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

DNA copying is important because it passes the parent's traits to the offspring.  OR  DNA copying is important because it gives the new baby the same instructions as the parents.

It helps the new cell or organism know how to grow and work.
Small changes during copying also create differences between individuals

Variation is useful for a species because it gives different members different qualities, so if the environment changes, some of them can still survive.
But a single individual doesn’t always benefit from the variation it has — it might help, or it might not make any difference.
So variation helps the species continue, even if it doesn’t always help each individual.

In binary fission, one organism splits into two equal parts, each becoming a new organism (like in amoeba).
In multiple fission, one organism divides into many daughter cells at the same time (like in Plasmodium).

So, binary fission produces two offspring, while multiple fission produces many in one go.

An organism benefits from spore formation because spores can survive harsh conditions like heat, dryness, or lack of food.
They are light and easily spread by wind or water, helping the organism spread to new places.
When conditions become favourable again, the spores grow into new organisms.

More complex organisms cannot reproduce by regeneration because their bodies are made of many specialised cells and organs that depend on each other.
If a big part is cut, the body cannot rebuild all those complex organs again.

Regeneration in such organisms only helps in healing wounds, not making a whole new individual.

Vegetative propagation is used because it produces new plants quickly and without seeds. The new plants are exactly like the parent plant, so good qualities are preserved.
It also helps grow plants that don’t produce viable seeds or take too long to grow from seeds.

DNA copying is essential because it passes the parent’s information and traits to the offspring.  It makes sure the new cell knows how to grow, function, and develop properly.

Without DNA copying, the new organism would not get the instructions it needs to live.

Pollination is the transfer of pollen from the anther to the stigma of a flower.  Fertilisation happens after pollination, when the male cell in the pollen fuses with the female egg cell inside the ovule.

So, pollination is about pollen reaching the stigma, while fertilisation is about fusion of male and female gametes.

The seminal vesicles and prostate gland produce fluids that mix with sperm to form semen.
These fluids give energy to the sperm and help them move smoothly.
They also protect the sperm and make it easier for them to travel through the female body.

Girls become taller, their breasts start to grow, and their hips get wider. Hair appears under the arms and in the private parts.
They also start their periods, showing that their body is becoming mature.

Inside the mother’s body, the embryo receives nourishment through a special organ called the placenta.
The placenta is attached to the wall of the uterus and connected to the embryo by the umbilical cord.
Food, oxygen, and other nutrients from the mother’s blood pass through the placenta and enter the baby’s blood without the two bloods mixing.
At the same time, waste materials from the embryo are carried back through the placenta to the mother’s blood for removal.
This exchange keeps the embryo healthy and helps it grow properly until birth.

No, a copper-T cannot protect a woman from sexually transmitted diseases. It only prevents pregnancy. To avoid STDs, using condoms is necessary because they block the transfer of germs during sex.

Sexual reproduction is better because it mixes the traits of two parents, so the offspring are all a little different from each other. These differences help the species survive if the environment changes, because some individuals may be better adapted.

In asexual reproduction, all offspring look the same, so if conditions become bad, the whole group can be affected.

The testis produces sperm, which are the male reproductive cells.
It also makes the male sex hormone testosterone, which controls changes at puberty and the development of male features.

So, the testis is responsible for making sperm and producing hormones.

Menstruation happens when the egg is not fertilised.
The thick wall inside the uterus is not needed, so it breaks and comes out as blood. This monthly bleeding is called menstruation.

Different methods of contraception include:

1. Barrier methods – like condoms, which stop sperm from reaching the egg.
2. Hormonal methods – like birth control pills, which prevent the release of an egg.
3. Intrauterine devices (IUDs) – like copper-T, which prevent fertilisation inside the uterus.
4. Surgical methods – like vasectomy in men and tubectomy in women, which permanently block the reproductive tubes.

These methods help prevent pregnancy in different ways.

In  unicellular organisms, reproduction is usually done by simple methods like binary fission or budding, where one cell divides to form a new organism.
In multicellular organisms, reproduction is more complex and usually involves special reproductive organs and cells (like sperm and egg).

So, unicellular organisms reproduce by simple cell division, while multicellular organisms use organised reproductive systems.

Reproduction helps keep a species stable because it makes new individuals to replace the ones that die. It also passes on the species’ traits from one generation to the next, so the group continues to survive.
By producing more members, reproduction keeps the population from disappearing.

People adopt contraceptive methods to avoid unwanted pregnancy and plan their family.
They help couples decide when they want to have children.
Contraceptives also protect the health of the mother by spacing births and, in some cases like condoms, help reduce the risk of sexually transmitted diseases.

