Chapter 3 — How do Organisms Reproduce? | Class 9 CBSE Biology 2026-27

Section 3.1

What is Reproduction? & DNA Copying

Reproduction is the process by which living organisms produce new individuals of their own kind. Unlike nutrition or respiration, it is not needed for an individual to survive — but it is essential for the continuity of the species.

Ensures continuity of life from one generation to the next.
Maintains the population of a species in its habitat.
Transfers genetic information (traits) from parents to offspring.
Introduces variation, helping populations survive in changing environments.

❖ DNA copying — the basic event

DNA in the chromosomes carries the instructions for making proteins, which control an organism's traits. Before a cell divides it copies its DNA, makes extra cell material, and then divides so each new cell gets one DNA copy and enough structures to work. DNA copying is very accurate but not perfect, so small errors cause variation — offspring resemble parents but are not identical.

💡 Did You Know?

A sprouting potato grows new shoots from its "eyes" without any seeds — reproduction from a single parent. The two broad types of reproduction are asexual (one parent) and sexual (two parents).

Section 3.2

Asexual Reproduction

Reproduction by a single parent without gametes. The offspring are genetically identical to the parent and have the same chromosome number. Common methods:

Binary fission: the parent's nucleus divides, then the cytoplasm splits, giving two identical daughter organisms

Method	How	Examples
Binary fission	Parent splits into two equal cells	Amoeba, Paramecium, bacteria (Leishmania splits lengthwise)
Multiple fission	Parent splits into many cells inside a cyst	Plasmodium (malaria parasite)
Fragmentation	Body breaks into pieces, each grows	Spirogyra
Regeneration	A body piece grows into a whole organism	Hydra, Planaria
Budding	A bud grows on the parent and detaches	Yeast, Hydra
Spore formation	Spores in sporangia grow into new organisms	Rhizopus, Mucor, ferns, mosses

Budding in yeast: a bulge (bud) forms, receives a nucleus by mitosis, grows, and detaches as a new yeast cell

🌱 Vegetative propagation (in plants)

New plants grow from vegetative parts (root, stem, leaf) without seeds. Natural: potato eyes & ginger (stem), onion (bulb), runners/stolons, Bryophyllum leaf-margin buds, sweet-potato roots. Artificial: stem cutting (rose, sugarcane), layering (jasmine, strawberry), grafting (mango, apple — a scion joined to a stock) and tissue culture (explant → callus → plantlet).

✓ Advantages of vegetative propagation

Plants flower and fruit earlier than from seed.
Grows seedless plants (banana, rose, jasmine).
Offspring are genetically identical to the parent.
Superior varieties produced quickly from one plant.

⚠️ Disadvantages

Needs skill/expertise; not possible for all plants.
If one plant is infected, the whole crop can be affected.
No variation, so poor adaptation to change.
Artificial methods can be expensive.

Section 3.3

Sexual Reproduction

New organisms are produced by the fusion of male and female gametes. Each gamete carries half the usual DNA, so the offspring gets a unique combination of traits from both parents.

Feature	Asexual	Sexual
Parents	One	Two
Gametes	Not formed	Formed
Fertilisation	Does not occur	Occurs
Offspring	Identical to parent	Show variation
Usually in	Lower organisms	Higher organisms

✓ Why variation matters

Because offspring differ, some are better suited to changing conditions. Sexual reproduction raises the survival rate, drives evolution and helps species adapt to new environments.

Section 3.4

Sexual Reproduction in Flowering Plants

The flower is the reproductive part. It has four whorls: sepals, petals, stamens (male) and the pistil/carpel (female).

Parts of a flower: sepals & petals (protective/attractive), stamen = anther + filament (male), pistil = stigma + style + ovary with ovules (female)

Stamen (male)

The anther makes pollen grains (with the male gametes); the filament holds it up. (The whorl is the androecium.)

Pistil/Carpel (female)

Stigma receives pollen, style is the tube, and the ovary holds ovules with the female gamete (egg). (The whorl is the gynoecium.)

💡 Did You Know?

A flower with both stamens and pistil is bisexual (rose, hibiscus); one with only stamens or only pistil is unisexual (papaya, maize, cucumber).

