Reproduction: How Life Continues — Detailed Notes
Reproduction is the biological process by which living beings produce new individuals of their own kind, ensuring the continuity of life and the transfer of genetic information. It occurs in two main ways — asexual (one parent, identical offspring) and sexual (two parents, offspring with mixed characters and variation).
11.1 Asexual reproduction
Seen in bacteria, amoeba, yeast, hydra, fungi and many plants. The offspring are genetically identical clones, produced by mitosis. Types:
• Vegetative propagation (plants) — new plants grow from vegetative parts: potato/ginger (underground stems), Bryophyllum (leaf plantlets), stem cuttings. Horticultural methods: cutting, grafting, layering, tissue culture — used to mass-produce desirable, identical crops (e.g. virus-free banana plantlets). • Budding — a bud grows on the parent and detaches (yeast, hydra). • Spore formation — fungi (Rhizopus, Aspergillus) make millions of light, airborne spores that germinate in moisture. (Louis Pasteur showed life comes only from pre-existing life.)
11.2 Sexual reproduction & meiosis
Two parents contribute genetic material. To stop the chromosome number doubling each generation, gametes are made by meiosis, which halves the chromosome number (diploid → haploid). Humans: body cells have 46 (23 pairs) chromosomes; gametes have 23. The random mixing of chromosomes in meiosis (and fertilisation) creates variation, which helps species adapt and evolve.
11.2.2 Sexual reproduction in flowering plants
A flower has four parts: sepals (green, protect the bud), petals (colourful, attract pollinators), stamen (male — filament + anther, makes pollen with male gametes), and pistil (female — stigma, style, ovary; ovary has ovules each with an egg).
Pollination = transfer of pollen from anther to stigma. Self-pollination (same/another flower of the same plant) or cross-pollination (between different plants of the same kind). Agents (pollinators): wind (wheat, maize, rice — light pollen, feathery stigma), water (Vallisneria, Hydrilla), insects (sunflower, hibiscus — bright, scented, nectar, sticky pollen), birds. Fertilisation: the pollen grows a pollen tube down the style; the male gamete fuses with the egg → zygote → embryo. The ovule becomes the seed and the ovary becomes the fruit.
11.3–11.4 Reproduction in animals
External fertilisation — outside the body, in water (fish, frogs); many eggs laid but low survival. Internal fertilisation — inside the female body (reptiles, birds, mammals); fewer eggs/young but higher survival (embryo better protected). Eggs of fish/amphibians/insects have yolk enough to form a larva (a feeding stage, e.g. butterfly: egg → larva → pupa → adult). Mammals develop the young inside the mother and feed them milk.
11.5 Reproduction in human beings
At puberty the reproductive organs mature and produce gametes. Male system: testes (make sperm + hormone, kept cool in the scrotum), vas deferens, urethra, glands (seminal vesicle, prostate). Female system: ovaries (make eggs + hormones), oviducts (fallopian tubes), uterus, cervix, vagina. Gametogenesis (by meiosis) makes millions of tiny motile sperm and few large non-motile eggs.
Menstrual cycle (≈28 days): the uterus lining thickens; ovulation (egg released) occurs around day 14. If a sperm fertilises the egg → a zygote forms, divides, and implants in the uterus → pregnancy (~9 months, 3 trimesters; embryo → foetus). If not fertilised, the lining sheds as menstruation (3–7 days).
Reproductive health: sexual maturity comes before emotional maturity; responsible choices prevent unplanned pregnancies and STIs (e.g. gonorrhoea, syphilis, HIV→AIDS). Contraceptive methods: barriers (condoms — also prevent STIs), oral pills (hormonal), IUDs (Copper-T), and surgical methods. Prenatal sex determination is illegal in India (to keep a healthy sex ratio).
Key Terms
reproduction, asexual/sexual, vegetative propagation, budding, spore formation, clone, mitosis, meiosis, gamete, variation, sepal/petal/stamen/pistil, pollination (self/cross), fertilisation, zygote, seed, fruit, external/internal fertilisation, puberty, gametogenesis, ovulation, menstrual cycle, pregnancy, STI, contraception.
Revise, Reflect, Refine — All NCERT Exercise Answers
1A flower’s anthers are removed before it matures; later pollen from another plant of the same species is dusted on its stigma and seeds form. Which process has been ensured?
(ii) Cross-pollination. Removing the anthers prevents the flower’s own pollen being used (no self-pollination); dusting pollen from another plant of the same species onto the stigma ensures pollen transfer between two different plants, which is cross-pollination. (Fertilisation then follows, but the step deliberately ensured here is cross-pollination.)
2Arrange in correct order: (i) pollen germination on stigma (ii) fertilisation (iii) pollination (iv) formation of zygote.
Correct order: (iii) Pollination → (i) Pollen germination on stigma → (ii) Fertilisation → (iv) Formation of zygote. Pollen first reaches the stigma (pollination), germinates to grow a pollen tube, the male gamete fuses with the egg (fertilisation), producing the zygote.
