Chapter 1 Biology

Chapter 1 Reproduction

By Aarish Sir

1. Introduction

• All organisms have a limited lifespan (time between birth and natural death).

• Death of individuals does not mean end of species → Reproduction ensures continuity of life.

• It is a fundamental feature of all living organisms.

• Main purpose of reproduction:

  1. Continuity of species.

  2. Transfer of genetic information from one generation to next.

  3. Variations → important for adaptation & evolution.

2. Lifespan of Organisms

• Every species has a definite lifespan.

• Examples:

  • Bacteria → Few minutes to hours.

  • Housefly → 1 month.

  • Crow → 15 years.

  • Cow → 20–25 years.

  • Human → Around 100 years.

  • Elephant → 60–70 years.

  • Parrot → More than 100 years.

  • Banyan tree → Several hundred years.

👉 Note: Lifespan is not related to size or complexity of organism.

3. Types of Reproduction

(A) Asexual Reproduction

• Involves only one parent.

• Offspring are exact copies of parent (clones).

• No gametes, no fertilization.

• Common in unicellular organisms and some plants & animals.

Methods of Asexual Reproduction:

1. Binary fission → Parent cell divides into 2 equal halves.

   • Example: Amoeba, Paramecium.

2. Multiple fission → Parent cell divides into many daughter cells.

   • Example: Plasmodium.

3. Budding → Outgrowth (bud) develops into new organism.

   • Example: Hydra, Yeast.

4. Fragmentation → Body breaks into pieces, each grows into new individual.

   • Example: Spirogyra (algae).

5. Sporulation → Formation of special spores.

   • Example: Rhizopus, Mucor (fungi).

6. Vegetative propagation → Parts of plants develop into new plants.

   • Examples:

     • Potato tuber (eye bud).

     • Onion bulb.

     • Bryophyllum leaves (leaf buds).

     • Sugarcane stem (nodes).

👉 Significance: Fast, produces many offspring, useful for stable environments.

(B) Sexual Reproduction

• Involves two parents (male & female).

• Formation of male and female gametes.

• Fertilization (fusion of gametes) → Zygote → New offspring.

• Offspring show variations → important for evolution.

Phases in Sexual Reproduction:

1. Juvenile phase – Period of growth before sexual maturity.

   • Example: Humans (childhood to puberty).

2. Reproductive phase – Organism can reproduce.

   • Example:

     • Seasonal breeders → Birds, Deer (reproduce in specific seasons).

     • Continuous breeders → Humans, Apes (reproduce throughout the year).

3. Senescent phase – Ageing, decline in fertility, ultimately death.

   • Example: Loss of fertility in women after menopause.

4. Events in Sexual Reproduction

1. Pre-fertilization Events

   • Gametogenesis → Formation of male and female gametes by meiosis.

     • Male gamete → Sperm, pollen.

     • Female gamete → Egg, ovum.

   • Gamete transfer → Bringing male and female gametes together.

     • External medium (water in algae/fishes).

     • Internal transfer (in animals through copulation).

2. Fertilization (Syngamy)

   • Fusion of male and female gametes → Zygote (diploid).

   • External fertilization → Outside body (fishes, frogs).

   • Internal fertilization → Inside body (reptiles, birds, mammals).

3. Post-fertilization Events

   • Zygote formation → Diploid, single-celled.

   • Embryogenesis → Zygote develops into embryo by mitosis + differentiation.

   • Types of animals:

     • Oviparous – Egg-laying (birds, reptiles, amphibians).

     • Viviparous – Give birth to young ones (mammals).

5. Differences Between Asexual & Sexual Reproduction

Asexual Reproduction	Sexual Reproduction
One parent	Two parents
No gamete formation	Gametes formed
No fertilization	Fertilization occurs
Offspring are clones	Offspring show variation
Fast & simple	Slower & complex
Seen in bacteria, algae, fungi	Seen in higher plants, animals

Important Question-Answers

Q1. Why is reproduction necessary for organisms?

Answer:
Reproduction ensures continuity of species, transfer of genetic material, and provides variations that help in survival and evolution of organisms.

Q2. Differentiate between lifespan and reproduction.

Answer:

• Lifespan → Time between birth and natural death.

• Reproduction → Process of producing new individuals to ensure survival of species.
  👉 Lifespan is limited, but reproduction ensures immortality of species.

Q3. List examples of organisms with very short and very long lifespans.

Answer:

• Short lifespan: Mayfly (1 day), Housefly (1 month).

• Long lifespan: Parrot (100+ years), Banyan tree (200+ years).

Q4. What are seasonal and continuous breeders? Give examples.

Answer:

• Seasonal breeders → Reproduce during specific seasons (e.g., Deer, Birds).

• Continuous breeders → Reproduce throughout the year (e.g., Humans, Rabbits).

Q5. Distinguish between oviparous and viviparous animals.

Answer:

• Oviparous → Lay eggs outside body (Birds, Reptiles). Development occurs in egg.

• Viviparous → Give birth to young ones (Mammals). Development occurs inside mother’s body.

Q6. Why is variation important in sexual reproduction?

Answer:
Variation helps organisms adapt to changing environments, provides resistance against diseases, and plays a key role in evolution.

Q7. Write a note on vegetative propagation with examples.

Answer:
Vegetative propagation is an asexual method of reproduction in which new plants are formed from vegetative parts (root, stem, leaf).
Examples:

• Potato (tuber → eye bud).

• Onion (bulb).

• Bryophyllum (leaf buds).

• Sugarcane (stem nodes).