How do Organisms Reproduce?

Reproduction is not needed for the survival of an individual the way nutrition or respiration is. A single Amoeba does not die if it never divides. Reproduction matters for the species, because it produces more individuals of the same kind and so keeps the population going.

Why do offspring look like their parents? Because the blueprint of body design sits in the DNA in the nucleus of the cell. DNA carries the information for making proteins, and proteins decide the body design. So the first and most basic event in any reproduction is DNA copying (replication): the cell makes a second copy of its DNA, and cellular machinery then separates the two copies into two cells. This is why reproduction at its most basic level is a cellular process.

The copying is remarkably accurate but never perfect. Small errors slip in. If the copied DNA is very different from the original, the proteins made from it will be different, the body design will not work and the new cell will die. But if the change is small, the new cell survives and is similar but subtly different from its parent. This is the origin of variation. NCERT's line to remember: DNA copying is accompanied by the creation of additional cellular apparatus, and the two are then separated.

The examiner wants two ideas in one sentence: DNA copying keeps the body design of the species stable from one generation to the next, and the inevitable small errors in copying are the source of variation on which evolution works.

A population lives in a particular niche: a set of conditions like temperature, water, food and light. Reproduction produces individuals suited to that niche because they carry the same basic design. But niches change. The temperature of the water may rise, a new disease may appear, a river may dry up. If every individual in a population were identical, one such change could wipe out the whole population.

Variation is the insurance. If a few individuals carry a variation that lets them survive at a higher temperature, they live on and the species continues, even though most individuals die. So variation is useful for the survival of the species over time, even though it does not guarantee the survival of any particular individual, and may not help an individual at all. This is exactly the wording of the NCERT in-text question, so learn to answer it: variation is beneficial to the species, not necessarily to the individual.

NCERT gives the example of bacteria living in temperate water. Suppose the water temperature rises because of global warming. Most bacteria would die, but a few variants resistant to heat would survive and grow. Without variation, none would.

Connect this with Chapter 8 (Heredity): variations that are inherited accumulate over generations, and the useful ones are selected by the environment. The board often asks a 1-mark Assertion–Reason pairing 'asexual reproduction gives little variation' with 'the offspring come from a single parent by DNA copying'. Both are true and the reason explains the assertion, so the answer is (a).

Asexual reproduction means a single parent produces offspring without gametes. The offspring are almost identical to the parent. The mode used depends on the body design of the organism.

Binary fission. Unicellular organisms simply split into two. In Amoeba the cell can divide in any plane: the nucleus divides first, then the cytoplasm pinches into two daughter cells. In Leishmania (the protozoan that causes kala-azar) there is a whip-like flagellum at one end of the cell, so the cell has a definite orientation and binary fission takes place in a definite plane, longitudinally, with respect to the flagellum. Bacteria and Paramecium also divide by binary fission.

Multiple fission. In Plasmodium, the malarial parasite, the nucleus divides repeatedly inside the cell to form many nuclei, each becomes surrounded by a little cytoplasm, and the parent cell then bursts to release many daughter cells at once. This is asexual reproduction. It usually happens under unfavourable conditions, when the cell is surrounded by a protective cyst.

The board asks for the difference in a table. Binary fission: parent splits into two, occurs in favourable conditions, no cyst, examples Amoeba, Leishmania, bacteria. Multiple fission: parent splits into many, often in unfavourable conditions, cell wall or cyst forms, example Plasmodium. A related 3-marker: 'Differentiate between binary fission in Amoeba and Leishmania.' Answer: Amoeba divides in any plane because it has no fixed body orientation; Leishmania divides in a definite plane because of the flagellum.

For the 5-mark version, draw the three stages of binary fission in Amoeba (single cell with nucleus, nucleus elongated and divided into two, cytoplasm constricting to form two daughter Amoebae) and describe them in words.

Some simple multicellular organisms with a very basic body organisation reproduce by fragmentation. The body breaks up into two or more pieces on maturation, and each piece grows into a complete new individual. The example every paper uses is Spirogyra, a filamentous green alga found in ponds. When a filament of Spirogyra attains considerable length it simply breaks into smaller fragments, and each fragment grows into a new filament.

