Revision: Reproduction >> Reproduction in Organisms Biology (Theory) ISC (Science) ISC Class 12 CISCE

Lifespan is the duration or period from birth to the natural death of an organism.

Reproduction is the process of formation of new individuals by sexual or asexual means, which can repeat the process in their own turn.

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Formation of new organism of same species by earlier existing organism is called as reproduction.

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Reproduction is a biological process that results in the formation of new individuals of the same kind, though usually with slight genetic, structural and physiological variations.

A mode of reproduction where a new organism is formed from a part of the parent body without involving gametes.

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The process of forming a new organism from an organism of the same species without the involvement of gametes is called asexual reproduction.

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When offspring is produced by a single parent by the formation of specialised asexual reproductive structures by the parent body, the reproduction is asexual.

Binary fission is an asexual reproduction process in single-celled organisms where a parent cell divides into two genetically identical daughter cells through coordinated nuclear and cytoplasmic division.

Simple binary fission is an asexual reproduction process in which a single Amoeba cell divides along a randomly oriented plane to form two genetically identical daughter cells.

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When a unicellular organism divides into two identical daughter cells in any plane, due to lack of definite shape, it is called simple binary fission.

Example: Amoeba

When a unicellular organism divides across the transverse (horizontal) axis, it is called transverse binary fission.

or

Transverse binary fission is an asexual reproduction process where the plane of division runs along the transverse (horizontal) axis of the individual, splitting it into two parts.

Example: Paramecium

When a unicellular organism divides along the longitudinal (vertical) axis, it is called longitudinal binary fission.

or

Longitudinal binary fission is an asexual reproduction process where the plane of division passes along the longitudinal (vertical) axis of the animal, splitting it into two parts.

Example: Euglena

The daughter individuals formed as a result of sporogony in Plasmodium are called sporozoites.

The end products of multiple fission that are enclosed in a hard, resistant covering are called spores.

When an Amoeba forms a protective covering around itself during adverse conditions and stops movement, it is called a cyst.

The process by which Amoeba withdraws its pseudopodia and forms a hard, protective, three-layered cyst around its body during unfavourable conditions is called encystation.

When a single organism divides into many daughter cells by forming multiple nuclei followed by cytoplasmic division, it is called multiple fission.

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The mode of asexual reproduction in which a single parent organism divides to produce many daughter organisms at the same time is called multiple fission.

The small daughter cells formed when each daughter nucleus produced during multiple fission in Amoeba gets surrounded by a small amount of cytoplasm are called amoebulae or pseudopodiospores.

The type of multiple fission in which the parent Amoeba forms a cyst and releases many daughter individuals on return of favourable conditions is called sporulation.

The process of multiple fission occurring in the oocyst of Plasmodium inside the mosquito is called sporogony.

In unicellular organisms like yeast, when a small bud forms on the parent cell, gets a nucleus, grows, and separates to become a new cell, it is called budding.

In multicellular organisms like Hydra, when a bud grows on the parent body through cell division and later becomes a new individual, this is called budding.

All freshwater sponges like Spongilla and some marine sponges reproduce asexually by the formation of specialised endogenous buds, known as gemmules (internal buds).

The process in which an organism develops a new individual from its body parts using specialised cells that multiply and differentiate into various tissues is called regeneration.

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When an organism regrows its lost or damaged body part, or forms a new organism from a body part, it is called regeneration.

When the body of a multicellular organism breaks into two or more fragments, and each fragment grows into a new individual, it is called fragmentation.

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The mode of asexual reproduction in which the body of an organism breaks into fragments, each capable of developing into a complete new organism, is called fragmentation.

Example: Ulothrix, Spirogyra, etc

Non-motile asexual spores produced externally and not inside a sporangium in many fungi are called conidia or conidiospores.

Non-motile spores with a typical cell wall and without flagella are called aplanospores.

Asexual spores produced inside a sporangium are called sporangiospores.

A special reproductive structure formed by simple plants and fungi that germinates directly to form a new individual is called a spore.

Spores that possess flagella and are capable of movement in water are called motile spores or zoospores.

Spores that lack flagella and are incapable of movement are called non-motile spores.

Reproduction in plants, in which a new plant can arise from a vegetative part such as a root, stem, or leaf without the help of any reproductive organ, is called vegetative propagation.

