Reproduction in Organisms — Biology Class 12 Notes (CBSE & HBSE)
Free NCERT Biology notes for Reproduction in Organisms (Class 12) on Siksha Sarovar, aligned to CBSE and Haryana Board (HBSE). This chapter is broken into 3 topics with clear explanations, formulas, solved examples and board-pattern practice — free to read, no sign-up required.
Board exam focus — Reproduction in Organisms (CBSE & HBSE)
CBSE focuses on types of asexual reproduction with real-life examples, significance of sexual reproduction, and terminology like gametogenesis and syngamy. HBSE emphasises direct definitions, diagrams of binary fission and budding, and comparative questions on asexual versus sexual reproduction. Both boards follow NCERT closely.
Asexual Reproduction
What is Reproduction?
Reproduction is the biological process by which organisms produce new individuals similar to themselves. It is essential for the continuity of species and is one of the defining characteristics of living organisms. Reproduction is broadly classified into asexual and sexual types.
Asexual Reproduction
In asexual reproduction, a single parent produces offspring that are genetically identical to the parent (called clones). It does not involve the formation or fusion of gametes.
Key Features:
- Involves only mitotic cell division
- Offspring are genetically identical (clones)
- Rapid colonisation of favourable habitats
- No need for a mate
- Common in lower organisms and plants
Types of Asexual Reproduction
1. Binary Fission The parent organism divides into two equal halves, each becoming a new individual.
- Amoeba — irregular binary fission (any plane)
- Paramecium — transverse binary fission
- Bacteria — simple binary fission
| Organism | Plane of Division |
|---|---|
| Amoeba | Any plane |
| Paramecium | Transverse |
| Euglena | Longitudinal |
2. Budding A small outgrowth (bud) forms on the parent body, grows, and detaches as a new organism.
- Hydra — the bud forms on the body wall, differentiates, and breaks off
- Yeast (Saccharomyces cerevisiae) — small bud forms at the cell wall, gains nucleus by mitosis, and separates
3. Sporulation Organisms produce special reproductive structures called spores that can survive harsh conditions and germinate under favourable conditions.
- Rhizopus (bread mould) — produces asexual sporangiospores in sporangia
- Ferns and mosses — produce spores (though part of alternation of generations)
4. Fragmentation The parent body breaks into fragments, each of which grows into a new individual.
- Spirogyra (filamentous alga) — any fragment can grow into a new filament
- Planaria — can be cut and each piece regenerates
5. Regeneration Specialised cells (neoblasts) in the organism have the ability to proliferate and differentiate to reconstruct the whole organism from a fragment.
- Planaria — each piece can form a complete organism
- Hydra — limited regeneration also occurs
Regeneration and fragmentation differ: fragmentation is the process of breaking, while regeneration is the underlying cellular mechanism that allows reconstruction.
6. Vegetative Propagation Natural vegetative propagation occurs through specialised vegetative structures:
| Structure | Example | Plant |
|---|---|---|
| Runner/Stolon | Grows horizontally above ground | Grass, strawberry |
| Rhizome | Underground horizontal stem | Ginger, turmeric, banana |
| Bulb | Fleshy underground stem | Onion, garlic, lily |
| Leaf buds (adventitious) | Buds on leaf margins | Bryophyllum |
| Offset | Short runner on water surface | Water hyacinth (Eichhornia) |
| Corm | Underground swollen stem | Colocasia, Crocus |
Bryophyllum is the classic NCERT example — leaf margins bear adventitious buds that drop and develop into new plants.
