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

Okay, here are the detailed notes for Chapter 1: Reproduction in Organisms, from the NCERT Class 12 Biology textbook, focusing on aspects relevant for government exam preparation.

Chapter 1: Reproduction in Organisms

Core Concept: Reproduction is a fundamental biological process by which living organisms produce offspring of their own kind, ensuring the continuity of the species, generation after generation.

1. Life Span

• Definition: The period from birth to the natural death of an organism.
• Key Points:
  • Life spans vary greatly among different organisms (e.g., Mayfly: 1 day; Crow: ~15 years; Parrot: ~140 years; Banyan tree: several hundred years).
  • Life span is not necessarily correlated with the size of the organism (e.g., Crow and Parrot are similar in size but have vastly different life spans).
  • Regardless of the life span, death of every individual organism is a certainty (except for single-celled immortal organisms like Amoeba, where the parent cell divides into daughter cells).
  • Life span involves phases: Birth -> Growth -> Maturity -> Senescence -> Death.

2. Reproduction: Basic Features

• Enables continuity of species.
• Based on whether one or two parents are involved, reproduction is of two types: Asexual and Sexual.

3. Asexual Reproduction

• Definition: Production of offspring by a single parent, without the involvement of gamete formation and fusion.
• Characteristics:
  • Offspring are genetically identical to the parent and to each other.
  • Such genetically identical individuals are called clones.
  • Common in single-celled organisms, plants, and animals with relatively simple organizations.
  • Rapid mode of multiplication.
• Types of Asexual Reproduction:
  • Fission: Division of the parent cell into two or more individuals.
    • Binary Fission: Parent divides into two halves (e.g., Amoeba, Paramecium - Bacteria). The plane of division varies (e.g., longitudinal in Euglena, transverse in Paramecium, simple/irregular in Amoeba). Considered 'immortal' as the parent body continues to live as daughter cells.
    • Multiple Fission: Parent divides into many daughter cells simultaneously, usually under unfavourable conditions (e.g., Plasmodium, Amoeba during encystment).
  • Budding: A bud (outgrowth) develops on the parent body, detaches, and grows into a new individual.
    • Examples: Yeast (unicellular fungus), Hydra (multicellular animal).
  • Spore Formation: Formation of microscopic, single-celled, thin or thick-walled propagules called spores.
    • Zoospores: Motile spores with flagella, common in aquatic fungi and algae (e.g., Chlamydomonas).
    • Conidia: Non-motile spores produced exogenously on special structures called conidiophores (e.g., Penicillium).
    • Other spores: Oidia, Chlamydospores.
  • Fragmentation: Parent body breaks into distinct pieces (fragments), each fragment grows into an adult.
    • Examples: Spirogyra (alga), Hydra, Fungi.
  • Gemmules: Internal buds; asexual reproductive structures resistant to desiccation and harsh conditions.
    • Example: Sponges (freshwater sponges like Spongilla).
  • Vegetative Propagation (in Plants): Asexual reproduction in plants where new plants arise from vegetative parts (roots, stems, leaves). The units of vegetative propagation are called vegetative propagules.
    • Natural Methods & Propagules:
      • Runners: Slender horizontal stems growing close to the ground (e.g., Grass - Cynodon, Strawberry).
      • Rhizomes: Underground horizontal stems (e.g., Ginger, Turmeric, Banana).
      • Suckers: Arise from the base of the main stem, grow horizontally beneath the soil, then emerge obliquely upwards (e.g., Mint, Chrysanthemum, Banana).
      • Tubers: Swollen underground stem tips with buds ('eyes') (e.g., Potato).
      • Offsets: Short, thick runners found in aquatic plants, one internode long (e.g., Water Hyacinth - Eichhornia crassipes, Pistia).
      • Bulbs: Condensed underground stems with fleshy leaves (e.g., Onion, Garlic).
      • Bulbils: Fleshy buds that can develop into new plants (e.g., Agave, Oxalis).
      • Leaves: Adventitious buds on leaves (e.g., Bryophyllum - leaf notches).
    • Note: Water Hyacinth (Eichhornia crassipes) is known as the 'Terror of Bengal'. It's an invasive aquatic weed that drains oxygen from water bodies, leading to fish mortality. It reproduces phenomenally fast via offsets.
    • Artificial Methods: Used in agriculture and horticulture.
      • Cutting: A piece of stem or root is cut and planted (e.g., Rose, Sugarcane, Bougainvillea).
      • Layering: A branch is bent and covered with soil while attached to the parent plant until roots develop (e.g., Jasmine, Lemon).
      • Grafting: Joining parts of two different plants (scion and stock) to grow as one (e.g., Mango, Apple, Rose). Stock is the rooted part, Scion is the part grafted onto it.
      • Micropropagation (Tissue Culture): Growing plant cells, tissues, or organs in a sterile culture medium under controlled conditions. Produces a large number of plants in a short time.