❖ Pollination

The transfer of pollen from the anther to the stigma. Self-pollination — within the same plant (pea, wheat). Cross-pollination — to a different plant of the same species (maize, apple), carried by wind, water, insects or animals.

After pollination, the pollen tube grows down the style carrying two male gametes; one fuses with the egg (zygote), the other with the polar nuclei (endosperm) — double fertilisation

One male gamete + egg → zygote (becomes the embryo).
Other male gamete + secondary nucleus (two polar nuclei) → endosperm (food for the embryo).
Two fusions = double fertilisation; the egg-cell fusion gives triple fusion (3n endosperm).

Section 3.4 (contd.)

From Flower to Seed & Fruit

After fertilisation the flower changes: the sepals, petals, stamens, style and stigma usually wither, while the ovary becomes the fruit and the ovule becomes the seed.

Before	After fertilisation
Ovary	Fruit (ovary wall → fruit wall/pericarp)
Ovule	Seed
Integuments	Seed coat
Zygote	Embryo (radicle + plumule)
Secondary nucleus	Endosperm (stored food)

Germination: the radicle grows down to form the root, the plumule grows up to form the shoot, using food stored in the cotyledons

🌿 Seed dispersal

Spreading seeds away from the parent reduces competition and helps colonise new areas. By wind (light/winged/hairy seeds — dandelion, maple), water (floating seeds — coconut, lotus), animals (eaten fruits, or hooked seeds clinging to fur — Xanthium) and explosion (bursting pods — pea, balsam, castor).

Section 3.5

Sexual Reproduction in Human Beings

Humans are unisexual — each person has one type of reproductive system. The systems produce gametes: sperms (male) and eggs/ova (female).

Schematic pathways: sperm travels testis → epididymis → vas deferens → urethra; in the female, the egg passes ovary → fallopian tube → uterus → vagina

♂ Male system

Testes (in the scrotum, ~2°C cooler) make sperms and the hormone testosterone; sperms mature in the epididymis, travel through the vas deferens and urethra. The seminal vesicles & prostate add fluid (semen) for energy.

♀ Female system

Ovaries release one egg a month (ovulation) and make hormones; the egg travels the fallopian tube (site of fertilisation) to the uterus (where the embryo develops), connected by the cervix to the vagina.

Gametes: the sperm is tiny and motile (head + nucleus, mitochondria-rich middle piece, tail); the ovum is large and non-motile with stored nutrients

Gametogenesis

Gametes are made by meiosis, so they are haploid (n). Spermatogenesis makes sperms in the testes (from puberty, continuously); oogenesis makes eggs in the ovaries (about one a month). At fertilisation the haploid gametes fuse to restore the diploid (2n) number.

Section 3.5 (contd.)

The Menstrual Cycle

A regular ~28-day sequence of changes in the ovary and uterus that prepares the female body for a possible pregnancy. It begins at puberty (menarche) and continues until menopause.

The four phases of a ~28-day cycle: menstruation, follicular (lining rebuilds), ovulation (egg released ~day 14) and luteal (corpus luteum maintains the lining)

Menstrual phase — if the egg was not fertilised, the thick uterine lining breaks down and is shed (3–5 days).
Follicular phase — an egg matures in a follicle while the uterine lining thickens again.
Ovulation — the mature egg is released (~day 14) into the oviduct.
Luteal phase — the empty follicle becomes the corpus luteum, which keeps the lining ready (progesterone).
If no pregnancy, the corpus luteum stops, the lining sheds, and the next cycle begins.

Section 3.5 (contd.)

Fertilisation, Implantation & Birth

In humans fertilisation is internal. Of the millions of sperms released, only one fuses with the egg in the fallopian tube to form a diploid zygote.

From fertilisation to implantation: sperm + egg → zygote → (divides into) blastocyst → implants in the uterine lining → develops into an embryo

Placenta

After implantation a disc-like tissue, the placenta, forms between the uterus wall and the embryo, connected by the umbilical cord. It supplies the foetus with nutrition and oxygen and removes wastes; amniotic fluid cushions the foetus.

Parturition

Childbirth (parturition) occurs after the gestation period (~9 months/280 days). Strong contractions of the uterus push the baby out through the vagina; the placenta is then expelled as the afterbirth.