The process in which a new plant develops from a vegetative part (stem, root, or leaf) of the parent plant under suitable conditions is called natural vegetative propagation.

The process in which a part of a plant is detached from the mother plant by humans and grown independently into a new plant to quickly produce plants while retaining the mother plant's qualities is called artificial vegetative propagation. It is done by cuttings, layering, and grafting.

A mode of reproduction involving the fusion of male and female gametes (sperm and egg) to form a zygote that develops into a new organism.

The period of growth and development before an organism becomes sexually mature is called the juvenile phase.

All the events of sexual reproduction prior to the fusion of gametes are referred as prefertilization events.

The fusion of the germ-cells (male and female) to form a zygote is called fertilisation.

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The process in which male and female gametes fuse to form a diploid zygote is called fertilization. The actual union of the male and female gametes during fertilization is called syngamy.

The process in which a zygote undergoes meiosis to form haploid spores in some algae and fungi is called zygotic meiosis.

The process by which the zygote develops into an embryo through mitotic divisions and cell differentiation is called embryogenesis.

All events occurring after fertilization and formation of the zygote during sexual reproduction are called post-fertilization events.

The process by which the embryo present in the seed develops into a seedling under appropriate conditions is called germination.

A unisexual female flower that bears only pistils is called a pistillate flower.

A plant that possesses both male and female reproductive structures on the same plant is called a bisexual plant.

A plant in which male and female reproductive structures are present on different plants is called a unisexual plant.

A unisexual male flower that bears only stamens is called a staminate flower.

A plant in which both male and female flowers are present on the same individual is called a monoecious plant.

Example: Cucurbits, coconut.

A plant in which male and female flowers are present on separate individuals is called a dioecious plant.

Example: Papaya, date palm.

The female gonad that produces ova (female gametes) is called the ovary.

Animals in which an individual possesses either male or female reproductive organs are called unisexual animals.

An organism that has both male and female reproductive organs in the same body is called a hermaphrodite.

Specialized reproductive organs that produce gametes in animals are called gonads.

The male gonad that produces sperms (male gametes) is called the testis.

The mode of reproduction in which a new individual develops from an unfertilized ovum without the participation of a sperm cell is called parthenogenesis.

An organism that develops from an unfertilized egg by parthenogenesis is called a parthenogenome or parthenote.

The periodic shedding of the uterine lining that occurs at the beginning of each menstrual cycle in human females is called menstruation

The reproductive cycle in most non-primate mammals in which females are sexually receptive and permit copulation only during a specific breeding period is called the oestrus cycle.

• Meaning - Reproduction is the production of young ones similar to their parents. It is an essential biological process for the continuation of the species and continuity of life.
• Lifespan = period from birth to natural death; ranges from 1 day (Mayfly) to 3,000–4,000 years (Sequoia).
• Two Types - All methods of reproduction fall into two categories: Asexual Reproduction and Sexual Reproduction.
• Single-celled organisms (Amoeba, Bacteria) do not undergo natural death — they divide by binary fission.
• Four life stages: Juvenile → Reproductive → Senescent → Death.
• In plants, the reproductive phase begins at flowering.
• Monocarpic plants flower once; Polycarpic plants flower every season.
• Asexual reproduction = one parent, clones produced, faster.
• Sexual reproduction = two parents, gametes, zygote formed, genetic variation.
• The three phases of sexual reproduction: Pre-fertilisation → Fertilisation → Post-fertilisation.

Modes of Reproduction refer to the various biological strategies by which organisms produce offspring to ensure the continuation of their species.

• There is enormous diversity in reproductive mechanisms across living organisms.
• The mode of reproduction an organism uses depends on:

• Meaning: Asexual reproduction involves only one parent, produces no fusion of gametes, and results in offspring that are genetically identical to the parent, called clones.
• Modes of Asexual Reproduction: Common methods include Binary Fission (Amoeba, Paramoecium), Fragmentation (Spirogyra), Budding (Yeast, Protosiphon), Spore Formation/Zoospores (Chlamydomonas), Conidia (Penicillium), and Gemmules (Marchantia).
• In Plants: Asexual reproduction in plants is called Vegetative Propagation. It can be Natural or Artificial (e.g., cutting, grafting, tissue culture).
• In Animals: Common in lower animals only. Does not involve meiosis or gamete formation. Lower animals reproduce asexually by budding and gemmule formation.
• Key Feature: Since only one parent is involved and no fusion occurs, all offspring are morphologically and genetically identical (clones) to the parent.