The endosperm provides nourishment to the developing embryo within the seed. If the endosperm fails to develop properly, the embryo will not receive the necessary nutrients, leading to the production of non-viable or empty seeds, even after successful fertilization.

Apomixis is a type of asexual reproduction in plants where seeds are produced without fertilization, resulting in offspring that are genetically identical to the parent plant, mimicking sexual reproduction but without genetic recombination.

Budding is a form of asexual reproduction where a new organism develops from an outgrowth or bud due to cell division at one particular site. This bud then detaches from the parent to form a new independent individual, as seen in yeast.

Plant fertilization is called double fertilization because two separate fusion events occur. One male gamete fuses with the egg cell to form the zygote, and the other male gamete fuses with the central cell (containing two polar nuclei) to form the primary endosperm nucleus, which develops into the nutritive endosperm tissue.

Surgically cutting and tying the vas deferens (vasectomy) prevents sperm from traveling from the testes to the urethra, thus preventing their release during ejaculation. This is a permanent surgical contraceptive method for males.

Reproduction via runners (vegetative propagation) would be more advantageous in stable, favorable environments where rapid colonization of a local area is beneficial. Since the offspring are genetically identical clones, they are well-suited to the parent's successful habitat, allowing for quick spread without the genetic risks and energy expenditure associated with sexual reproduction (seeds).

Tissue culture would be the most suitable technique. It allows for the rapid production of a large number of genetically identical plantlets (micropropagation) from a small piece of parent tissue, ensuring that all desired traits like high yield and disease resistance are uniformly passed on.

The transfer of pollen grains, known as pollination, is critical because pollen grains contain the male gametes. Without pollination, the male gametes cannot reach the ovule to fuse with the egg cell, thus preventing fertilization and the subsequent formation of seeds and fruits, which are essential for sexual reproduction.

Fungal spores typically reproduce asexually through spore formation. Each spore, upon landing on a suitable moist surface with sufficient nutrients, germinates and develops into a new fungal mycelium. This mode of reproduction is favored by conditions that are conducive to growth (warmth, moisture, food) but may not allow for the genetic recombination of sexual reproduction, enabling rapid colonization.

Regeneration is primarily the ability of an organism to regrow lost or damaged body parts, whereas reproduction is the process of producing new individual organisms. While both can result in new individuals (e.g., Planaria cut into pieces each regrows into a whole organism), regeneration fundamentally serves to restore an existing individual, while reproduction's primary goal is species perpetuation.

Under abundant nutrient conditions, the organism exhibits binary fission, which allows for rapid population growth when resources are plentiful. When nutrients are scarce, it performs multiple fission within a cyst, a survival strategy that protects the organism during adverse conditions and allows for mass reproduction once conditions become favorable again.

The Fallopian tube is the site where fertilization typically occurs, as it transports the ovum from the ovary to the uterus. If the Fallopian tube becomes blocked, it prevents the sperm from reaching the ovum and the fertilized egg (zygote) from reaching the uterus, leading to infertility or an increased risk of ectopic pregnancy.

This plant is most likely adapted for insect pollination. Bright colors, strong scents, and nectar are all typical adaptations to attract insects (like bees or butterflies) which act as pollinators, transferring pollen from one flower to another as they seek nectar.

Pollen grains are male gametophytes and are haploid (n), so n=12. The primary endosperm nucleus is formed by the fusion of one male gamete (n) with two polar nuclei (n+n), making it triploid (3n). Therefore, 3n = 3 * 12 = 36.

A strong root system provides better anchorage, absorption of water and minerals, and often allows access to deeper water reserves. For seed-reproducing plants, this is crucial for the establishment of the seedling in potentially new or variable environments, enhancing its chances of survival and growth where resources might be patchily distributed, unlike the typically more localized and less exploratory growth of vegetatively propagated clones.

Both testes (males) and ovaries (females) are the primary reproductive organs (gonads) responsible for producing gametes (sperm and ova, respectively) and sex hormones (androgens and estrogens/progesterone, respectively). They differ in the type of gamete produced, the hormones secreted, and their anatomical location and cyclical activity (testes produce sperm continuously, ovaries release ova cyclically).

Cutting relies on the plant's ability to undergo vegetative propagation. A section of the stem, leaf, or root is cut and placed in suitable conditions, stimulating the growth of adventitious roots and shoots. This method is successful because plant cells retain totipotency (the ability to differentiate into various cell types), allowing a new, genetically identical plant (a clone) to develop from the parent tissue.