Why can Spirogyra do this when a dog cannot? Because Spirogyra's body is a simple chain of similar cells with no tissues or organs. Any piece already contains all it needs to grow. The 2017 board question 'Name the method by which Spirogyra reproduces under favourable conditions' expects the one-word answer fragmentation. The 2022 question 'What happens when a mature Spirogyra filament attains considerable length?' expects: it breaks into fragments and each fragment grows into a new individual.

Note the contrast NCERT makes: multicellular organisms are not just random collections of cells. Specialised cells are organised into tissues, tissues into organs, organs into organ systems, each placed at a definite position in the body. Cell-by-cell division cannot rebuild that organisation, so complex organisms cannot reproduce by fragmentation. They need a single cell type that can proliferate and produce all the other cell types in the right order. This is the answer to the NCERT in-text question on why complex organisms cannot give rise to new individuals through regeneration, and it is also the reason for the 2024 'give a reason' 5-marker.

Many fully differentiated organisms can give rise to new individuals from their body parts: if the organism is somehow cut or broken into pieces, many of the pieces grow into separate individuals. This is regeneration. The textbook examples are Hydra and Planaria (a flatworm). Cut a Planaria into pieces and each piece grows into a complete organism.

How it works: regeneration is carried out by specialised cells. These cells proliferate (divide rapidly) and make large numbers of cells. From this mass of cells, different cells undergo changes to become various cell types and tissues, and these changes take place in an organised sequence known as development.

NCERT is careful to say that regeneration is not the same as reproduction, because most organisms would not normally depend on being cut up to reproduce. Hydra, for instance, reproduces by budding as its regular method; regeneration is a capacity it has, not its usual mode. Mention this distinction in a 3-mark answer and you look like you have read the book.

Why can't complex animals do it? Regeneration works in organisms with a simple body organisation. In a human or a dog, cells are organised into tissues, organs and organ systems in a fixed pattern; a lost limb cannot rebuild a whole body, and a random piece of the body does not contain the organising instructions or the cell types to make all the others. Complex organisms therefore need a single specialised cell type (the germ cell) that can proliferate and give rise to all other cell types in the correct sequence.

Typical 3-marker (CBSE 2017): 'What happens when Planaria gets cut into many pieces?' Answer: each piece regenerates the missing parts and grows into a complete Planaria by regeneration, because specialised cells proliferate and then differentiate in an organised sequence.

In budding a small outgrowth, the bud, forms on the parent body by repeated cell division at one specific site. The bud develops into a tiny individual and, when fully mature, detaches from the parent body and becomes a new independent individual.

Hydra uses regenerative cells for this. A bud develops as an outgrowth on the body wall, grows tentacles and a mouth while still attached, and then separates. Several buds can be present on one Hydra at once. The 5-mark question 'Name and explain two modes of asexual reproduction in Hydra' (CBSE 2023) wants budding as the regular mode and regeneration as the second, each explained in two or three lines, with the budding diagram.

Yeast is the unicellular example. A single-celled fungus, yeast forms a small bulb-like projection (bud) on the cell; the nucleus divides and one nucleus moves into the bud; the bud grows and either separates or stays attached to form a chain of buds. This is the answer to NCERT exercise question 1: asexual reproduction through budding takes place in yeast, not in Amoeba, Plasmodium or Leishmania.

How to draw budding in Hydra for full marks: three or four stages. (1) Parent Hydra with tentacles and a small bulge on the side of the body. (2) The bulge enlarged, now with tiny tentacles. (3) A fully formed small Hydra still attached. (4) The daughter detached, living independently. Label parent, bud, developing tentacles, daughter Hydra. In the exam the diagram plus three sentences of explanation earns the full 3 marks for that part.

In many plants, parts like the root, stem and leaves develop into new plants under appropriate conditions. This is vegetative propagation. It is natural in some plants and used artificially by farmers and gardeners through layering (bending a stem to the ground so it roots) and grafting (joining a shoot of one plant to the rooted stem of another). Sugarcane, roses and grapes are grown this way; so are banana, jasmine, potato, ginger and orange.