4. Sexual Reproduction

• Definition: Reproduction involving the formation of male and female gametes, either by the same individual or by different individuals of the opposite sex, followed by the fusion of these gametes (fertilization/syngamy) to form a zygote.

• Characteristics:

  • Involves two parents (typically) of opposite sexes (biparental). Can be uniparental in bisexual organisms.
  • Involves formation and fusion of gametes (meiosis is usually involved).
  • Offspring are genetically different from parents and from each other, leading to variation.
  • Elaborate, complex, and slower process compared to asexual reproduction.

• Phases in Life Cycle: Organisms pass through distinct phases before they can reproduce sexually.

  • Juvenile Phase: Period of growth from birth until reproductive maturity. Called Vegetative Phase in plants. Duration varies greatly.
  • Reproductive Phase: Organism is capable of reproduction.
    • In plants: Marked by flowering.
      • Annuals: Complete life cycle (vegetative, reproductive, senescent) in one year/season (e.g., Wheat, Rice, Marigold).
      • Biennials: Complete life cycle in two years (vegetative first year, flowering/fruiting second year) (e.g., Carrot, Radish, Cabbage).
      • Perennials: Live for many years, flower repeatedly (e.g., Mango, Apple, Hibiscus). Some show unusual flowering:
        • Bamboo: Flowers only once in its lifetime (after 50-100 years), produces large number of fruits, and dies.
        • Strobilanthes kunthiana (Neelakurinji): Flowers once in 12 years (last flowered 2006, 2018). Found in hills of Kerala, Karnataka, Tamil Nadu.
    • In animals: Juvenile phase followed by morphological/physiological changes prior to active reproductive behaviour. Birds in nature lay eggs seasonally; poultry birds can be made to lay eggs throughout the year (commercial exploitation).
    • Oestrous Cycle: Cyclical changes in reproductive activity in non-primate mammals (e.g., cows, sheep, rats, deer, dogs, tigers). Females receptive only during oestrus/heat period.
    • Menstrual Cycle: Cyclical changes in reproductive activity in primates (e.g., monkeys, apes, humans). Female receptivity may not be restricted to a specific period.
    • Seasonal Breeders: Reproduce only during favourable seasons (e.g., frogs, lizards, most birds).
    • Continuous Breeders: Reproductively active throughout their reproductive phase (e.g., humans, poultry).
  • Senescent Phase: End of reproductive phase; period of ageing. Leads to deterioration, structural and functional changes, and ultimately death. Hormones regulate transitions between phases.

• Events in Sexual Reproduction: Sequentially ordered into three stages:

  • I. Pre-fertilization Events: Processes occurring before the fusion of gametes.

    • (a) Gametogenesis: Process of formation of the two types of gametes – male and female.
      • Gametes are haploid (n) cells.
      • Homogametes (Isogametes): Morphologically similar gametes (cannot categorize into male/female) (e.g., some algae like Cladophora).
      • Heterogametes: Morphologically distinct gametes (male and female).
        • Male gamete: Antherozoid or Sperm. Usually smaller and motile.
        • Female gamete: Egg or Ovum. Usually larger and non-motile, contains stored food.
        • Examples: Fucus (alga), Humans, most sexually reproducing organisms.
      • Sexuality in Organisms:
        • Bisexual (Monoecious/Hermaphrodite): Both male and female reproductive structures present in the same individual.
          • Plants: Monoecious (e.g., Chara, Cucurbits, Coconut). Flowers may be unisexual (male/female) or bisexual (both stamens and carpels).
          • Animals: Hermaphrodite (e.g., Earthworm, Sponge, Tapeworm, Leech).
        • Unisexual (Dioecious): Male and female reproductive structures present on different individuals (male and female individuals are distinct).
          • Plants: Dioecious (e.g., Marchantia, Papaya, Date palm). Plants are either male or female.
          • Animals: Most animals including Cockroach, Humans.
        • Fungi: May be homothallic (bisexual condition) or heterothallic (unisexual condition).
      • Cell Division during Gamete Formation:
        • If the parent body is haploid (n) (e.g., Monera, Fungi, Algae, Bryophytes), gametes are produced by Mitosis.
        • If the parent body is diploid (2n) (e.g., Pteridophytes, Gymnosperms, Angiosperms, most Animals), gametes are produced by Meiosis (reductional division). Specialized cells called meiocytes (gamete mother cells) undergo meiosis.
        • Important: Meiocytes are diploid (2n), Gametes are haploid (n). (Refer to Table 1.1 in NCERT for chromosome numbers in meiocytes and gametes of various organisms - e.g., Human: Meiocyte 46, Gamete 23; Housefly: Meiocyte 12, Gamete 6; Fruit fly: Meiocyte 8, Gamete 4; Ophioglossum (a fern): Meiocyte 1260, Gamete 630; Rice: Meiocyte 24, Gamete 12; Maize: Meiocyte 20, Gamete 10; Potato: Meiocyte 48, Gamete 24; Onion: Meiocyte 16, Gamete 8).
    • (b) Gamete Transfer: Bringing male and female gametes together physically to facilitate fusion (fertilization).
      • In most organisms, the male gamete is motile, and the female gamete is stationary. Exceptions exist (some fungi and algae where both are motile).
      • A medium is needed for male gamete movement.
        • Water is the medium in simple plants like Algae, Bryophytes, and Pteridophytes. Large numbers of male gametes are produced to compensate for loss during transport.
      • In Seed Plants (Gymnosperms and Angiosperms):
        • Pollen grains carry the male gametes. Ovule contains the egg cell.
        • Pollination: Transfer of pollen grains from the anther to the stigma.
          • Self-pollination: Transfer within the same flower or to another flower on the same plant (occurs in bisexual plants).
          • Cross-pollination: Transfer to the stigma of a flower on a different plant (occurs in unisexual/dioecious plants and also in bisexual plants with mechanisms preventing self-pollination).
        • Pollen germinates on the stigma, pollen tube carries male gametes to the ovule for fertilization.
      • In dioecious animals: Requires copulation or simultaneous release of gametes for successful transfer and fertilization.

