Class 12 NCERT Solution : Chapter-1 Reproduction in Organism -

Class 12 NCERT Solution : Chapter-1 Reproduction in Organism

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1. Why is reproduction essential for organisms?

Reproduction is a fundamental process for all living organisms that ensures the continuation of a species over time. It is essential for the following reasons:

Perpetuation of species: Reproduction enables the perpetuation of a species by creating new individuals that carry the genetic information necessary for the survival of the species.
Genetic diversity: Reproduction allows for the creation of genetic diversity within a population, which is important for adaptation and evolution. Sexual reproduction, in particular, leads to the mixing of genetic material from two individuals, resulting in offspring with unique combinations of traits.
Maintenance of population size: Reproduction ensures that the population size of a species remains stable. Without reproduction, the population size of a species would gradually decline over time, leading to its eventual extinction.
Repair and replacement of damaged or dead cells: Some organisms, such as unicellular organisms and some plants, reproduce asexually to repair and replace damaged or dead cells.
Metabolic activities: In some organisms, such as bacteria, reproduction is essential for metabolic activities, such as the production of enzymes and the breakdown of nutrients.

Reproduction is essential for the perpetuation of a species, genetic diversity, maintenance of population size, repair and replacement of damaged cells, and metabolic activities. Without reproduction, living organisms would not be able to survive and thrive in their environments.

2. Which is a better mode of reproduction: sexual or asexual? Why?

Both sexual and asexual reproduction have their own advantages and disadvantages, and which mode of reproduction is better depends on various factors such as environmental conditions, availability of resources, and the evolutionary history of the organism.

Asexual reproduction involves the creation of offspring that are genetically identical to the parent, and it is a relatively simple and efficient process. Asexual reproduction is often beneficial for organisms living in stable environments with little variation, as it allows for rapid reproduction and colonization of new territories without the need for a mate.

However, asexual reproduction does not allow for genetic diversity and creates a population of genetically identical individuals that may be vulnerable to disease or changes in the environment. Moreover, in the long term, asexual reproduction can lead to a lack of genetic diversity, making the population less able to adapt to changing environments.

Sexual reproduction, on the other hand, involves the fusion of gametes from two individuals, resulting in offspring with unique genetic combinations. This leads to genetic diversity within the population, which is beneficial for adaptation to changing environmental conditions. Additionally, sexual reproduction can allow for the elimination of harmful mutations through the process of natural selection.

However, sexual reproduction is often a more complex and energy-intensive process than asexual reproduction, requiring the production and maintenance of specialized reproductive structures, and the need to find a mate. Moreover, sexual reproduction can lead to a reduction in the rate of reproduction due to the need to find a mate, which can be a disadvantage in some environments.

In conclusion, whether sexual or asexual reproduction is better depends on various factors, including environmental conditions and the evolutionary history of the organism. While asexual reproduction may be beneficial in some environments, sexual reproduction generally leads to greater genetic diversity, adaptability, and long-term survival of a species.

3. Why is the offspring formed by asexual reproduction referred to as clone?

Offspring produced by asexual reproduction are referred to as clones because they are genetically identical to the parent organism. This is because asexual reproduction involves only one parent, and the offspring are produced by the division of the parent’s cells, without the fusion of gametes.

During asexual reproduction, the genetic material of the parent is replicated, and then divided equally between the daughter cells, resulting in two genetically identical cells. This process continues, resulting in the production of multiple genetically identical cells, which eventually develop into new individuals.

Since there is no mixing of genetic material from two different individuals, the offspring are genetically identical to the parent, and to each other. This is in contrast to sexual reproduction, where the offspring inherit a combination of genetic material from both parents, resulting in unique genetic combinations.

The genetic identity of clones has many practical applications, such as in agriculture, where plants are propagated asexually to produce clones with desirable traits, or in medical research, where genetically identical animals are used for experiments to minimize variability in results.

In conclusion, the offspring produced by asexual reproduction are referred to as clones because they are genetically identical to the parent organism and to each other. This process of reproduction ensures that the genetic information is passed on to the next generation without any variation, which can be advantageous in certain environments or for certain applications.

4. Offspring formed due to sexual reproduction have better chances of survival. Why? Is this statement always true?

Offspring formed due to sexual reproduction have better chances of survival because they inherit a combination of genetic traits from both parents, which increases their genetic diversity and ability to adapt to changing environments. This also reduces the chances of inheriting harmful genetic mutations from a single parent.