The famous natural example is Bryophyllum (the 'leaf of life'). Buds are produced in the notches along the leaf margin. When a leaf falls on moist soil, each bud develops into a new plant. In the 2017 3-marker 'What happens when a Bryophyllum leaf falls on wet soil?' the expected answer is exactly this. Money plant and sweet potato are other examples the board accepts.

Advantages, which the board asks for almost every year (list three for a 5-marker):

• Plants that have lost the capacity to produce seeds, such as banana, orange, rose and jasmine, can still be propagated.
• Plants raised this way bear flowers and fruits earlier than plants raised from seed.
• All the plants produced are genetically similar to the parent, so a desirable variety (a good mango, a seedless grape) is preserved exactly.
• It is a quicker and easier method that does not depend on pollinators or seed germination.

Disadvantage: no variation, so the crop is uniformly vulnerable to the same disease or environmental change. The 2025 MCQ asked which statement about vegetative propagation is incorrect; the wrong one is 'it produces variations'.

NCERT also describes tissue culture: cells or tissue from the growing tip of a plant are placed in an artificial medium, they divide to form a callus (an unorganised mass of cells), the callus is transferred to a medium with hormones for growth and differentiation, and the plantlets are then transplanted. Ornamental plants are commonly grown this way.

NCERT describes the bread mould Rhizopus. On a slice of wet bread left in a warm, moist place, thin thread-like structures spread over the surface. These threads are the hyphae, and they are not reproductive parts; they absorb food from the bread. Standing up from the hyphae are tiny stalks (sporangiophores) that carry blob-like knobs, the sporangia. Each sporangium contains hundreds of spores, which are the reproductive parts. So for the 2022 and 2023 board questions: vegetative or non-reproductive part = hyphae; reproductive part = sporangium (bearing spores).

The spores are covered by thick walls that protect them until they meet a moist surface. When the sporangium bursts on maturation (CBSE 2017), the spores are released and carried away by air or water. On moist, warm bread a spore germinates and grows into a new Rhizopus individual. This is why the 2018 MCQ listed the factors for rapid spread of bread mould as a large number of spores plus availability of moisture and nutrients on the bread.

How is the organism benefited by spores? Answer for the NCERT in-text question, three points: spores are produced in very large numbers, so the chance of at least some surviving is high; the thick wall protects the spore from drying and other unfavourable conditions until conditions improve; and spores are light and are dispersed far by air and water, so the organism spreads to new places.

Diagram for the 5-marker (CBSE 2024): draw a slice of bread with a mesh of thin hyphae over it, a few upright stalks, each with a round sporangium at the tip, and show one sporangium burst with spores scattering. Label hyphae, sporangiophore (stalk), sporangium, spores. Also note that Rhizopus is a fungus and that this is asexual reproduction.

Sexual reproduction involves two individuals (or two sexes) producing gametes that fuse to form a zygote, which develops into the new individual. The advantage over asexual reproduction comes from variation. DNA copying gives a low but steady rate of variation whatever the mode. Sexual reproduction adds much more, because the offspring combines DNA from two different individuals, so new combinations of variations arise in every generation. This is the answer to NCERT exercise question 4: sexual reproduction produces more variation, and variation helps the species survive changes in the environment.

The chromosome-number argument is asked almost every year, so learn it as a derivation. Each cell has a fixed number of chromosomes (humans: 46, in 23 pairs). If two ordinary cells simply fused, the offspring would have double the number, and it would double again in the next generation. So the germ cells (gametes) are made by a special kind of cell division that halves the chromosome number (meiosis). A human sperm has 23 chromosomes and a human egg has 23; when they fuse the zygote has 46, the normal number. So a zygote has two sets of chromosomes, one from each parent, and the chromosome number of the species stays constant across generations. The 2024 MCQ asked the chromosome number of a human zygote: 46. The ratio of chromosomes in egg to zygote is 1 : 2.

