Class 12 Biology Notes: Reproduction in Organisms

1. Asexual Reproduction

• Definition: A mode of reproduction involving a single parent, producing genetically identical offspring (clones).
• Occurrence: Common in unicellular organisms (e.g., Amoeba, Paramecium) and some plants.
• Methods:
  • Binary Fission: Cell divides into two identical halves (e.g., Amoeba).
  • Budding: Unequal cell division forms buds (e.g., yeast, Hydra).
  • Spore Formation: Zoospores (algae), conidia (Penicillium).
  • Vegetative Propagation:
    • Natural: Runners (grass), rhizomes (ginger), tubers (potato), bulbs (onion).
    • Artificial: Cutting, grafting (used in horticulture).

2. Sexual Reproduction

• Definition: Involves fusion of male and female gametes, leading to genetic variation.
• Phases:
  • Juvenile Phase: Non-reproductive growth phase.
  • Reproductive Phase: Maturity for reproduction.
• Cycles:
  • Oestrus Cycle: Non-primates (e.g., cows, dogs).
  • Menstrual Cycle: Primates (humans, apes).
• Breeding Types:
  • Continuous Breeders: Reproduce throughout the year (e.g., humans).
  • Seasonal Breeders: Reproduce in favorable seasons (e.g., frogs).

Key Point: Sexual reproduction enhances biodiversity due to genetic recombination.

1. Gametogenesis: Formation of haploid gametes.
   • Isogametes: Similar gametes (algae).
   • Heterogametes: Distinct male (sperm) and female (egg) gametes.
   • Cell Division:
     • Haploid parents: Mitosis.
     • Diploid parents: Meiosis (via meiocytes).
2. Gamete Transfer:
   • In Plants: Pollination (pollen grain → stigma).
   • In Animals: Male gametes are motile; external (water) or internal (terrestrial) transfer.

4. Fertilisation (Syngamy)

• Definition: Fusion of gametes to form a diploid zygote.
• Types:
  • External Fertilisation: In water (e.g., fish, frogs).
  • Internal Fertilisation: Inside female body (e.g., mammals).
• Parthenogenesis: Zygote forms without fertilisation (e.g., honeybees, rotifers).

Key Point: External fertilisation risks high offspring mortality due to predators.

5. Post-Fertilisation Events

1. Zygote Formation:
   • Diploid zygote develops into an embryo.
   • In oviparous animals, zygote forms a hard shell (e.g., birds).
2. Embryogenesis:
   • Oviparous: Egg-laying (e.g., reptiles).
   • Viviparous: Live birth (e.g., mammals).
   • In Plants:
     • Zygote → Embryo → Seed → Fruit (dispersal).