Multiple fission allows organisms like Plasmodium to rapidly produce a large number of offspring within a short period, especially during the infectious stages inside a host. This strategy helps them quickly increase their population to overwhelm the host's defenses or ensure successful transmission to new hosts, which is more efficient than binary fission for explosive growth.

The testes are directly responsible for producing sperm through spermatogenesis. The seminal vesicles and the prostate gland are crucial accessory glands that contribute fluids to semen, providing nutrients (like fructose) and maintaining the optimal pH for sperm motility and viability.

This scenario describes protandry, a mechanism that promotes cross-pollination by preventing self-pollination. Its adaptive advantage is to increase genetic diversity within the plant population, enhancing the species' ability to adapt to changing environmental conditions and reducing inbreeding depression.

At puberty, the pituitary gland increases the secretion of gonadotropins (Follicle-Stimulating Hormone and Luteinizing Hormone). In males, these hormones stimulate the testes to produce testosterone, leading to secondary sexual characteristics. In females, they stimulate the ovaries to produce estrogen and progesterone, initiating the menstrual cycle and the development of female secondary sexual characteristics.

The uterus is a muscular organ responsible for receiving, implanting, and nourishing the fertilized egg, allowing the embryo and fetus to develop until birth. Ova production occurs in the ovaries, fertilization in the Fallopian tubes, and hormone secretion mainly in the ovaries.

The scrotum is located outside the abdominal cavity because sperm production (spermatogenesis) requires a temperature slightly lower (about 2-3°C cooler) than the normal body temperature. This anatomical arrangement provides the optimal cooler environment necessary for the healthy development and viability of sperm.

The organism is exhibiting budding for reproduction, where a new individual develops from an outgrowth. It also possesses high regenerative ability, meaning it can regrow lost parts. Hydra is a classic example of an organism that reproduces by budding and also has remarkable regenerative capabilities.

Genetic variation, introduced through sexual reproduction, provides the raw material for natural selection. It allows some individuals within a population to possess traits that make them more adaptable to changing environmental conditions, diseases, or predators. This adaptability increases the likelihood that the species will survive and evolve over generations.

After double fertilization, the ovule develops into the seed, containing the embryo and endosperm, enclosed within a protective seed coat. The ovary, which surrounds the ovules, develops and matures into the fruit, which often aids in seed dispersal.

Grafting is successful because it exploits the plant's ability to heal wounds and form vascular connections between the tissues of the scion and the rootstock. The cambium layers of both parts must align and fuse, allowing the transport of water, nutrients, and sugars between them. It's artificial vegetative propagation because it involves human intervention to produce a new plant that is genetically identical to the scion.

External fertilization occurs outside the body, making the gametes and zygotes vulnerable to predation, environmental fluctuations (temperature, pH), and being washed away. Producing a large number of gametes increases the statistical probability that at least some gametes will meet, fertilize, and survive to develop, ensuring the continuation of the species despite high losses.

Folic acid is crucial for the proper development of the neural tube in the early embryo, which forms the brain and spinal cord. Adequate folic acid intake significantly reduces the risk of neural tube defects like spina bifida, highlighting its importance for healthy fetal development even before conception.

STIs pose a significant public health challenge because they can lead to long-term health complications like infertility, chronic pain, and certain cancers if left untreated. They can also be transmitted from mother to child during pregnancy or childbirth, affecting newborn health, and their spread can be asymptomatic, making control and prevention difficult across populations.

Oral contraceptive pills contain hormones (estrogen and/or progestin) that primarily work by inhibiting the release of FSH and LH from the pituitary gland, thereby preventing the maturation of follicles and ovulation in the ovaries.

Vegetative propagation (e.g., cutting, layering, grafting, tissue culture) would be suitable. These asexual methods produce offspring that are genetically identical to the parent plant, ensuring that the specific desirable trait is perfectly conserved without any genetic variation introduced by sexual reproduction.

The seminal vesicles secrete a fluid rich in fructose, calcium, and certain enzymes that provide nourishment and activation for sperm. The prostate gland secretes a slightly acidic fluid containing citrate, enzymes, and prostate-specific antigen, which also contributes to sperm motility and provides nutrients.

During pregnancy, the developing embryo and placenta produce hormones, particularly human chorionic gonadotropin (hCG) and later progesterone. These hormones maintain the uterine lining, prevent the breakdown of the corpus luteum (which continues to produce progesterone), and inhibit the pituitary gland from secreting FSH and LH, thereby preventing ovulation and menstruation.