Advantages of Asexual Reproduction
- Speed: Very rapid multiplication
- Efficiency: No need to find a mate
- Genetically identical: Desirable traits are preserved exactly
- Used in horticulture: Cutting, grafting (artificial vegetative propagation)
Disadvantages
- No genetic variation → less adaptability to environmental changes
- Disease can wipe out all clones simultaneously (no resistant individuals)
Comparison: Types of Asexual Reproduction
| Type | Division | Organism | Special Feature |
|---|---|---|---|
| Binary fission | Equal halves | Bacteria, Amoeba | Very rapid |
| Budding | Unequal (bud) | Hydra, yeast | Bud separates |
| Sporulation | Spores | Rhizopus | Spores resist drought |
| Fragmentation | Body pieces | Spirogyra | Each fragment grows |
| Regeneration | Neoblasts | Planaria, Hydra | Molecular mechanism |
| Vegetative propagation | Vegetative parts | Plants | Used in agriculture |
Diagram Indicator: [Labelled diagrams of binary fission in Amoeba (showing irregular cleavage plane), budding in Hydra (with bud formation stages), and Bryophyllum leaf with adventitious buds on notches]
Sexual Reproduction
Sexual Reproduction — Overview
Sexual reproduction involves the fusion of two haploid gametes (male and female) to produce a diploid zygote, which then develops into a new organism. Unlike asexual reproduction, it requires two parents (usually) and results in genetically varied offspring.
Sexual reproduction is observed in virtually all eukaryotes — algae, fungi, plants, and animals.
Three Phases of Sexual Reproduction
Sexual reproduction can be divided into three distinct phases:
| Phase | Events | Key Terms |
|---|---|---|
| Pre-fertilisation | Gametogenesis + gamete transfer | Gametes, meiosis |
| Fertilisation | Fusion of gametes | Syngamy, zygote |
| Post-fertilisation | Zygote development | Embryogenesis, embryo |
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Phase 1: Pre-fertilisation Events
This includes two important processes:
A. Gametogenesis — the process of formation of gametes by meiotic division of gamete mother cells (GMCs). Results in haploid gametes (n).
B. Gamete Transfer — transfer of male gamete to the female gamete for fertilisation.
- In many aquatic organisms (algae, fish, frogs): water medium is used for gamete transfer
- In terrestrial organisms: pollen grains (in plants), sperm cells (in animals) are transferred via various mechanisms
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Phase 2: Fertilisation (Syngamy)
Syngamy = fusion of male and female gametes → zygote (2n)
The zygote is the beginning of a new individual.
Types of Fertilisation based on location:
| Type | Definition | Examples |
|---|---|---|
| External fertilisation | Gametes released into external medium; fusion outside the body | Frogs (Rana), most fishes, Fucus (algae) |
| Internal fertilisation | Fusion inside the female body | Reptiles, birds, mammals, seed plants |
External fertilisation requires water as medium and involves synchrony — both organisms release gametes simultaneously (often triggered by pheromones, season, or moonlight).
Parthenogenesis — development of an unfertilised egg into an organism (e.g., honeybee drones, some lizards, Apis workers). Not true sexual reproduction but related.
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Phase 3: Post-fertilisation Events
After fertilisation:
- Zygote (2n) is formed
- Embryogenesis — zygote undergoes mitotic cell divisions (cleavage) + cell differentiation → embryo
- Embryo develops into the new organism
Oviparous organisms: Embryo develops inside an egg outside the mother (reptiles, birds)
Viviparous organisms: Embryo develops inside the mother's uterus (most mammals, including humans)
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Significance of Sexual Reproduction
- Genetic recombination: New combinations of alleles → variation in offspring
- Adaptation: Varied offspring → better chance that some survive new conditions
- Evolutionary drive: Provides raw material for natural selection
- Repair of harmful mutations: Complementation between two parents
Comparison: Asexual vs Sexual Reproduction
| Feature | Asexual | Sexual |
|---|---|---|
| Parents required | One | Two (usually) |
| Gametes | Not formed | Formed (haploid) |
| Meiosis | Absent | Present |
| Genetic variation | None (clones) | Yes (recombination) |
| Speed | Fast | Slower |
| Organisms | Lower organisms, plants | Most eukaryotes |
Important Terminology
- Gamete: Haploid reproductive cell (sperm, egg, pollen)
- Gametogenesis: Process of gamete formation
- Syngamy: Fusion of male and female gametes
- Zygote: Product of syngamy (2n)
- Embryogenesis: Development of zygote into embryo
- Clone: Genetically identical offspring
Diagram Indicator: [Flow diagram showing three phases of sexual reproduction: Pre-fertilisation (gametogenesis → gametes) → Fertilisation (syngamy → zygote) → Post-fertilisation (embryogenesis → embryo → organism), with diploid/haploid states labelled]
Pre-fertilisation Events: Gametogenesis and Gamete Types
Pre-fertilisation Events — Detailed Study
Pre-fertilisation events are critical steps that prepare organisms for successful fertilisation. They include gametogenesis (formation of gametes) and gamete transfer (transport of gametes to the site of fertilisation).