In simple organisms the two gametes may look alike. In most organisms they are of two types: the male gamete is small, motile and produced in large numbers (sperm, pollen); the female gamete is large, non-motile and stores food for the zygote (egg, ovum). Sexual reproduction therefore also requires organs to make the gametes and a mechanism to bring them together.

Difference expected in a 3-marker: asexual reproduction needs one parent, no gametes, no fertilisation, offspring identical, fast; sexual reproduction needs two parents (usually), gametes, fertilisation, offspring show variation, slower. Which has better survival chances (CBSE 2018)? The sexually reproducing species, because its variation lets some individuals survive a changed niche.

The reproductive parts of an angiosperm are in the flower. A flower has four whorls: sepals (green, protect the bud), petals (coloured, attract insects), stamens and carpel (pistil). Sepals and petals are the non-reproductive parts (CBSE 2015, 2023). The stamen is the male part: a stalk called the filament carrying the anther, which produces pollen grains. Pollen grains are yellowish and contain the male germ cells. The carpel is the female part, in the centre of the flower: a swollen base, the ovary, which contains the ovules; a slender middle part, the style; and a sticky tip, the stigma, which receives pollen. Each ovule contains one egg cell (female germ cell).

A flower with both stamens and carpel is bisexual (Hibiscus, mustard). A flower with only one of them is unisexual (papaya, watermelon).

Pollination is the transfer of pollen grains from the anther to the stigma. If the pollen lands on the stigma of the same flower (or another flower of the same plant) it is self-pollination; if it lands on the stigma of a flower on a different plant of the same species it is cross-pollination. The transfer is done by agents: wind, water, insects and other animals. Cross-pollination gives more variation in the offspring because the two parents are different plants (this was the 2021 MCQ: maximum variation from cross-pollination between different plants). Self-pollination does not depend on an agent and preserves parental characters, but gives little variation.

The distinction between pollination and fertilisation is a fixed 2-marker: pollination is the transfer of pollen from anther to stigma, occurs outside the ovule, needs an agent and comes first; fertilisation is the fusion of the male germ cell with the egg cell, occurs inside the ovule, needs no agent and comes after pollination. Fertilisation is not possible without pollination (CBSE 2016) because the male germ cell can reach the egg only if the pollen is first placed on the stigma.

After a pollen grain lands on a suitable stigma it germinates. A pollen tube grows out of the pollen grain, pushes down through the style, enters the ovary and reaches the ovule through a small opening (the micropyle). The male germ cell travels down the tube. In the ovule the male germ cell fuses with the egg (female germ cell). This fusion is fertilisation, and its product is the zygote. So: site of fertilisation = ovule (inside the ovary); product = zygote (CBSE 2019, 2025). This is the diagram the board asks for 'with a labelled diagram, explain how the male germ cell reaches the ovary' (CBSE 2021): draw a carpel in section, pollen grain on the stigma, pollen tube going down the style to an ovule inside the ovary, and label stigma, style, pollen grain, pollen tube, ovary, ovule, male germ cell, female germ cell.

Post-fertilisation changes, in the order the marking scheme lists them:

1. The zygote divides several times to form the embryo within the ovule.
2. The ovule develops a tough coat and becomes the seed.
3. The ovary grows rapidly and ripens to form the fruit.
4. The petals, sepals, stamens, style and stigma shrivel and fall off. (In some plants the sepals persist, as in brinjal and tomato.)

The seed contains the future plant, the embryo, which develops into a seedling under appropriate conditions: this is germination. In a soaked gram seed you can see two fleshy cotyledons (store food and feed the embryo during germination), the plumule (future shoot) and the radicle (future root). The 2024 3-marker asked why part B, the cotyledon, is important during germination: it stores food that nourishes the growing embryo until the seedling can photosynthesise.

So, for the 2019 question 'Summarise the events after fertilisation in a flowering plant', a four-line list (zygote to embryo, ovule to seed, ovary to fruit, other parts wither) is a full-marks answer.

Humans use sexual reproduction. The body grows to a certain size and then, in the early teens, resources are diverted to maturing the reproductive tissue. This period is puberty, and the changes, which are triggered by the hormones testosterone (in boys) and oestrogen (in girls), signal that the person is sexually mature.