External fertilisation	Internal fertilisation
Outside the female body	Inside the female body
Needs water	Does not need water
Many eggs produced	Few eggs produced

Section 3.6 – 3.8

Reproductive Health, Birth Control & ART

Reproductive health is complete physical, mental and social well-being relating to the reproductive system — not just reproduction, but hygiene, preventing infection and making informed choices.

🧼 Reproductive hygiene

Regular bathing and cleanliness of the external area (avoid harsh chemicals).
Menstrual hygiene: use clean sanitary pads/cloth, change regularly, dispose properly.
Clean, breathable cotton clothing changed daily.
Notice signs of infection (itching, unusual discharge, pain) and seek medical help.
A balanced diet and exercise support hormonal balance and immunity.

❖ Birth control (contraception)

Methods to prevent unwanted pregnancy, important for family planning and population control. Barrier (condoms — also reduce STIs), hormonal (oral pills — stop egg release, under medical advice), IUDs (Copper-T, placed in the uterus) and surgical/permanent (vasectomy in males, tubectomy in females).

💡 Did You Know?

Barrier methods such as condoms also help prevent sexually transmitted infections (STIs) — bacterial (gonorrhoea, syphilis) and viral (HIV/AIDS, genital warts).

🧬 Assisted Reproductive Technologies (ART)

Medical methods that help couples facing infertility (from hormonal imbalance, blocked tubes, low sperm count, infections or age). Examples: IVF (in-vitro fertilisation — fertilisation in the lab, then the embryo is placed in the uterus) and artificial insemination. Heavy metals and excess heat can harm fertility, so a healthy environment matters.

Glossary

Key Terms

Quick definitions for fast revision.

Reproduction: Producing new individuals of the same species.
DNA / Gene / Chromosome: Genetic material / a part of DNA controlling a trait / thread-like structure carrying genes.
Variation: Differences in traits among individuals of a species.
Asexual reproduction: One parent; offspring genetically identical.
Sexual reproduction: Two parents and gamete fusion; offspring show variation.
Binary / Multiple fission: Parent splits into two / into many (in a cyst).
Budding / Fragmentation / Regeneration: Bud grows into new organism / body breaks into pieces / a part regrows the whole.
Spore formation: Reproduction by protected spores (fungi, ferns).
Vegetative propagation: New plants from root/stem/leaf (cutting, layering, grafting, tissue culture).
Pollination: Transfer of pollen from anther to stigma (self or cross).
Fertilisation: Fusion of male and female gametes to form a zygote.
Double fertilisation / Triple fusion: Two fusions in plants; the second forms 3n endosperm.
Ovulation: Release of an egg from the ovary (~day 14).
Menstrual cycle: ~28-day cycle preparing the uterus for pregnancy.
Placenta: Tissue linking foetus and mother for nutrition, gas exchange and waste removal.
Contraception / ART: Methods to prevent pregnancy / methods (e.g. IVF) to help with infertility.

Exam Strategy

Top Exam Tips

High-yield reminders that catch most students out.

DNA copying = variation

Copying is accurate but not perfect; tiny errors create the variation that drives evolution.

Fission count

Binary fission → two (Amoeba); multiple fission → many (Plasmodium, in a cyst).

Match the organism

Budding = yeast/Hydra; fragmentation = Spirogyra; regeneration = Planaria; spores = Rhizopus.

Flower parts

Stamen = anther + filament (male); pistil = stigma + style + ovary (female).

Double fertilisation

Gamete 1 + egg → zygote; gamete 2 + polar nuclei → endosperm (triple fusion, 3n).

After fertilisation

Ovary → fruit, ovule → seed, zygote → embryo, secondary nucleus → endosperm.

Where fertilisation happens

In humans it's the fallopian tube (oviduct), not the uterus.

Gametes are haploid

Made by meiosis (n); fusion restores diploid (2n) in the zygote.

Self-Evaluation

Practice Question Bank — 150 Questions

CBSE-pattern questions across all five sections with toggle answers. Section A: 50 MCQ & Assertion-Reason (1 mark) · Section B: 30 Very Short Answer (2 marks) · Section C: 30 Short Answer (3 marks) · Section D: 20 Long Answer (5 marks) · Section E: 20 Case-Based (4 marks).