• Simple binary fission is an asexual reproduction method in which a unicellular organism, such as an Amoeba, divides along a random plane to produce two genetically identical daughter cells.
• The process begins with cell growth and DNA replication, ensuring that the parent cell synthesises the necessary proteins and creates an exact duplicate of its genetic material.
• Following replication, the cell undergoes nuclear division (mitosis), during which the nuclear membrane dissolves and the duplicated chromosomes separate into two distinct daughter nuclei.
• The final step is cytokinesis, wherein a cleavage furrow forms randomly, pinching the cell membrane inward to divide the cytoplasm and release two independent, fully functional daughter cells.

• Transverse binary fission is an asexual reproduction process in which an organism divides horizontally along its transverse axis to produce two distinct individuals.
• In Paramecium, the larger macronucleus divides amitotically while the smaller micronucleus divides mitotically, distributing one pair of each to the anterior and posterior ends of the cell.
• Following nuclear division, a horizontal constriction forms in the middle of the Paramecium and deepens until the cell splits completely into two independent daughter paramecia.
• Planaria (flatworms) also utilise this method by developing a constriction behind the pharynx and splitting in two, with each resulting half regenerating its missing segments to form a complete organism.

• A unicellular organism divides along its longitudinal (vertical) axis, splitting into two daughter cells - an asexual reproduction process, as seen in Euglena.
• The cell first suspends locomotion (flagellum sometimes withdrawn), then the blepharoplast splits, followed by eumitotic nuclear division where the nuclear membrane fully dissolves.
• A longitudinal cleavage furrow begins at the reservoir on the anterior end and proceeds down the body, gradually dividing the cell into two.
• The furrow deepens until two independent daughter euglenae are formed, with any lost parts regenerated immediately after division.

• In multiple fission, a single parent divides into many daughter cells at once by first forming multiple nuclei, followed by cytoplasmic division around each.
• In Amoeba, under unfavourable conditions, it withdraws its pseudopodia and secretes a three-layered chitinous cyst around itself - a process called encystation.
• Inside the cyst, the nucleus divides repeatedly into many daughter nuclei, each gathering a little cytoplasm to form tiny daughter cells called amoebulae (pseudopodiospores), which are released as adults when favourable conditions return.
• In Plasmodium, schizogony occurs in the human liver and red blood cells, producing many merozoites by multiple fission.
• In Plasmodium, sporogony occurs in the oocyst inside the female Anopheles mosquito, where multiple fission produces sporozoites.

• In unicellular organisms like yeast, a small bud forms on the parent cell, receives a nucleus, grows, and separates to become a new cell.
• A small projection appears on the parent cell's surface and enlarges into a bud, while the nucleus divides and one nucleus moves into the bud.
• The bud continues to grow while attached to the parent, then the mature bud separates to become an independent new yeast cell.

• In multicellular organisms like Hydra, a bud grows on the parent body through cell division and later detaches to become a new individual.
• The bud arises from repeated multiplication of epidermal interstitial cells, contains both epidermis and gastrodermis, and its cavity stays continuous with the parent's gastrovascular cavity.
• As it grows, the bud develops a mouth and tentacles, then separates and attaches to a surface by its basal disc to live independently as a new Hydra.

• Freshwater sponges like Spongilla (and some marine sponges) reproduce asexually by forming specialised internal buds called gemmules, unlike Hydra, which reproduces by external budding.
• A gemmule has a central mass of archaeocytes - undifferentiated cells capable of forming a new sponge - surrounded by a resistant protective coat secreted by amoebocytes.
• Under unfavourable conditions such as drying or thermal stress, archaeocytes aggregate within the sponge body, and a protective layer forms around them, developing into a dormant, resistant gemmule.
• When favourable water and temperature conditions return, the gemmule germinates, and its archaeocytes become active, forming a new sponge.

• Regeneration is the process by which an organism regrows lost or damaged body parts - or forms a complete new individual from a body fragment - using specialised cells that multiply repeatedly and differentiate into various tissues.
• In Hydra and Planaria, a cut body fragment can regenerate the missing parts and grow into a complete organism (genetically similar clones), making it a form of asexual reproduction.
• Starfish also show strong regenerative ability, where a body fragment can, in some cases, grow into a complete new starfish, reflecting the high regenerative capacity of lower animals.

