NCERT Solutions Class 10 Science Chapter 7 How do Organisms Reproduce?

DNA (deoxyribonucleic acid) is the chemical inside the cell that carries the information for building and running the body. This information decides the body design of the organism. During reproduction a parent must hand a copy of this information to its offspring so that the new individual is of the same kind.

• Reproduction begins with the copying of the DNA in the parent cell, which then divides so each new cell gets one full set.
• Because the DNA is copied, the offspring inherit the same basic body design, which is why a dog gives birth to a puppy, not a kitten.
• DNA copying also makes the cell build extra cellular material, so the two new cells each have enough to work normally.
• Copying is never perfectly exact; the small differences that creep in are the source of variation, the raw material for long-term survival of the species.

Answer: DNA copying passes the body-design information from parent to offspring, so the new individual is of the same kind; the tiny copying errors also create useful variations.

Variation means the small differences that appear among the offspring of the same parents. A species is the whole group of similar organisms, while an individual is just one member. Whether a variation helps depends on the surroundings and on the time scale we look at.

• Conditions in nature keep changing: temperature, water, food and disease can all shift, so a variation that is useless today may become a life-saver tomorrow.
• For the species, a pool of varied individuals is an insurance: if a sudden change wipes out the ordinary types, the few with a suitable variation can survive and continue the species.
• For a single individual, that same variation may give no advantage in its own lifetime, because the individual lives in present conditions only.
• So variation works at the level of the group over many generations, not for one organism in one lifetime.

Answer: Variation gives the species a better chance of surviving environmental change, since some varied individuals may cope; but a single individual may gain no benefit from its variation in its own lifetime.

Fission is an asexual mode in which a single unicellular organism splits to form new individuals. The difference between the two kinds lies in how many offspring form at one time and what happens to the nucleus.

Answer: Binary fission forms two daughter cells from one parent (one nuclear division), as in Amoeba; multiple fission forms many daughter cells at once (the nucleus divides repeatedly first), as in Plasmodium.

Spores are tiny asexual reproductive units that form inside a case called the sporangium, as in the bread mould Rhizopus. Each spore can grow into a new individual on its own, and their structure gives several survival advantages.

• Spores are produced in very large numbers, so a single parent gives rise to a great many offspring.
• Each spore has a thick, hard wall that protects the living material inside from heat, cold and lack of water, letting it survive long bad spells.
• Spores are extremely small and light, so air, water and animals carry them easily, spreading the organism to new places.
• When a spore lands somewhere moist and suitable, the wall breaks and it germinates into a new individual.

Answer: Spores let an organism produce many offspring that are protected by a thick wall, can survive harsh conditions, and spread easily through air and water to new places.

Regeneration is the ability of some simple organisms, such as Hydra and Planaria, to grow a complete new individual from a small cut piece of the body. It depends on cells that can still become any kind of cell. Complex organisms are built differently.

• Complex organisms have a highly organised body of many specialised tissues and organs (heart, lungs, brain, liver), each doing one fixed job.
• These specialised cells have largely lost the ability to change into other cell types, so a stray body part cannot rebuild the missing organs.
• The organs work together as one system; a small part on its own cannot organise and coordinate the formation of a whole new, balanced body.
• So complex organisms reproduce through reproductive organs and gametes; regeneration in them, if any, only heals wounds.

Answer: Complex organisms have specialised tissues and organs whose cells cannot change back into other cell types, so a body part cannot reorganise into a complete new individual; they reproduce through dedicated reproductive organs instead.

Vegetative propagation is an asexual method in which a new plant grows from a part of the parent plant, a root, stem or leaf, not from a seed. Growers use it on purpose for several practical gains.

• Plants grown this way can flower and bear fruit earlier than plants raised from seeds, which is useful for the farmer.
• The new plants are exact copies (clones) of the parent, so all its good qualities (taste, colour, size) are kept unchanged.
• Some plants, such as banana, orange, rose and jasmine, have lost the ability to make viable seeds, so this is the only practical way to grow more of them.
• It is often cheaper and faster to raise many identical plants this way, for example through cuttings, layering and grafting.