Changes common to both boys and girls (the 2022 2-marker): thick hair grows in new parts of the body, armpits and the genital area, and this hair is darker; thinner hair appears on legs, arms and face; the skin often becomes oily and pimples develop; the person becomes aware of their body and of others' bodies in a new way.

Changes in girls (NCERT in-text question): breast size begins to increase and the skin of the nipples darkens; menstruation begins (menarche), which is the event that marks the onset of the reproductive phase (CBSE 2016 MCQ). Changes in boys: thick hair on the face; the voice begins to crack; the penis occasionally becomes enlarged and erect.

Two points NCERT makes that examiners like: these changes take place slowly over months and years, and not everyone shows them at the same time or to the same degree, so it is normal to be different from one's friends. Also, all these changes are signals that the sexual reproductive tissue is maturing; the reproductive organs are now ready to produce gametes.

If the question is 'What is puberty? List two changes common to boys and girls', write the definition (the age at which the reproductive system becomes functional, roughly 10–14 in girls and 12–16 in boys) and any two common changes, and you have the full 2 marks.

The male reproductive system consists of the parts that produce the germ cells and the parts that deliver them to the site of fertilisation.

Testes (singular testis): a pair, located outside the abdominal cavity in a bag of skin called the scrotum. Functions (NCERT exercise question 5): they produce the male germ cells, sperms, and they secrete the hormone testosterone, which regulates sperm formation and brings about the changes in appearance seen in boys at puberty. Why outside the body? Because sperm formation requires a temperature lower than normal body temperature, about 2–3 °C lower; the scrotum provides this. Hence the 2018 MCQ and the 2024 'analyse the impact if the testes cannot descend into the scrotum': sperms would not form properly, causing infertility.

Vas deferens: a tube that carries sperms from each testis and unites with the tube coming from the urinary bladder to form the urethra, which is a common passage for both sperms and urine, opening through the penis. If the vas deferens is plugged (as in a vasectomy, or in the 2024 question), sperms cannot be delivered, so fertilisation cannot occur; hormone secretion and sexual function are unaffected.

Seminal vesicles and prostate gland: along the path of the vas deferens these glands add their secretions to the sperms. The secretions make the mixture fluid, so that sperms are transported easily, and they provide nutrition (fructose) for the sperms. Sperms plus these secretions form semen. If the glands are non-functional, sperms would lack nutrition and a fluid medium, and would not be delivered effectively.

Sperm: a tiny cell consisting mainly of genetic material (in the head) and a long tail that helps it move towards the female germ cell. It carries 23 chromosomes, one of which is either X or Y (the sex-determining chromosome).

Path of the sperm (CBSE 2023 2-marker): testis → vas deferens → urethra → penis → vagina → uterus → fallopian tube (oviduct), where it meets the egg.

The ovaries, a pair, produce the female germ cells (eggs or ova) and secrete the hormone oestrogen, which controls the changes at puberty. A girl is born with thousands of immature eggs already in her ovaries; from puberty one egg matures and is released every month by one of the ovaries (ovulation). These two points, egg production and hormone production, are the 'two functions of the ovary' asked in 2016.

The egg is carried from the ovary to the womb through a thin tube, the fallopian tube or oviduct. The two oviducts unite into an elastic bag-like structure, the uterus, which opens into the vagina through the cervix. NCERT exercise question 2: the vas deferens is not part of the female system.

Sperms enter through the vaginal passage during sexual intercourse and travel up to reach the egg in the fallopian tube. This is the site of fertilisation. If fertilisation occurs, the zygote begins dividing to form an embryo, which moves down and gets implanted in the lining of the uterus (a frequently asked implantation-site MCQ). Every month the uterus prepares for this: its lining becomes thick and spongy, richly supplied with blood, ready to nourish an embryo (CBSE 2022, 2023 'give reason').