• When the body of a multicellular organism breaks into two or more fragments, each fragment grows into a new individual - a common, efficient method of asexual reproduction.
• In filamentous algae like Ulothrix and Spirogyra, the thallus breaks into small fragments, and each grows independently into a new thallus.
• In fungi, a small fragment of the hypha detaches from the parent mycelium and develops into a new individual.

• A spore is a reproductive structure in simple plants and fungi that germinates directly into a new individual; it may be motile (zoospores) or non-motile (aplanospores).
• In algae like Chlamydomonas, the protoplast divides into 4–8 parts, each forming a zoospore that grows into a new individual.
• Fungi reproduce asexually by two spore types - sporangiospores and conidia.
• Sporangiospores form inside a sporangium: motile zoospores (e.g., Pythium) and non-motile aplanospores (e.g., Rhizopus).
• Conidia are non-motile spores formed externally on conidiophores, as in Aspergillus and Penicillium.

• Vegetative propagation is an asexual method where a new plant regenerates from root, stem, leaf, or bud, forming offspring genetically identical to the parent (a clone).
• It is rapid, easy, and inexpensive, useful for plants with poor seed viability or that cannot form seeds, like banana, seedless grapes, and rose.
• It preserves the parent's characters (mango, citrus, litchi) and aids grafting and disease-free micropropagation.
• Drawbacks: propagules decay easily, are prone to disease, show no variation, and cause overcrowding due to poor dispersal.

• Sexual reproduction involves the formation and fusion of male and female gametes (amphimixis) through meiotic division to create a diploid zygote.
• Life cycles progress from an initial juvenile phase focused purely on vegetative growth and development to a mature reproductive phase in which sex organs become functional.
• The reproductive process is strictly regulated by hormones and sequence-driven, consisting of three sequential stages: pre-fertilisation, fertilisation, and post-fertilisation.
• Pre-fertilisation focuses on gamete production and transfer, while post-fertilisation events track zygote development and the formation of embryonic structures.
• Meiotic cell division and cross-fertilisation blend parental genetic material, generating the vital variations needed to drive species adaptation and organic evolution.

• Pre-fertilization events occur before fertilization and mainly include gametogenesis and gamete transfer.
• Gametogenesis produces haploid gametes, which may be isogametes or heterogametes.
• In haploid organisms, gametes are formed by mitosis; in diploid organisms, they are formed by meiosis.
• In lower plants and animals, gamete transfer often occurs through water.
• In seed plants, pollen grains carry male gametes to the ovule through pollination.
• In higher animals, sperms are transferred directly to the female body during copulation.

• Fertilization is the fusion of male and female gametes to form a diploid zygote; the actual union of the gametes is called syngamy.
• In some organisms (rotifers, honeybees, and turkeys), the female gamete develops into a new organism without fertilization, which is called parthenogenesis.
• External fertilization occurs outside the body in water (algae, bony fishes, amphibians); many gametes are released, but offspring are vulnerable to predators.
• Internal fertilization occurs inside the female body (bryophytes to angiosperms; reptiles, birds, mammals); motile male gametes are produced in large numbers.
• In flowering plants, the embryo sac lies inside the ovule, so pollen on the stigma forms a pollen tube to carry the male gametes to it.

• After the juvenile phase, animals undergo changes leading to reproductive maturity, then produce gametes in organs called gonads - testis in males (sperms), ovary in females (ova).
• By distribution of reproductive organs, animals are grouped into unisexual and hermaphrodite forms.
• In unisexual animals, sexes are separate, as in cockroaches, humans, and birds.
• In hermaphrodite animals, both sex organs occur in the same individual (earthworm, tapeworm, leech), but cross-fertilization is common.

• Parthenogenesis is the development from an unfertilized egg.
• Sperm cells do not participate in this process.
• Charles Bonnet discovered it in animals in 1745.
• It occurs naturally in insects, crustaceans, rotifers, some lizards, and platyhelminthes.
• It is rare in vertebrates.
• Honey bee is the standard example: drones arise from unfertilized eggs, while queen and workers arise from fertilized eggs.