However, this statement may not always be true in certain situations such as when the parents have genetic disorders or when the environment remains stable over a long period of time, reducing the need for genetic diversity. Additionally, asexual reproduction can also be advantageous in certain environments or for certain species that have adapted to it over time.

5. How does the progeny formed from asexual reproduction differ from those formed by sexual reproduction?

Progeny formed from asexual reproduction are genetically identical to the parent organism, as they are produced through the process of mitosis, which involves the replication of the parent’s genetic material and division of the cell into two identical daughter cells. Asexual reproduction occurs in single-celled organisms such as bacteria, as well as in some multicellular organisms such as plants and animals.

In contrast, progeny formed from sexual reproduction inherit a unique combination of genetic material from both parents through the process of meiosis, which involves the reduction of the parent’s genetic material and the formation of gametes (sperm and egg cells) with half the usual number of chromosomes. When these gametes fuse during fertilization, they create a zygote with a new combination of genetic material, resulting in genetic diversity in the offspring. Sexual reproduction occurs in most animals and plants.

Overall, asexual reproduction results in offspring that are genetically identical to the parent, while sexual reproduction results in offspring that are genetically diverse and have a better chance of surviving in changing environments.

6. Distinguish between asexual and sexual reproduction. Why is vegetative reproduction also considered as a type of asexual reproduction?

Asexual reproduction and sexual reproduction are two different methods by which organisms produce offspring.

Asexual reproduction is the process of producing offspring from a single parent, without the involvement of gametes (sperm and egg cells). It results in offspring that are genetically identical to the parent, as they are produced through the process of mitosis. Asexual reproduction is common in single-celled organisms, such as bacteria and protists, as well as in some multicellular organisms, such as plants and animals.

In contrast, sexual reproduction involves the fusion of gametes from two parents to produce offspring that have a unique combination of genetic material from both parents. Sexual reproduction results in genetic diversity among offspring, which can help them adapt to changing environments.

Vegetative reproduction is a type of asexual reproduction that occurs in plants. In vegetative reproduction, new plants are produced from non-reproductive parts of the parent plant, such as roots, stems, or leaves. This can happen through processes such as budding, where new plants grow as outgrowths from the parent plant, or fragmentation, where the parent plant breaks apart and each fragment develops into a new plant. Vegetative reproduction is considered a type of asexual reproduction because it produces offspring that are genetically identical to the parent plant, without the involvement of gametes.

7. What is vegetative propagation? Give two suitable examples.

Vegetative propagation is a method of asexual reproduction in plants, where new plants are produced from non-reproductive parts of the parent plant, such as roots, stems, or leaves. This method of reproduction does not require the production of seeds or the fusion of gametes.

Two examples of vegetative propagation are:

Runners: Runners are specialized stems that grow horizontally above the ground, giving rise to new plants at their nodes. For example, strawberry plants produce runners that grow along the ground and produce new plantlets at their nodes. These plantlets eventually develop roots and form new strawberry plants.
Rhizomes: Rhizomes are underground stems that grow horizontally, producing new shoots and roots at their nodes. For example, ginger plants produce rhizomes that can be harvested and replanted to produce new ginger plants. These rhizomes also store nutrients, allowing the new plant to grow and develop before it produces its own leaves and roots.

8. Define
(a) Juvenile phase,
(b) Reproductive phase,
(c) Senescent phase.

Answer :

(a) Juvenile phase: The juvenile phase is the early stage of growth and development of an organism, where it is not yet capable of reproduction. In plants, this phase is characterized by rapid vegetative growth, with a focus on developing roots, stems, and leaves. Once the juvenile phase is over, the plant becomes capable of producing flowers and reproducing.

(b) Reproductive phase: The reproductive phase is the stage of growth and development where an organism becomes capable of producing offspring. In plants, this phase is characterized by the production of flowers and fruits, which contain the reproductive structures necessary for fertilization and seed production. Once the reproductive phase is over, the organism may enter a senescent phase, where it begins to show signs of aging and decline.

