How do Organisms Reproduce – Class 10 Biology Chapter 7 NCERT Notes

Chapter 7: How do Organisms Reproduce

7.1 DNA Copying and Variation

The process of reproduction begins with DNA replication in the cell. While the copying is mostly accurate, small variations (errors) occur. These variations are important because they provide the raw material for evolution and adaptation.

7.2 Asexual Reproduction

In asexual reproduction, a single parent produces offspring without the fusion of gametes. The offspring are genetically identical to the parent (clones).

Types of Asexual Reproduction

• Fission:
  • Binary fission: The parent cell divides into two equal halves. Example: Amoeba, bacteria.
  • Multiple fission: The parent cell divides into many daughter cells simultaneously. Example: Plasmodium (malaria parasite).
• Budding: A small outgrowth (bud) develops on the parent body, grows, and eventually detaches as a new organism. Example: Hydra, yeast.
• Spore Formation: Organisms produce spore-bearing structures. Spores are released, and each can germinate into a new individual under favourable conditions. Example: Rhizopus (bread mould), ferns.
• Regeneration: The ability to regrow a complete organism from a body fragment. Example: Planaria, Hydra. (Note: Regeneration is not the same as reproduction — it is a repair mechanism that can result in new organisms.)
• Fragmentation: The parent body breaks into two or more fragments, each growing into a new organism. Example: Spirogyra (filamentous algae).
• Vegetative Propagation: New plants grow from vegetative parts (roots, stems, leaves) without seeds. Examples: potato (tuber), bryophyllum (leaf buds), rose (stem cutting), jasmine (layering).

7.3 Sexual Reproduction

In sexual reproduction, two parents contribute specialised cells called gametes. The male gamete (sperm) fuses with the female gamete (ovum/egg) in a process called fertilisation, producing a zygote that develops into a new organism.

Sexual reproduction introduces greater genetic variation because the offspring receives DNA from two different parents.

7.4 Sexual Reproduction in Flowering Plants

The flower is the reproductive organ of a plant. A typical flower has four whorls:

• Sepals (Calyx): Green, outermost; protect the flower bud.
• Petals (Corolla): Colourful; attract pollinators.
• Stamens (Androecium): Male reproductive part. Each stamen has an anther (produces pollen grains) and a filament.
• Pistil/Carpel (Gynoecium): Female reproductive part. Consists of stigma (receives pollen), style (connects stigma to ovary), and ovary (contains ovules).

Pollination

• Self-pollination: Pollen from the anther of a flower reaches the stigma of the same flower or another flower on the same plant.
• Cross-pollination: Pollen from one plant reaches the stigma of a flower on a different plant of the same species. Agents: wind, water, insects, birds.

Fertilisation and Seed Formation

1. Pollen grain lands on the stigma and germinates, forming a pollen tube that grows down through the style.
2. The male gamete travels through the pollen tube and fuses with the egg cell in the ovule — this is fertilisation.
3. The fertilised egg (zygote) develops into an embryo.
4. The ovule develops into a seed (containing the embryo).
5. The ovary develops into a fruit (enclosing the seeds).

7.5 Human Reproductive System

Male Reproductive System

• Testes: Primary reproductive organs; produce sperm and the hormone testosterone. Located outside the body in the scrotum (sperm production requires a temperature 2-3°C lower than body temperature).
• Vas deferens: Tube that carries sperm from testes to the urethra.
• Seminal vesicles and Prostate gland: Secrete fluids that nourish sperm and form semen.
• Urethra: Common passage for urine and semen (not simultaneously).

Female Reproductive System

• Ovaries: Primary reproductive organs; produce eggs (ova) and hormones estrogen and progesterone. One egg is released every month (ovulation).
• Fallopian tubes (Oviducts): Carry the egg from the ovary to the uterus. Fertilisation usually occurs here.
• Uterus (Womb): The fertilised egg implants here and develops during pregnancy. Has a thick, blood vessel-rich lining.

Menstrual Cycle

If the egg is not fertilised, the thickened uterine lining breaks down and is expelled along with blood through the vagina. This is called menstruation and occurs roughly every 28 days.