The embryo gets nourishment from the mother's blood through a special disc-like tissue, the placenta, embedded in the uterine wall. On the embryo's side it has finger-like villi; on the mother's side there are blood spaces surrounding the villi. This gives a large surface area for glucose and oxygen to pass from the mother to the embryo, and for waste substances from the embryo to pass into the mother's blood. This answers the NCERT in-text question on how the embryo is nourished, and the 2024 question on the functions of the placenta. If the placenta does not attach properly, the embryo is under-nourished and the pregnancy may fail.

Development of the child takes about nine months (gestation), and the child is born as a result of rhythmic contractions of the uterine muscles. The 2024 5-marker 'State briefly the changes in the fertilised egg till birth' wants exactly this sequence: zygote → embryo → implantation → placenta → foetus grows over nine months → birth.

If the egg is not fertilised it lives for about one day. Because the ovary releases one egg every month, the uterus also prepares itself every month to receive a fertilised egg: its lining becomes thick and spongy, ready to support and nourish an embryo. When no fertilisation happens this thick lining is not needed any longer. It slowly breaks down and comes out through the vagina as blood and mucous. This cycle takes roughly a month and lasts about two to eight days. It is called menstruation.

This is NCERT exercise question 6, 'Why does menstruation occur?', and the answer is graded on three points: (1) the uterus prepares each month by thickening its lining; (2) the egg is not fertilised and dies within a day; (3) the lining is no longer needed, breaks down and is shed with blood and mucous through the vagina. Add that this happens roughly monthly and lasts 2–8 days.

A frequently set 3-marker (CBSE 2025) combines three parts: name the site of fertilisation (fallopian tube), how the uterus prepares (lining becomes thick, spongy and richly supplied with blood), and what happens if the egg is not fertilised (menstruation as above). Write it as three short labelled parts.

Two background facts help the case-based questions: the first menstruation (menarche) marks the start of the reproductive phase at puberty, and the cycles stop at menopause, around 45–50 years of age, which ends the reproductive phase. The cycle is controlled by hormones, which is why hormonal pills can interfere with it.

Sexually transmitted diseases (STDs) are infections passed on through sexual contact. NCERT names bacterial ones, gonorrhoea and syphilis, and viral ones, warts and HIV-AIDS. Using a covering such as a condom during sex prevents transmission of many of these infections. A Copper-T does not protect from STDs (NCERT in-text question): it only prevents pregnancy.

Contraception means preventing pregnancy. The board wants the categories with an example and a drawback for each:

• Mechanical / barrier methods: condoms (for men or women) and other coverings stop the sperm from reaching the egg. Advantages: no side effects, and they also prevent STDs; that is why the 2024 question says condoms benefit both sexes.
• Chemical / hormonal methods: oral contraceptive pills change the hormonal balance of the body so that eggs are not released, and fertilisation cannot occur. Drawback: because they change hormonal balance, they can cause side effects.
• Intrauterine contraceptive devices: the loop or Copper-T is placed in the uterus and prevents implantation of the embryo. Drawback: it can cause irritation, bleeding or infection of the uterus, and gives no STD protection.
• Surgical methods: in the male the vas deferens is blocked (vasectomy), so sperm transfer is prevented; in the female the fallopian tube is blocked (tubectomy), so the egg cannot reach the uterus. These are permanent and safe in the long run, but surgery has risk of infection and other problems if done badly.

Reasons for adopting contraception (NCERT exercise 11, CBSE 2024): to avoid unwanted pregnancy, to space children so the mother's health is protected, to keep family size small so children get proper care and resources, to check population growth, and to prevent STDs (barrier methods).

Sex-selective abortion. Surgical methods can also be used to remove unwanted pregnancies, and this facility is misused for female foeticide after prenatal sex determination. NCERT states that this is illegal in India (the PCPNDT Act) and that it leads to an alarming decline in the child sex ratio, with serious social consequences: fewer women in society, imbalance and injustice. A healthy society needs a balanced sex ratio, which is why prenatal sex determination is prohibited.

Reproductive health also includes maturity of the mind and body, the fact that sexual maturity does not mean the person is ready for children, and that pregnancy has demands on the mother's body that she must be prepared for.