Answer: Vegetative propagation is practised because it gives earlier flowering and fruiting, produces exact copies of a desirable parent, and is the only way to grow plants like banana and rose that do not make viable seeds.

The instructions for the entire body design of an organism are stored in its DNA. Reproduction must hand a complete set of these instructions to the offspring, which is only possible if the DNA is first copied.

• Every reproduction event starts with the parent cell copying its DNA, so each new cell receives a full set of instructions.
• Because the offspring receive a copy of the parent's DNA, they develop the same basic body design and belong to the same species.
• Along with copying the DNA, the cell builds extra cell material so each daughter cell is complete and able to function.
• The small errors during copying provide the variation that helps the species adapt and survive over generations.

Answer: DNA copying is essential because it passes the complete body-design information to the offspring so they are of the same kind, and the small errors in copying also create the variation a species needs.

In flowering plants the male part is the stamen (its anther makes pollen) and the female part is the carpel (its ovary holds the ovules). Pollination and fertilisation are two separate steps: one moves the pollen, the other fuses the gametes.

Answer: Pollination is the transfer of pollen from anther to stigma (no fusion yet); fertilisation is the fusion of the male gamete with the female gamete inside the ovule to form a zygote, and it occurs after pollination.

In the human male reproductive system, the sperms made in the testes travel through the vas deferens. Two glands, the seminal vesicles and the prostate gland, open into this path and add their fluids to the sperms.

• The seminal vesicles and prostate gland pour their secretions into the vas deferens, so the sperms travel suspended in this fluid.
• The sperms together with these secretions form a fluid called semen, which makes the transport of sperms much easier.
• The secretions provide nutrition to the sperms, containing sugars such as fructose that give energy to move.
• The fluid keeps the sperms active and helps them swim, improving their chance of reaching the egg.

Answer: The seminal vesicles and prostate gland add their secretions to the sperms in the vas deferens, forming semen; this fluid nourishes the sperms (with sugars such as fructose) and makes their transport and movement easier.

Puberty is the stage when the body becomes sexually mature and capable of reproduction. It is brought about by hormones. In girls, several visible and internal changes appear.

• The breasts develop and increase in size, and the region around the nipple darkens.
• New hair grows in the armpits and the genital (pubic) area, and thinner hair may appear on the arms and legs.
• Menstruation begins; the ovaries start releasing eggs and the monthly menstrual cycle starts.
• The skin often becomes oily and pimples or acne may appear on the face.

Answer: At puberty in girls the breasts develop, hair grows in the armpits and pubic region, menstruation begins, and the skin becomes oily, often with pimples.

After fertilisation the embryo is embedded in the lining of the uterus and develops there. It is fed through a special tissue called the placenta, which connects the embryo to the mother.

• The embryo gets its nourishment from the mother's blood through the placenta.
• The placenta is a disc-like tissue: on the embryo's side it has finger-like projections called villi; on the mother's side there are blood spaces around these villi.
• This gives a large surface area for exchange, so glucose, oxygen and other nutrients pass from the mother's blood to the embryo.
• Waste products such as carbon dioxide pass the other way, from the embryo into the mother's blood, to be removed by her body.

Answer: The embryo is nourished by the mother's blood through the placenta; villi on the embryo's side and blood spaces on the mother's side give a large surface so glucose and oxygen pass to the embryo and wastes like carbon dioxide pass back to the mother.

A copper-T (an intra-uterine device, or IUD) is a contraceptive placed inside the uterus. Sexually transmitted diseases (STDs) are infections passed from one person to another during sexual contact. We must check whether a copper-T can block such infections.

