORMONAL/ORAL PILLS: change the hormonal balance so eggs are not released (may have side effects). (3) IUCD (copper-T / loop): a device placed in the uterus. (4) SURGICAL: VASECTOMY — the vas deferens is blocked in males; TUBECTOMY — the fallopian tube is blocked in females.

Determining the SEX of an unborn child before birth (prenatal sex determination) is PROHIBITED BY LAW in India, because its misuse — FEMALE FOETICIDE — has badly disturbed the SEX RATIO (the number of girls per 1000 boys).

REPRODUCTIVE HEALTH means physical, mental and social well-being in all matters relating to reproduction — not merely the absence of disease. Family planning and safe sexual practices are part of it.

Why reproduce at all

DNA copy → offspring; copying errors → VARIATION → survival of the species

Reproduction is not needed for an individual to live, only for the species to continue.

Asexual vs sexual

Asexual: 1 parent, clones, little variation. Sexual: 2 parents, DNA combined, MUCH more variation

This contrast is the answer to 'Why is variation beneficial to the species?'

Modes of asexual reproduction

fission • fragmentation • regeneration • budding • spore formation • vegetative propagation

Learn one named example for each — that is how the question is asked.

Named examples

Amoeba & Leishmania (binary) • Plasmodium (multiple) • Spirogyra • Planaria • Hydra & yeast • Rhizopus • Bryophyllum

Leishmania divides in a DEFINITE plane; Amoeba in any plane.

Parts of a flower

Sepal → Petal → Stamen (anther + filament) → Pistil (stigma + style + ovary → ovules)

Stamen = male, Pistil = female. Bisexual has both; unisexual has one.

Fertilisation in plants

pollen on stigma → pollen tube down the style → male germ cell + egg in the ovule = ZYGOTE

Pollination is only the TRANSFER of pollen; fertilisation is the FUSION of germ cells.

After fertilisation

zygote → embryo; ovule → SEED; ovary → FRUIT

Petals, sepals, stamens and style dry up and fall off. Guaranteed marks — learn as a set.

Male reproductive path

Testes (in scrotum) → vas deferens → urethra → penis

Testes make sperms + testosterone. Seminal vesicles and prostate add nourishing fluid.

Female reproductive path

Ovary → fallopian tube (oviduct) → uterus → cervix → vagina

Ovaries make eggs + oestrogen and progesterone. FERTILISATION happens in the oviduct.

Placenta

villi (embryo's side) → large surface → glucose + O₂ in, wastes out; ~9 months

The disc embedded in the uterine wall that connects embryo to mother.

Contraception — four methods

Barrier (condom) • Hormonal (oral pills) • IUCD (copper-T) • Surgical (vasectomy / tubectomy)

Vasectomy = vas deferens blocked (male); tubectomy = fallopian tube blocked (female).