Reproduction in Organisms

Reproduction is essential for organisms because: 1. Continuation of species: It ensures the survival of a species across generations. Without reproduction, a species would become extinct when individual organisms die. 2. Replacement of dead individuals: Since all organisms have a finite life span, reproduction replaces dead individuals and maintains population size. 3. Transmission of genetic information: Reproduction passes genetic information (DNA) from one generation to the next, ensuring continuity of traits. 4. Evolution and adaptation: Sexual reproduction introduces genetic variation, which is the raw material for natural selection and evolution, enabling species to adapt to changing environments. Note: Reproduction is NOT essential for the survival of an individual organism (unlike nutrition and respiration), but it is essential for the survival of the species.

Both modes have advantages depending on the context. However, sexual reproduction is considered 'better' in an evolutionary sense: Advantages of Sexual Reproduction: 1. Produces genetic variation (through meiosis and random fertilisation) 2. Genetic variation is the raw material for natural selection and evolution 3. Helps populations adapt to changing environments 4. Harmful mutations can be eliminated over generations 5. Produces offspring with new gene combinations Advantages of Asexual Reproduction: 1. Faster — produces many offspring in a short time 2. Only one parent needed 3. Produces genetically identical offspring (useful when parent is well-adapted) 4. No need to find a mate 5. Energetically cheaper Conclusion: Sexual reproduction is evolutionarily superior due to genetic variation it creates. However, asexual reproduction is advantageous in stable environments and for rapid population growth.

The offspring formed by asexual reproduction are referred to as clones because: 1. They are produced from a single parent without the fusion of gametes. 2. Since no meiosis or fertilisation occurs, there is no recombination of genetic material. 3. The offspring are genetically identical to the parent and to each other — they have the exact same DNA sequence. Definition: A clone is a group of genetically identical individuals derived from a single individual by asexual means. Examples: • A bacterial cell dividing by binary fission — both daughter cells are clones • A Hydra reproducing by budding — the bud is a clone of the parent • Vegetative propagation in plants — all new plants are clones of the parent Note: Mutations can occasionally cause minor differences, but clones are essentially genetically identical.

(a) Mango (Mangifera indica): Vegetative reproduction — though mango reproduces mainly sexually via seeds. Asexual propagation by grafting (stem cutting) is done horticulturally, but natural asexual structures are limited. (b) Yeast: Budding — small outgrowths (buds) develop on the parent cell, enlarge, and eventually detach as new yeast cells. Under unfavourable conditions, yeast forms spores. (c) Planaria (flatworm): Fragmentation and Regeneration — the body breaks into fragments, and each fragment regenerates into a complete new organism. (d) Moss (Bryophyta): • Gemmae — small, multicellular, disc-like asexual propagules produced in gemma cups (e.g., in Marchantia, a liverwort) • Fragmentation of protonema • Vegetative propagules

Vegetative propagation is a type of asexual reproduction in plants where new plants develop from vegetative parts (roots, stems, leaves, buds) of the parent plant — NOT from seeds. Key features: • New plants are genetically identical to parent (clones) • Used widely in horticulture and agriculture • Faster than seed germination • Preserves desirable characteristics of parent Examples: 1. Potato (Solanum tuberosum): The underground stem (tuber) bears 'eyes' (axillary buds). When the tuber is planted, each eye grows into a new plant. This is stem-based vegetative propagation. 2. Bryophyllum (Sprout-leaf plant): Adventitious buds develop along the notches of leaves. When the leaf falls to the ground, each bud grows into a new plant. This is leaf-based vegetative propagation. Other examples: Runners in strawberry, bulbs in onion and garlic, rhizomes in ginger.

(a) Juvenile phase (also called vegetative phase in plants): The period of growth from birth/germination until the organism becomes capable of sexual reproduction. The organism develops physically but cannot reproduce sexually. Example: A child growing until puberty; seedling growing until it can flower. (b) Reproductive phase: The period of life during which the organism is capable of sexual reproduction. It begins at sexual maturity. • In humans: from puberty to menopause (in females) / old age (in males) • In plants: the flowering and fruiting phase • In some organisms: breeding seasons (c) Senescent phase (old age): The phase following the reproductive phase, characterised by progressive deterioration of body functions, reduced metabolism, and eventual death. • Cells lose their ability to divide • Physiological processes slow down • Immunity decreases • Ends with the death of the organism

Oestrus Cycle: • Occurs in non-primate mammals (e.g., dogs, cows, sheep, rats, deer) • The uterine endometrium is reabsorbed if fertilisation does not occur (no bleeding) • Female is sexually receptive only during oestrus (heat period) • Seasonal in some species (e.g., sheep — once a year) • Little or no visible bleeding Menstrual Cycle: • Occurs in primates (humans, apes, monkeys) • If fertilisation does not occur, the thickened endometrium is shed along with blood — menstruation (visible bleeding) • Female is not restricted to specific periods of sexual receptivity • Cycle length: ~28 days in humans • First menstruation: menarche; last: menopause Key difference: In oestrus, unfertilised endometrium is reabsorbed; in menstrual cycle, it is shed as menstrual flow.

(a) Asexual reproduction through fragmentation is observed in: Spirogyra (filamentous algae), Planaria (flatworm), some fungi and lichens. (b) Offspring formed by sexual reproduction have better chances of survival than those formed by asexual means — because genetic variation (produced by sexual reproduction) helps in adaptation to changing environments. They can better withstand diseases and environmental changes. (c) In higher plants, the unit of sexual reproduction is the flower (specifically, the seed after fertilisation, or the gametes — pollen grain and ovule).