• A copper-T works only inside the uterus, stopping a pregnancy by preventing the fertilised egg from settling and developing.
• It is not a barrier between the two partners, so the body fluids that carry the germs of STDs still pass freely.
• Therefore a copper-T does not protect a woman from STDs such as HIV/AIDS, gonorrhoea or syphilis.
• To prevent STDs, a barrier method like a condom is needed, because it physically stops the exchange of body fluids.

Answer: No. A copper-T only prevents pregnancy; it is not a barrier and does not stop the exchange of body fluids, so it cannot protect against sexually transmitted diseases. A condom is needed for that.

Budding is an asexual mode in which a small outgrowth, or bud, forms on the parent body, grows, and then detaches as a new individual. We must pick the organism that reproduces this way.

• Amoeba reproduces by binary fission, not budding.
• Yeast reproduces by budding: a small bud appears on the parent cell, enlarges, and separates as a new yeast cell. So (b) is correct.
• Plasmodium reproduces by multiple fission.
• Leishmania reproduces by binary fission in a definite plane.

Answer: Correct option (b) Yeast. Yeast reproduces by budding, while Amoeba and Leishmania use binary fission and Plasmodium uses multiple fission.

The female reproductive system in humans is made of the ovaries, the fallopian tubes (oviducts), the uterus and the vagina. We must find the part that does not belong to it.

• The ovary makes eggs and female hormones, so it is part of the female system.
• The uterus is where the embryo develops, so it is part of the female system.
• The vas deferens carries sperms in the male body, so it is part of the male system, not the female. Hence (c) is the answer.
• The fallopian tube carries the egg and is where fertilisation occurs, so it is part of the female system.

Answer: Correct option (c) Vas deferens. It belongs to the male reproductive system; the ovary, uterus and fallopian tube are all parts of the female system.

The stamen is the male part of a flower. It has two parts: a stalk called the filament and a swollen top called the anther. We must recall what the anther holds.

• Sepals are the green leaf-like parts at the base of the flower; they are not inside the anther.
• Ovules are inside the ovary of the carpel (the female part), not in the anther.
• The pistil (carpel) is the whole female part of the flower, not something held inside the anther.
• The anther produces and contains pollen grains, which carry the male gametes. So (d) is correct.

Answer: Correct option (d) pollen grains. The anther is the part of the stamen that produces and holds the pollen grains.

In asexual reproduction a single parent makes offspring that are exact copies, so there is almost no variation. In sexual reproduction two parents contribute, and the mixing of their DNA creates new combinations. This difference is the source of every advantage.

• Sexual reproduction produces a large amount of variation among the offspring, because the genetic material of two parents is combined.
• This variation helps the species adapt to changes in the environment and improves its chances of long-term survival.
• It promotes the gradual evolution of the species, as useful new combinations of traits can appear and be passed on.
• Errors in the DNA that might build up in asexual copies are reduced, because new gene combinations help mask and remove harmful ones, giving healthier offspring.

Answer: Sexual reproduction mixes the DNA of two parents, so it creates much more variation than asexual reproduction; this variation helps the species adapt to a changing environment, drives evolution, and yields healthier offspring.

The testes are the main male reproductive organs, present as a pair in the scrotum outside the abdomen. They carry out two key functions, one to do with cells and one to do with hormones.

• The testes produce the male gametes, that is, the sperms, by cell division.
• The testes also produce the male sex hormone called testosterone.
• Testosterone controls the formation of sperms and brings about the male secondary sexual characters at puberty, such as a deeper voice, facial and body hair, and broad shoulders.
• The testes lie in the scrotum, outside the body cavity, because sperm formation needs a temperature lower than normal body temperature.

Answer: The testes perform two functions: they produce the male gametes (sperms), and they secrete the male sex hormone testosterone, which controls sperm formation and the male secondary sexual characters at puberty.

Each month the uterus prepares for a possible pregnancy by building a thick, soft, blood-rich lining to receive a fertilised egg. Menstruation is what happens to this lining when no pregnancy occurs.