  • II. Fertilization (Syngamy): The fusion of male and female gametes to form a diploid zygote (2n).

    • Most vital event in sexual reproduction.
    • Parthenogenesis: Development of a new organism (female gamete/egg) without fertilization (e.g., Rotifers, Honeybees (drones develop this way), some Lizards, Turkey bird).
    • Types of Fertilization based on location:
      • External Fertilization: Syngamy occurs outside the body of the organism, in an external medium (usually water).
        • Examples: Most aquatic organisms like Algae, Fish, Amphibians.
        • Characteristics: Organisms show great synchrony in sex cell release; produce a large number of gametes.
        • Disadvantage: Offspring are highly vulnerable to predators, threatening survival.
      • Internal Fertilization: Syngamy occurs inside the body of the organism (usually the female body).
        • Examples: Many terrestrial organisms like Fungi, Reptiles, Birds, Mammals; also majority of plants (Bryophytes, Pteridophytes, Gymnosperms, Angiosperms).
        • Characteristics: Egg formed inside the female body. Male gamete is motile and reaches the egg. Number of sperms produced is very large; number of eggs produced is reduced.
        • Advantage: Increased protection for the developing offspring. In seed plants, non-motile male gametes are carried to the female gamete by the pollen tube.

  • III. Post-fertilization Events: Events occurring after the formation of the zygote.

    • (a) The Zygote (2n):
      • The first diploid cell formed after fertilization. Universal in sexually reproducing organisms.
      • Forms the vital link ensuring continuity between generations.
      • Development depends on the organism's life cycle and environment.
      • In organisms with external fertilization, zygote is formed in the external medium (water).
      • In organisms with internal fertilization, zygote is formed inside the parent body.
      • In Fungi and Algae, the zygote often develops a thick wall resistant to desiccation and damage, undergoing a period of rest before germination.
      • In organisms with a haplontic life cycle (e.g., Algae), the diploid zygote undergoes meiosis to form haploid spores that grow into haploid individuals.
      • In organisms with diplontic or haplo-diplontic life cycles (e.g., Seed plants, Animals), the diploid zygote undergoes mitosis to develop into a diploid embryo and then the individual.
    • (b) Embryogenesis: The 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 Location (in Animals):
        • Oviparous: Animals that lay fertilized or unfertilized eggs. Development of the zygote occurs outside the female parent's body. Eggs are often covered by a hard calcareous shell for protection (e.g., Reptiles, Birds). Risk of predation is higher.
        • Viviparous: Animals that give birth to young ones. Development of the zygote occurs inside the female parent's body into a young one.
          • Examples: Majority of Mammals, including humans.
          • Advantage: Better embryonic care and protection inside the mother's body; increases chances of survival.
      • In Flowering Plants (Angiosperms):
        • Zygote develops inside the ovule.
        • Zygote -> Embryo
        • Ovule -> Seed
        • Ovary -> Fruit
        • Ovary wall -> Pericarp (protective wall of the fruit).
        • After dispersal, seeds germinate under favourable conditions to produce new plants. (Diagram of seeds and pericarp in different fruits like pea, tomato, cucumber is relevant - Fig 1.8 NCERT).

This covers the essential details from Chapter 1 for exam preparation. Remember to refer to the NCERT textbook for diagrams and specific examples mentioned in tables. Good luck!