Fragmentation is an asexual mode of reproduction where an organism's body breaks into two or more fragments, each of which develops into a new, complete individual, as seen in filamentous algae like Spirogyra.

Sexual dimorphism refers to distinct differences in size, appearance, or behavior between males and females of the same species, beyond their sexual organs. This often relates to reproductive strategies, where elaborate male traits (e.g., bright plumage, large antlers) or behaviors are evolved through sexual selection to attract mates and signal genetic fitness, thereby increasing reproductive success.

The timing of implantation is critical because the uterine lining (endometrium) must be in a specific receptive state, primed by hormones, to allow the embryo to attach and embed. If implantation occurs too early or too late, the uterine lining may not be conducive, leading to implantation failure or an unsuccessful pregnancy.

Spore formation in organisms like Rhizopus provides an evolutionary advantage through efficient dispersal, as spores are typically lightweight and can be carried by wind over long distances to new habitats. They also have thick walls that protect them from unfavorable conditions (e.g., desiccation, extreme temperatures), allowing the organism to survive until conditions become favorable for germination and growth.

Unisexual flowers contain either stamens (male reproductive parts) or pistils (female reproductive parts), but not both (e.g., papaya, watermelon). Bisexual flowers (also called perfect flowers) contain both stamens and pistils within the same flower (e.g., hibiscus, mustard).

A zygote is the single-celled diploid structure formed immediately after the fusion of sperm and egg during fertilization. An embryo is the multicellular structure that develops from the zygote through repeated cell divisions (cleavage and differentiation) after implantation in the uterus, representing the early stages of organismal development.

The plant breeder would employ sexual reproduction through cross-pollination and subsequent selective breeding. This strategy allows for the combination of genetic material from two different parent plants, leading to offspring with a mix of traits from both, and then selecting individuals that express the desired combination (large fragrant flowers AND disease resistance).

Some STIs are difficult to diagnose early because they can be asymptomatic or have very mild, non-specific symptoms, making individuals unaware they are infected. Delayed diagnosis and treatment can lead to severe long-term complications such as infertility, chronic pelvic pain, ectopic pregnancies, increased risk of other STIs (including HIV), and potentially life-threatening systemic infections.

The placenta serves a dual function: it facilitates the transfer of essential substances like oxygen, nutrients, and antibodies from the mother's blood to the developing fetus. Simultaneously, it removes waste products such as carbon dioxide and urea from the fetal blood and transfers them to the mother's circulation for excretion.

In sexually reproducing weeds, genetic variation is introduced with each generation through recombination. Some individuals might have naturally possessed a gene for herbicide resistance. When exposed to the herbicide, these resistant individuals survive and reproduce, passing on their advantageous trait, leading to a rapid increase in the proportion of resistant weeds in the population over successive generations due to natural selection.

The amniotic fluid serves several crucial roles: it cushions the fetus from external shocks and physical injury, maintains a stable temperature around the fetus, allows for freedom of movement which aids in muscle and skeletal development, and prevents adhesion of the fetus to the amniotic sac.

Sperm are produced in the testis, mature and are stored in the epididymis, then travel through the vas deferens, and finally exit the body via the urethra.

Seed dispersal mechanisms reduce competition between parent plants and their offspring for resources like light, water, and nutrients by moving seeds away from the parent. It also allows for the colonization of new habitats, increases the species' geographical range, and helps prevent the accumulation of pests and diseases specific to the parent location, enhancing survival and genetic mixing.

Using a sterile nutrient medium and maintaining aseptic conditions are crucial to prevent contamination by microorganisms (bacteria, fungi). Contaminants would outcompete the plant tissue for nutrients, introduce toxins, or directly damage the plant cells, leading to the failure of the culture and the death of the plantlets.

This flower is most likely adapted for wind pollination. Dull petals, lack of scent, and abundant, lightweight pollen are all characteristic features. The large amount of powdery pollen is a structural adaptation to increase the chances of airborne pollen reaching a receptive stigma, as wind pollination is less targeted than animal pollination.

If a human egg cell is not fertilized within 12-24 hours after ovulation, it begins to degenerate and eventually disintegrates. This is followed by the shedding of the uterine lining, marking the beginning of menstruation, as the body prepares for a new cycle.

Ethical considerations for surgical contraceptive methods (vasectomy, tubal ligation) often revolve around issues of informed consent, voluntariness, and permanence. It is crucial that individuals fully understand the irreversible nature of these procedures and are not coerced. Other ethical aspects include access to these services, potential for misuse, and the balance between individual reproductive autonomy and societal concerns.