Class 12 Biology Notes Chapter 1 (Reproduction in Organisms) – Examplar Problems Book

Detailed Notes with MCQs of Chapter 1: Reproduction in Organisms from your NCERT Exemplar. This chapter lays the foundation for understanding how life perpetuates itself, a crucial topic for many competitive exams.

Chapter 1: Reproduction in Organisms - Detailed Notes

1. Life Span:

• Definition: The period from birth to the natural death of an organism.
• Key Points:
  • Highly variable among different species (e.g., Mayfly: 1 day; Parrot: 140 years; Banyan tree: several hundred years).
  • Not necessarily correlated with the size of the organism (e.g., Crow and Parrot are similar in size but have vastly different life spans).
  • Death is a certainty for all individuals (except single-celled organisms like Amoeba, which are considered 'immortal' as the parent cell divides into daughter cells).
• Phases: Juvenile phase (growth), Reproductive phase (maturity), Senescent phase (aging leading to death).

2. Reproduction:

• Definition: A biological process in which an organism gives rise to young ones (offspring) similar to itself.
• Purpose: Enables the continuity of the species, generation after generation.
• Basic Features: Replication of DNA, cell division, formation of reproductive units, development of new individuals.
• Types: Asexual and Sexual Reproduction.

3. Asexual Reproduction:

• Definition: Reproduction involving a single parent, without the formation and fusion of gametes.
• Characteristics:
  • Offspring are genetically identical to the parent (clones).
  • Rapid mode of multiplication.
  • Common in single-celled organisms, plants, and animals with relatively simple organizations.
• Modes of Asexual Reproduction:
  • Fission: Parent cell divides into two or more individuals.
    • Binary Fission: Division into two halves (e.g., Amoeba, Paramecium). Can be simple, longitudinal, or transverse.
    • Multiple Fission: Parent cell divides into many daughter cells simultaneously, usually under unfavorable conditions (e.g., Amoeba, Plasmodium). Often involves cyst formation.
  • Budding: A small bud arises from the parent body, detaches, and develops into a new individual.
    • Exogenous Budding: Bud forms on the outer surface (e.g., Hydra, Yeast).
    • Endogenous Budding (Gemmules): Internal buds formed to survive unfavorable conditions (e.g., Sponges like Spongilla).
  • Spore Formation: Formation of microscopic, single-celled, thin or thick-walled propagules called spores.
    • Zoospores: Motile spores, usually flagellated (e.g., Chlamydomonas, Algae, Fungi).
    • Conidia: Non-motile spores produced exogenously on conidiophores (e.g., Penicillium, Aspergillus).
    • Other types: Aplanospores (non-motile, thin-walled), Hypnospores (thick-walled), Chlamydospores (thick-walled resting spores).
  • Fragmentation: Parent body breaks into distinct pieces (fragments), each fragment grows into an adult (e.g., Spirogyra, Hydra, Fungi, some flatworms).
  • Vegetative Propagation (in Plants): Formation of new plants from vegetative parts (roots, stem, leaves). Units of vegetative propagation are called vegetative propagules.
    • Natural Methods:
      • Runners: Horizontal stems growing close to the ground (e.g., Grass, Oxalis).
      • Rhizomes: Underground horizontal stems (e.g., Ginger, Turmeric, Banana).
      • Suckers: Arise from the base of the main stem (e.g., Mint, Chrysanthemum).
      • Tubers: Swollen underground stem tips with 'eyes' (axillary buds) (e.g., Potato).
      • Offsets: Short, horizontal branches with rosettes of leaves and roots (e.g., Pistia, Eichhornia - 'Terror of Bengal').
      • Bulbs: Condensed underground stems with fleshy leaves (e.g., Onion, Garlic).
      • Leaves: Adventitious buds on leaf margins (e.g., Bryophyllum).
    • Artificial Methods (Horticulture):
      • Cutting: Stem or leaf cuttings planted to grow roots (e.g., Rose, Sugarcane).
      • Layering: A branch is induced to root while still attached to the parent plant (e.g., Jasmine).
      • Grafting: Joining parts of two different plants (scion and stock) to grow as one (e.g., Mango, Apple). Requires cambium alignment.
      • Micropropagation (Tissue Culture): Growing plant cells, tissues, or organs in a sterile culture medium under controlled conditions. Produces a large number of plants (somaclones) quickly.