(c) Senescent phase: The senescent phase is the final stage of growth and development in an organism, where it begins to show signs of aging and decline. In plants, this phase is characterized by a decrease in growth and productivity, as well as an increase in susceptibility to diseases and environmental stresses. In animals, this phase is marked by a decline in physical and cognitive abilities, as well as an increased risk of age-related diseases. Ultimately, the senescent phase leads to the death of the organism.

9. Higher organisms have resorted to sexual reproduction in spite of its complexity. Why?

Higher organisms have resorted to sexual reproduction in spite of its complexity because it confers several evolutionary advantages. Sexual reproduction leads to the formation of offspring that are genetically diverse due to the combination of genetic material from two parents. This genetic diversity allows for the production of offspring that have a wider range of traits and characteristics, which can be beneficial in adapting to changing environments and in resisting diseases.

Sexual reproduction also helps to prevent the accumulation of harmful mutations in the genome. The process of meiosis, which is involved in sexual reproduction, shuffles and recombines the genetic material from both parents, leading to the creation of new combinations of genetic material in each offspring. This can help to remove harmful mutations from the genome, reducing the risk of genetic diseases.

Finally, sexual reproduction also allows for the evolution of new traits and adaptations through the process of natural selection. Offspring with beneficial traits that help them survive and reproduce are more likely to pass those traits on to their own offspring, leading to the evolution of new species and the diversification of life on Earth.

10. Explain why meiosis and gametogenesis are always interlinked?

Ans : Meiosis and gametogenesis are always interlinked because meiosis is the process by which cells undergo two rounds of division to produce haploid cells with half the number of chromosomes as the parent cell, which then undergo gametogenesis to form gametes.

Gametogenesis is the process by which haploid gametes are produced from diploid germ cells. This process involves meiosis, where the diploid germ cells undergo two rounds of division to produce four haploid cells. These haploid cells then differentiate into male or female gametes, such as sperm or eggs.

Without meiosis, the germ cells would not be able to divide and differentiate into gametes with half the number of chromosomes as the parent cell. Likewise, without gametogenesis, the haploid cells produced by meiosis would not be able to differentiate into mature gametes capable of fertilization.

Therefore, meiosis and gametogenesis are interlinked processes that are essential for sexual reproduction in organisms.

11. Identify each part in a flowering plant and write whether it is haploid
(n) or diploid (2n).
(a) Ovary ———————————
(b) Anther ———————————
(c) Egg ———————————
(d) Pollen ———————————
(e) Male gamete ———————————
(f) Zygote ———————————

Ans : (a) Ovary – diploid (2n) (b) Anther – diploid (2n) (c) Egg – haploid (n) (d) Pollen – haploid (n) (e) Male gamete – haploid (n) (f) Zygote – diploid (2n).

12. Define external fertilization. Mention its disadvantages.

External fertilization is a type of fertilization where the fusion of the sperm and egg occurs outside the body of the organisms. This type of fertilization is typically seen in aquatic organisms such as fish, amphibians, and many invertebrates.

One of the major disadvantages of external fertilization is that it requires a large number of gametes to increase the chances of fertilization. This is because a large number of gametes are lost due to dilution in the water or predation by other organisms, reducing the chances of successful fertilization.

External fertilization is also dependent on environmental conditions, such as temperature and water pH, which can affect the survival of the gametes and the success of fertilization. Additionally, external fertilization does not allow for any parental care of the offspring, leaving them vulnerable to predation and other environmental factors.

Finally, external fertilization can result in low genetic diversity in populations, as the same set of genes is repeatedly combined in each generation, leading to reduced adaptability to changing environments and a higher risk of genetic diseases.

13. Differentiate between a zoospore and a zygote.

A zoospore is a type of asexual spore that is produced by some organisms, such as algae, fungi, and protozoa. Zoospores are motile and can move through water using flagella. They are genetically identical to the parent organism and can give rise to new individuals through asexual reproduction.

On the other hand, a zygote is a cell that results from the fusion of two gametes during sexual reproduction. The zygote contains genetic material from both parents and is the first cell of the new individual. It undergoes mitotic cell division to produce a multicellular organism.

Therefore, the main difference between a zoospore and a zygote is that a zoospore is a product of asexual reproduction and is genetically identical to the parent organism, while a zygote is formed as a result of sexual reproduction and contains genetic material from both parents. Additionally, zoospores are motile, while zygotes are not.