3Assertion: the zygote formed after fertilisation immediately attaches to the uterus wall. Reason: the uterus wall is always prepared to receive the zygote. Choose the option.
Both A and R are false. The zygote does not attach immediately — it first undergoes several mitotic divisions while travelling down the oviduct and implants in the uterus only after some days. And the uterus wall is not always prepared — its lining thickens cyclically (around ovulation) and sheds during menstruation if there is no fertilisation. So neither statement is correct. (Of the given options none fits a ‘both false’ case; the key point is that both A and R are false.)
4Why does asexual reproduction produce offspring that are genetically identical to the parent?
Asexual reproduction involves a single parent and uses mitosis, which produces daughter cells with exactly the same number and kind of chromosomes (same DNA) as the parent cell. Since there is no mixing of genetic material from two parents, the offspring are genetically identical copies of the parent, called clones.
5Explain why the menstrual cycle stops during pregnancy.
If fertilisation occurs, the zygote implants in the thick uterine lining, which is needed to nourish and protect the developing embryo. Hormones maintain this lining so it is not shed, and they also stop further ovulation. Since menstruation is the shedding of the unused lining when there is no pregnancy, it does not occur while the lining is supporting the growing baby — so the menstrual cycle pauses during pregnancy.
6Why are night-blooming flowers usually white or light-coloured compared to day-blooming flowers?
Night-blooming flowers are pollinated by night-active pollinators (such as moths and bats). In the dark, bright colours cannot be seen, but white or light-coloured petals reflect the little available light and stand out, making the flowers easy to locate. (They also rely strongly on strong fragrance to attract pollinators at night.)
7Why are vegetatively propagated plants more vulnerable to diseases than sexually reproduced plants?
Vegetatively propagated plants are clones — all genetically identical, with no variation. So if a disease can attack one plant, it can attack all of them equally, and a single epidemic can wipe out the whole crop. Sexually reproduced plants have genetic variation, so at least some individuals are likely to resist a given disease, helping the population survive.
8If all flowers of a plant were capable of only self-pollination, how would genetic diversity change over generations? Explain.
With only self-pollination, the offspring receive genes from a single parent each time, so there is very little mixing of different genetic material. Over many generations the population becomes more and more genetically uniform — genetic diversity decreases. This makes the plants less able to adapt to changing conditions and more vulnerable to diseases and environmental stress.
9A farmer wants many genetically identical plants quickly. Suggest suitable methods and explain why they work.
Use asexual (vegetative) methods — cuttings, grafting, layering, or tissue culture (micropropagation). They work because asexual reproduction uses mitosis and a single parent, so every new plant is a genetically identical clone that keeps the parent’s desirable traits. Tissue culture in particular can rapidly produce large numbers of healthy, disease-free, identical plantlets (as in banana farming).
10Suresh germinates pollen in different sugar concentrations (0, 2.5, 5, 7.5, 10%). (i) hypotheses that can be tested (ii) parameters to keep the same.
(i) Hypotheses: ‘Pollen germination depends on the sugar concentration of the medium’; and ‘there is an optimum sugar concentration at which germination is maximum’ (too low or too high reduces germination). (ii) Keep the same (controls): the same type and age of pollen, the same amount of pollen per slide, the same temperature, humidity, light, time of observation and method of preparing the slides — only the sugar concentration should vary.
11Identify the likely pollination type: tomato (stamens cover stigma, flowers open after pollination), wheat, papaya (male & female flowers on different trees).
Tomato — the stamens enclose the stigma and the flower opens only after pollination, so pollen reaches its own stigma → self-pollination. Wheat — light pollen carried by wind to other flowers → cross-pollination (wind). Papaya — male and female flowers are on different trees, so pollen must travel from one tree to another → cross-pollination.
12Apple orchards: Place A uses natural pollinators, Place B adds beekeeping. Fruit-setting and fruit-drop data are compared (Fig. 11.24). (i) hypothesis (ii) parameters (iii) compare (iv) inference.
(i) Hypothesis: increasing pollinators (by beekeeping) improves apple yield — it raises fruit setting and lowers fruit drop. (ii) Parameters: the independent variable is the pollination method (natural vs beekeeping); the dependent variables are fruit setting (%) and fruit drop (%); controlled variables are apple variety, climate, soil and orchard care. (iii) Comparison: Place B (with beekeeping) shows higher fruit setting and lower fruit drop than Place A. (iv) Inference: more pollinators lead to better pollination and higher yield, so beekeeping (mixed farming) improves apple production while also giving honey.
13A student claims ‘in humans, ovulation always happens on day 14 of the menstrual cycle.’ Is this correct? Give at least two reasons.
The claim is not correct. Reason 1: the menstrual cycle length varies from person to person (typically 21–35 days, not always 28), so day 14 is only the midpoint of an idealised 28-day cycle. Reason 2: the exact day of ovulation varies even in the same person, affected by stress, health, nutrition and hormonal changes. So ovulation is only around the middle of the cycle, not fixed on day 14.