4. Sexual Reproduction:

• Definition: Reproduction involving the formation and fusion (fertilization) of male and female gametes to form a zygote, which develops into a new organism.
• Characteristics:
  • Involves two parents (usually) or sometimes one (bisexual organisms).
  • Offspring are genetically different from parents and from each other, leading to variation.
  • More complex and slower process than asexual reproduction.
  • Involves meiosis (gamete formation) and syngamy (fusion).
• Phases in Life Cycle (related to reproduction):
  • Juvenile Phase: Period of growth before reproductive maturity. Called vegetative phase in plants.
  • Reproductive Phase: Organism is capable of reproduction. Marked by specific events like flowering in plants.
    • Seasonal Breeders: Reproduce only during favorable seasons (e.g., most birds, frogs, lizards).
    • Continuous Breeders: Reproduce throughout their reproductive phase (e.g., humans, poultry).
    • Flowering Plants:
      • Annuals/Biennials: Clear-cut vegetative, reproductive, and senescent phases.
      • Perennials: Inter-flowering period can be complex. Some flower seasonally, some throughout the year. Unusual flowering: Strobilanthes kunthiana (Neelakurinji) flowers once in 12 years; Bamboo species flower once in their lifetime (50-100 years).
    • Placental Mammals (Females): Exhibit cyclical changes in ovaries, accessory ducts, and hormones during the reproductive phase.
      • Oestrous Cycle: In non-primates (cows, sheep, rats, deer, dogs, tigers). Characterized by 'heat' periods.
      • Menstrual Cycle: In primates (monkeys, apes, humans).
  • Senescent Phase: End of reproductive phase; period of aging leading to deterioration and ultimately death. Concomitant changes occur (e.g., slowing metabolism).
• Events in Sexual Reproduction: Sequentially ordered into three stages.
  • I. Pre-fertilization Events: Processes occurring before the fusion of gametes.
    • (a) Gametogenesis: Formation of gametes (male and female).
      • Gametes are haploid (n).
      • Isogametes (Homogametes): Morphologically similar gametes (e.g., Cladophora - an alga).
      • Heterogametes: Morphologically distinct gametes.
        • Antherozoid/Sperm: Male gamete (usually smaller, motile).
        • Egg/Ovum: Female gamete (usually larger, non-motile, stores food). (e.g., Fucus - an alga, Humans, most sexually reproducing organisms).
      • Sexuality in Organisms:
        • Homothallic/Monoecious: Both male and female reproductive structures present in the same individual (e.g., Fungi like Rhizopus, Plants like Cucurbits, Coconuts, Chara). Bisexual condition.
        • Heterothallic/Dioecious: Male and female reproductive structures on different individuals (e.g., Fungi like Mucor, Plants like Papaya, Date palm, Marchantia). Unisexual condition.
        • Animals:
          • Hermaphrodite/Bisexual: Both male and female reproductive organs in the same animal (e.g., Earthworm, Sponge, Tapeworm, Leech).
          • Unisexual: Sexes are separate (e.g., Cockroach, Humans, most higher animals).
      • Cell Division during Gamete Formation:
        • If parent body is haploid (n) (e.g., Monera, Fungi, Algae, Bryophytes), gametes are formed by mitosis.
        • If parent body is diploid (2n) (e.g., Pteridophytes, Gymnosperms, Angiosperms, most Animals), gametes are formed by meiosis (reductional division) in specialized cells called meiocytes (gamete mother cells). Meiocytes are diploid.
    • (b) Gamete Transfer: Bringing male and female gametes together for fertilization.
      • In most organisms, the male gamete is motile, and the female gamete is stationary. Exception: some fungi and algae where both are motile.
      • Algae, Bryophytes, Pteridophytes: Water is the medium for gamete transfer (zooidogamy). Large number of male gametes produced to compensate for loss.
      • Seed Plants (Gymnosperms & Angiosperms): Pollen grains carry male gametes. Ovule contains the egg cell. Pollination (transfer of pollen to stigma) is required, followed by pollen tube growth. Water is not required for gamete transfer itself.
      • Animals: Usually requires copulation (insemination) for internal fertilization. External fertilization relies on water.
  • II. Fertilization (Syngamy): Fusion of male and female gametes to form a diploid zygote (2n).
    • External Fertilization: Syngamy occurs outside the body, in an external medium (usually water) (e.g., most algae, fishes, amphibians like frogs). Requires synchrony in gamete release; produces large numbers of gametes; offspring vulnerable to predators.
    • Internal Fertilization: Syngamy occurs inside the body of the female organism (e.g., fungi, reptiles, birds, mammals, majority of plants like bryophytes, pteridophytes, gymnosperms, angiosperms). Male gamete is motile and reaches the egg. Fewer eggs produced; greater protection for developing embryo.
    • Parthenogenesis: Development of an egg cell into a complete individual without fertilization (e.g., Rotifers, honeybees (drones), some lizards, birds like Turkey). A modification of sexual reproduction, sometimes considered asexual.
  • III. Post-fertilization Events: Events occurring after zygote formation.
    • (a) The Zygote:
      • Vital link ensuring continuity between generations.
      • Universally diploid (2n) in sexual reproduction.
      • Development depends on the life cycle and environment. In fungi and algae, it may develop a thick wall (zygospore) to resist desiccation/damage and undergo a resting period before germination.
      • In organisms with haplontic life cycle (e.g., algae), the zygote undergoes meiosis to form haploid spores.
      • In organisms with diplontic or haplodiplontic life cycles (e.g., seed plants, animals), the zygote undergoes mitosis to develop into an embryo.
    • (b) Embryogenesis: Process of development of the embryo from the zygote.
      • Involves:
        • Cell Division (Mitosis): Increases the number of cells in the developing embryo.
        • Cell Differentiation: Cells undergo modifications to form specialized tissues and organs.
    • Based on Zygote Development Site:
      • Oviparous Animals: Lay fertilized or unfertilized eggs. Development of the zygote occurs outside the female parent's body (e.g., Reptiles, Birds). Eggs are usually covered by a hard calcareous shell (protection).
      • Viviparous Animals: Give birth to young ones. Development of the zygote occurs inside the female parent's body (e.g., Majority of mammals, including humans). Embryo gets nourishment and protection inside the mother's body. Better chances of survival.
    • In Flowering Plants:
      • Zygote develops into the embryo.
      • Ovules develop into seeds.
      • Ovary develops into the fruit.
      • The fruit wall (pericarp) develops from the ovary wall and is protective.
      • After dispersal, seeds germinate under favorable conditions to produce new plants.