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Gametogenesis
Gametogenesis is the process of formation of male and female gametes by meiotic division of gamete mother cells (GMCs). The process ensures gametes are haploid (n), so that upon fusion (syngamy), the diploid (2n) condition of the species is restored.
Location of gametogenesis:
- In animals: gonads — testes (male gametes/spermatozoa), ovaries (female gametes/ova)
- In plants: anthers (pollen/male gametes), ovules (egg/female gamete)
General steps:
- Mitotic proliferation of germ cells → large number of GMCs
- Meiosis I of GMC → two haploid secondary cells
- Meiosis II → four haploid cells (not always complete before fertilisation in females)
- Differentiation of cells into mature gametes
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Types of Gametes
A. Homogametes (Isogametes)
- Both male and female gametes are morphologically identical (same size and structure)
- Cannot be distinguished as male or female on structural basis
- Found in: Cladophora (green alga), some fungi
B. Heterogametes (Anisogametes)
- Male and female gametes are morphologically distinct
- Female gamete is usually larger (more cytoplasm, yolk reserves)
- Male gamete is usually smaller and motile
- Found in: most animals, higher plants
Special case — Oogamy:
- An extreme form of anisogamy
- Female gamete: large, non-motile egg (ovum)
- Male gamete: small, motile sperm
- Found in: humans, most animals, angiosperms (though pollen tube replaces motility in plants)
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Motility of Gametes
| Feature | Male Gametes | Female Gametes |
|---|---|---|
| Size | Small | Large (stores nutrients) |
| Motility | Usually motile (flagellated) | Usually non-motile |
| Numbers | Millions produced | Few produced |
| Examples | Sperm in animals, antherozoids in bryophytes | Egg in animals, ovum in flowering plants |
Exceptions:
- In fungi and algae with isogamy: both gametes may be motile (flagellated)
- In angiosperms: male gametes (in pollen) are non-motile — transported by pollen tube; water is NOT required
This is why gymnosperms and angiosperms are called Embryophyta — they protect the embryo.
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Gamete Transfer
After formation, gametes must be brought together for fertilisation.
In water (aquatic organisms):
- Millions of gametes released into water
- Water current facilitates transfer
- Synchrony critical (pheromones, season, lunar cycle)
- Huge numbers needed to ensure at least some fusions occur
In plants:
- Pollination — transfer of pollen from anther to stigma
- Agents: wind (anemophily), water (hydrophily), insects (entomophily), birds (ornithophily), bats (chiropterophily)
- After pollination, pollen tube grows to carry male gametes to egg
In animals (internal fertilisation):
- Sperm transferred during copulation
- Accessory reproductive organs (penis, vagina) facilitate transfer
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Compatibility Mechanisms
Not all gametes can fuse — compatibility barriers prevent:
- Interspecific fusion (between different species)
- Self-fertilisation in plants (self-incompatibility mechanisms — stigma rejects own pollen)
Self-incompatibility in plants: controlled by S-locus genes — prevents inbreeding and promotes genetic variation.
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Summary Table: Key Terms
| Term | Definition |
|---|---|
| Gametogenesis | Formation of gametes by meiosis |
| Isogamy | Both gametes morphologically identical |
| Anisogamy | Gametes morphologically different |
| Oogamy | Large non-motile egg + small motile sperm |
| Syngamy | Fusion of gametes → zygote |
| Parthenogenesis | Development of unfertilised egg |
Diagram Indicator: [Diagram showing three types of gamete fusion: isogamy (two equal gametes), anisogamy (unequal sized), and oogamy (large egg + small sperm); arrows showing fusion → zygote for each type]
Frequently asked questions
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Concept explanations, key formulas and definitions, fully solved examples and board-pattern practice questions for Reproduction in Organisms.