14. Differentiate between gametogenesis from embryogenesis.

Gametogenesis and embryogenesis are two different processes involved in sexual reproduction.

Embryogenesis, on the other hand, is the process of development of the zygote into a multicellular organism. After fertilization, the zygote undergoes mitotic cell division to form a blastula, which then undergoes further differentiation and development into an embryo.

Therefore, the main difference between gametogenesis and embryogenesis is that gametogenesis is the process of producing gametes, while embryogenesis is the process of developing the zygote into a multicellular organism. Gametogenesis occurs in the reproductive organs, while embryogenesis occurs in the developing embryo. Additionally, gametogenesis involves meiosis, while embryogenesis involves mitosis.

Post-fertilization changes in a flower refer to the events that occur after the fertilization of the egg cell by the sperm cell.

After fertilization, the zygote is formed, which later develops into an embryo. The ovary enlarges and develops into a fruit, while the other parts of the flower, such as petals and sepals, wither and fall off. The ovules develop into seeds, which contain the embryo and endosperm.

The endosperm, which is formed by the fusion of the polar nuclei and a sperm cell, provides nutrients to the developing embryo. In some plants, such as monocots, the endosperm remains in the seed and serves as a food source for the developing seedling.

The embryo undergoes further development and differentiation, giving rise to the various parts of the plant, such as the shoot, root, and leaves. The seed coat protects the embryo and endosperm and also helps in seed dispersal.

Overall, post-fertilization changes in a flower involve the development of the fruit and seed, which protect and nourish the developing embryo, and the withering of other flower parts that are no longer needed.

16. What is a bisexual flower? Collect five bisexual flowers from your neighbourhood and with the help of your teacher find out their common and scientific names.

A bisexual flower is a flower that has both male and female reproductive structures, namely the stamen and the pistil, respectively.

Five examples of bisexual flowers that can be found in the neighbourhood are:

Hibiscus – Hibiscus rosa-sinensis
Rose – Rosa sp.
Sunflower – Helianthus annuus
Marigold – Tagetes sp.
Jasmine – Jasminum sp.

All of these flowers have both stamens and pistils and can self-pollinate or be cross-pollinated by insects or other animals.

17. Examine a few flowers of any cucurbit plant and try to identify the staminate and pistillate flowers. Do you know any other plant that bears unisexual flowers?

Cucurbit plants, such as pumpkin, cucumber, and watermelon, produce both male and female flowers on the same plant. The male flowers, or staminate flowers, have long, thin stalks with a single, large, yellow anther, while the female flowers, or pistillate flowers, have a bulbous ovary at the base of the flower.

To identify the staminate and pistillate flowers, one can observe the flower closely and look for the presence of a long stalk with a single anther (stamen) or the presence of an ovary (pistil) at the base of the flower.

There are many other plants that bear unisexual flowers, meaning they produce either male or female flowers, but not both on the same plant. Examples of such plants include:

Papaya – Carica papaya
Kiwi – Actinidia deliciosa
Date palm – Phoenix dactylifera
Willows – Salix sp.
Asparagus – Asparagus officinalis

In these plants, male and female flowers are produced on separate plants, and pollination must occur between the male and female plants for fertilization and reproduction to take place.

18. Why are offspring of oviparous animals at a greater risk as compared to offspring of viviparous animals?

Oviparous animals are those that lay eggs, while viviparous animals give birth to live young. Offspring of oviparous animals are at a greater risk compared to offspring of viviparous animals because the eggs are exposed to the environment and do not receive the protection and nutrients provided by the mother as in viviparous animals.

In oviparous animals, the eggs must be laid in a suitable location for incubation, and the developing embryos are vulnerable to predation, extreme temperatures, and other environmental factors. Additionally, the yolk in the egg provides only limited nutrients to the developing embryo, which means that the offspring may be born smaller and less developed compared to viviparous offspring.

In contrast, viviparous offspring receive continuous nourishment and protection from the mother’s body, which helps to ensure their survival. The mother provides nutrients, oxygen, and other essential resources to the developing embryo through the placenta or other specialized structures, which allows the offspring to grow and develop in a more controlled and stable environment.

Class 12 NCERT Solution : Chapter-1 Reproduction in Organism

Chapter 1 Reproduction in organism Part-1

Sexual Reproduction in flowering plants

Human Reproduction