• Once a month an ovary releases an egg (ovulation), and the uterus lining becomes thick and well supplied with blood, ready to nourish an embryo.
• If the egg is not fertilised, no embryo is formed and the thick lining is no longer needed.
• The lining then breaks down and is shed through the vagina as blood and mucus. This monthly flow is menstruation.
• The cycle lasts about 28 days and repeats, with the uterus building a fresh lining each time.

Answer: Menstruation occurs because, when the released egg is not fertilised, the thick blood-rich uterine lining built up to support a pregnancy is no longer needed; it breaks down and is shed through the vagina as blood and mucus, about every 28 days.

A flower is the reproductive part of a plant. A longitudinal section (a cut from top to bottom) shows the four whorls: the green sepals, the coloured petals, the male stamens (each an anther on a filament) and the female carpel (stigma, style, ovary and ovules).

• The carpel (stigma, style, ovary, ovules) is the female part of the flower.
• The stamen (anther, filament) is the male part.
• The sepals and petals are accessory whorls that protect the bud and attract pollinators.

Answer: A correctly labelled longitudinal section shows the sepal, petal, stamen (anther and filament), and carpel (stigma, style, ovary and ovule), as in the diagram above.

Contraception means the deliberate prevention of pregnancy. The methods work by stopping one of the steps that lead to pregnancy: the meeting of sperm and egg, the change of hormones, the implantation of the embryo, or the supply of gametes.

Answer: Contraception methods are barrier methods (condom, diaphragm), chemical methods (hormonal oral pills), intra-uterine devices (copper-T), and surgical methods (vasectomy in males, tubectomy in females).

A unicellular organism is made of a single cell, so reproduction and body growth are the same act of cell division. A multicellular organism has many specialised cells, so it needs special cells or organs set apart for reproduction.

• In unicellular organisms (such as Amoeba, bacteria, yeast, Plasmodium), the whole single cell divides, by binary fission, multiple fission or budding.
• Here there are no separate reproductive organs; the entire body takes part in producing offspring.
• In multicellular organisms the body has many specialised cells, so the whole body cannot simply split; they use fragmentation, regeneration, spore formation, vegetative propagation, or sexual reproduction.
• In complex multicellular organisms, reproduction is carried out by special reproductive organs that produce gametes; only these cells, not the whole body, make the offspring.

Answer: Unicellular organisms reproduce by simple division of the whole single cell (binary fission, multiple fission, budding), while multicellular organisms use specialised cells, tissues or reproductive organs (such as spores, vegetative parts or gametes) because their bodies are made of many specialised cells.

A population stays stable when its numbers and basic features do not crash over time. Reproduction keeps a population stable by replacing the individuals that die and by passing on a reliable body design, while also supplying variation.

• Reproduction makes new individuals of the same kind, replacing those lost to death, disease or predators, so the number stays roughly steady.
• Through DNA copying, reproduction passes on the same body design from one generation to the next, keeping the population's characteristic features and identity.
• The variation produced (especially in sexual reproduction) lets the population adapt to environmental change, so it is not wiped out when conditions shift.
• Together, replacement of numbers, continuity of body design and adaptability give the population stability over many generations.

Answer: Reproduction provides stability by continually replacing dead individuals with new ones of the same kind, passing on a reliable body design through DNA copying, and supplying variation so the population can adapt to environmental change and survive over generations.

Contraceptive methods prevent unwanted pregnancy and, in some cases, infection. People and societies adopt them for several health and social reasons.

• To prevent unwanted pregnancy and to space and limit the number of children, so a family can be cared for properly.
• To protect the health of the mother, as too many or too closely spaced pregnancies weaken her body.
• To help control the rapid growth of population, so resources like food, water and jobs are not overstretched.
• To prevent the spread of sexually transmitted diseases, since barrier methods such as condoms stop the exchange of body fluids that carry the germs.

Answer: Contraceptive methods are adopted to prevent unwanted pregnancy and space births, to protect the mother's health, to control the rapid growth of population, and (with barrier methods) to prevent sexually transmitted diseases.