Key Differences: Asexual vs. Sexual Reproduction

Multiple Choice Questions (MCQs)

1. Which of the following vegetative propagules are found in Pistia and Eichhornia?
   (a) Rhizome
   (b) Sucker
   (c) Offset
   (d) Runner

2. Meiosis occurs in specialized cells called meiocytes. In a potato plant (2n=48), what will be the chromosome number in its gametes?
   (a) 48
   (b) 24
   (c) 12
   (d) 96

3. Identify the group of organisms that exhibit external fertilization.
   (a) Reptiles, Birds, Mammals
   (b) Bryophytes, Pteridophytes, Gymnosperms
   (c) Most Fishes, Amphibians, Most Algae
   (d) Fungi, Angiosperms, Insects

4. The term 'clone' cannot be applied to offspring formed by sexual reproduction because:
   (a) Offspring do not possess exact copies of parental DNA
   (b) DNA of only one parent is copied and passed on to the offspring
   (c) Offspring are formed at different times
   (d) DNA of parent and offspring are completely different

5. Which of the following flowers only once in its lifetime, generally after 50-100 years, produces large number of fruits and dies?
   (a) Mango
   (b) Neelakurinji (Strobilanthes kunthiana)
   (c) Bamboo species
   (d) Jackfruit

6. In which mode of reproduction are variations most likely to occur?
   (a) Binary Fission
   (b) Budding
   (c) Parthenogenesis
   (d) Sexual Reproduction

7. Gemmules are specialized structures produced for asexual reproduction in:
   (a) Hydra
   (b) Amoeba
   (c) Spongilla
   (d) Chlamydomonas

8. Monoecious condition (having both male and female reproductive structures on the same plant) is found in:
   (a) Papaya
   (b) Date Palm
   (c) Coconut
   (d) Marchantia

9. In oviparous animals like reptiles and birds, the fertilized eggs are covered by a hard calcareous shell. This provides:
   (a) Nourishment to the embryo
   (b) A medium for gas exchange
   (c) Protection to the developing embryo
   (d) A site for waste removal

10. The process of development of an embryo from the zygote through mitotic cell divisions and cell differentiation is called:
    (a) Gametogenesis
    (b) Syngamy
    (c) Parthenogenesis
    (d) Embryogenesis

Answer Key:

1. (c) Offset
2. (b) 24
3. (c) Most Fishes, Amphibians, Most Algae
4. (a) Offspring do not possess exact copies of parental DNA
5. (c) Bamboo species
6. (d) Sexual Reproduction
7. (c) Spongilla
8. (c) Coconut
9. (c) Protection to the developing embryo
10. (d) Embryogenesis

Make sure you revise these concepts thoroughly. Pay special attention to the examples provided for each type of reproduction and life cycle event, as questions are often framed around them. Good luck with your preparation!