How do Organisms Reproduce? — 50 Q&A

Reproduction is the biological process by which organisms produce new individuals of the same kind. It is necessary for species survival and continuity across generations.

There are asexual reproduction (single parent; no fusion of gametes) and sexual reproduction (involves fusion of male and female gametes).

• Binary fission (e.g., amoeba)
• Budding (e.g., Hydra, yeast)
• Fragmentation (e.g., Spirogyra)
• Spore formation (e.g., fungi)
• Vegetative propagation (plants — runners, tubers, bulbs).

Binary fission is division of a single organism into two equal daughter organisms. Example: Amoeba divides its nucleus followed by cytoplasmic division to form two amoebae.

In budding, a new organism develops as an outgrowth (bud) from parent and may detach (yeast) or remain attached forming colonies (Hydra).

Vegetative propagation is asexual reproduction in plants using vegetative parts. Natural: runners in strawberry, tubers in potato. Artificial: cutting, grafting, layering used in horticulture.

Fungi produce numerous spores that disperse through air or water; when conditions are favourable, spores germinate to form new fungal individuals.

Fragmentation is breaking of an organism into pieces, each growing into a new individual. Spirogyra filaments break into fragments and each fragment develops into a new filament.

Advantages: fast, no mate needed, many offspring. Disadvantages: low genetic variation (less adaptability), harmful mutations passed on.

Sexual reproduction involves meiosis to form gametes, fusion (fertilisation) of male & female gametes to form a zygote. It produces genetic variation among offspring.

Gametes are haploid sex cells. Male gamete: sperm (motile in many animals); female gamete: egg or ovum (non-motile). Fusion restores diploid chromosome number.

External fertilisation: gametes fuse outside body (e.g., many fishes, frogs) — often many gametes released. Internal fertilisation: fusion inside female body (e.g., humans, birds), usually fewer gametes and more parental care.

Pollination is transfer of pollen from anther to stigma. Agents include wind (anemophily), insects (entomophily), birds, bats, and humans (artificial pollination).

Self-pollination: pollen from anther to stigma of same flower/plant (less variation). Cross-pollination: pollen to stigma of different plant (increases genetic variation).

Male: stamen (anther + filament) produces pollen. Female: pistil/carpel (stigma + style + ovary containing ovules). Petals & sepals protect & attract pollinators.

Double fertilisation: one male nucleus fuses with egg to form zygote (embryo) and another fuses with two polar nuclei to form endosperm (food reserve) — unique to flowering plants.

After fertilisation, the zygote develops into embryo inside ovule, endosperm forms as food, seed coat forms — ovary matures into fruit protecting seeds and aiding dispersal.

• Wind (anemochory): dandelion, coconut (also water)
• Water (hydrochory): coconut
• Animals (zoochory): burrs cling, fruits eaten & dispersed
• Explosive (ballistic): Balsam
• Gravity: seeds falling near parent.

Fragmentation allows parts of algae/seaweeds to regrow into full individuals, helping rapid colonisation and recovery after breakage.

Regeneration is regrowth of lost body parts (e.g., starfish regrow arms, planaria). Capacity varies — some animals can regenerate whole bodies from fragments, others cannot.

Parthenogenesis is development of an egg without fertilisation (virgin birth). Seen in some insects, bees (haploid males from unfertilised eggs) and some lizards.

Hermaphrodites possess both male and female reproductive organs (e.g., earthworm). Many hermaphrodites still practice cross-fertilisation between two individuals.

Alternation of generations: plant life cycle alternates between a multicellular haploid gametophyte (produces gametes) and a multicellular diploid sporophyte (produces spores). Seen in bryophytes and pteridophytes.

Meiosis produces haploid gametes and introduces variation via crossing-over (exchange of chromosome segments) and independent assortment of chromosomes — leading to diverse offspring.

• Two modes: asexual (one parent) and sexual (gametes, meiosis, fertilisation).
• Plants: pollination, double fertilisation, seed & fruit formation.
• Humans: internal fertilisation, placenta, menstruation, contraception, reproductive health.
• ART & cloning are modern technologies with benefits and ethical issues.