Question 1. Why is reproduction essential for organisms?
Sol. Reproduction, as the characteristic feature of all living organisms, is the ability to produce a young one similar to itself.
It is essential for organisms because it helps in survival and ensures the
continuance of various species on the earth. In its absence, the species will not be able to exist for a long time and may soon disappear from the earth.
Question 2. Which is a better mode of reproduction: sexual or asexual? Why?
- Sexual reproduction is a better mode of reproduction as compared to asexual reproduction.
- It is because of the following reasons:
- Asexual reproduction allows very little or no variation throughout their generation, due to which the individuals produced are exact copies of their parents and themselves.
- Sexual reproduction allows the formation of new variants by combining the DNA of two different parents. The offspring produced from the fusion of male and female gamete are not identical to their parents and to themselves. Due to such variation, individuals constantly adapt the changing and challenging environment for their survival which leads to the evolution of better-suited organisms which ensures the greater survival of a species.
Note: The variation that sexual reproduction creates among offspring is very important to the survival and reproduction of the population.
Question3. Why is the offspring formed by asexual reproduction referred to as clone?
Sol. Asexual reproduction is a type of reproduction in which a single individual is capable of producing offspring without involving fusion of male and female gametes. Offspring produced by asexual reproduction is referred to as clones because clone are genetically and morphologically similar and the offspring produced by asexual reproduction are not only genetically and morphologically similar to one another but also similar to their parent.
Question 4. Offspring formed due to sexual reproduction have better chances of survival. Why? Is this statement always true?
Sol. • Yes, the offspring produced by sexual reproduction have better chance of survival because they are not only genetically identical to their parents but also exhibit variations which lead them to adapt the changing environment for their better survival. Asexually produced organisms are genetically identical and all organism show similar adaptations. So, during any calamity, there is a possibility that the whole generation would destroyed leading to extinction of species.
• However, the given statement – “the offsprings produced due to sexual reproduction has better chance of survival” is not always true because of the following reasons –
(i) There are some inborn genetic disorders due to which organism have a risk in their survival, e.g., haemophilia.
(ii) Asexual reproduction is a fast and a quick mode of reproduction which does not consume much time and energy as compared to sexual reproduction.
Question 5. How does the progeny formed from asexual reproduction differ from those formed by sexual reproduction?
Sol. The progeny formed from asexual reproduction differ from those formed by sexual reproduction because, in asexual reproduction offspring (progeny) is produced by a single parent without the formation and fusion of gametes and involves only mitotic cell division that gives rise the daughter cells which are genetically identical to the parent cell. It does not show any variation among offsprings hence called as clones. Whereas, in sexual reproduction offspring is produced by two parents. It involves meiotic cell divisions producing haploid nuclei which on fusion produce offspring that are genetically different from their parents. Organism thus produced show variation from each other and their parents.
Question 6. Distinguish between asexual and sexual reproduction. Why is vegetative reproduction also considered as a type of asexual reproduction?
Sol. The difference between asexual and sexual reproduction are as follows :
Vegetative reproduction is considered asexual reproduction in plants because it does not involve the fusion of male & female gamete to produce new plant, rather vegetative plant parts like rhizome, runner, sucker, tuber, bulb all are capable of producing new offspring. These parts give rise to daughter individuals without the involvement of two parents.
Question 7. What is vegetative propagation? Give two suitable examples.
Sol.• Vegetative propagation, a method of asexual reproduction in plants, is a process which does not involve the production of seeds or spores for the propagation of new plants. In this propagation, the vegetative parts of plants (like runners, rhizomes, suckers, tubers etc.) other than seeds are used as vegetative propagules. This method is carried out by those plants which have lost their capacity to produce seeds or produce non viable seeds (e.g., Banana, seedless grapes, rose, pineapple, etc.)
• Examples of vegetative propagation:
(i) Underground modified stems such as rhizomes (e.g., Ginger, Eichhornia or water hyacinth, etc.), corms (e.g., Colocasia, Banana, etc.), bulbs (e.g., Garlic, onion, etc.), etc. are able to give rise new offsprings. Rhizomes are root-like stems that grow horizontally under the ground. New roots and shoots form at the nodes with shoots growing upwards to form new plantlets. Lateral buds grow out to form new rhizomes.
(ii) Vegetative propagation by leaves in Bryophyllum, Begonia etc. Bryophyllum have adventitious buds coming out from the notches of the leaves. In these plants, plantlets develop from the buds present on the marginal notches of the intact leaves. These plantlets become detached and develop into independent plants.
Question 8. Define:
(a) Juvenile phase
(b) Reproductive phase
(c) Senescent phase.
Sol. (a) Juvenile phase : All organisms have to reach a certain stage of growth and maturity in their life before they can reproduce sexually. That period of growth is called juvenile phase. However, this phase is known as vegetative phase in plants.
(b) Reproductive phase : The end of juvenile/vegetative phase marks the beginning of reproductive phase. During this phase, the organisms reproduce sexually and produce offspring.
(c) Senescent phase: It begins from the end of the reproductive phase. It is the period when an organisms grows old and lose the ability to reproduce.
Question 9. Higher organisms have resorted to sexual reproduction in spite of its complexity. Why?
Sol. Although sexual reproduction involves more time and energy, higher organisms have resorted to sexual reproduction in spite of its complexity because sexual reproduction results in multiplication and perpetuation of species and also contributes to evolution of species by introducing variation much more faster as compare to asexual reproduction in a particular population. Sexual reproduction enables higher organisms to survive during unfavourable conditions.
Question 10. Explain why meiosis and gametogenesis are always interlinked?
Sol. • Meiosis (a process which occurs during gametogenesis, involves the reductional division in which the amount of genetic material is reduced) and gametogenesis (the process of the formation of gametes) are always interlinked because gametes produced by organisms are haploids (containing one set of chromosomes), while the body of an organisms is diploid.
• Therefore, for producing haploid gametes (gametogenesis) the germ cells of an organisms undergoes meiosis. During the process of meiosis the meiocytes of an organisms undergoes two successive nuclear and cell divisions with a single cycle of DNA replication to form haploid gametes. Thus, gametes are formed as a result of meiosis so that their chromosome number remains constant.
Question 11. Identify each part in a flowering plant and write whether it is haploid (n) or diploid (2n).
(e) Male gamete____
(c) n (d) n
(e) n (f) 2n
Question 12. Define external fertilisation. Mention its disadvantages.
Sol. • When fusion of the gametes takes place outside the body of the organisms in an external medium, generally water, it is called external fertilisation or external syngamy. This form is found in many aquatic animals like fishes, amphibians, majority of algae. During the process, parents release eggs and sperms in the surrounding water, then fertilisation and development of offspring occur externally.
• Disadvantages of external fertilisation
(i) It occurs only in aquatic medium.
(ii) A chance factor is involved requiring synchronous release of gametes nearby and absence of turbulence of water.
(iii) There is no protection to young ones which results in a low rate of survival of the progenies and are vulnerable to a number of predators.
(iv) Eggs have less chances offertilisation. This can lead to the wastage of a large number of eggs produced during the process.
Note: Animals using external fertilisation strategy does not rely on hormones, mating rituals and behavioural factors to ensure that the male and female have sexual intercourse.
Question 13. Differentiate between a zoospore and a zygote.
Sol. • Zoospores are the microscopic, flagellated (motile) special asexual reproductive structures found in certain members of the kingdom fungi and simple plants like algae whereas zygote is a diploid cell formed by the fusion of male and female gametes. The zygote is usually nonflagellated.
• Zoospores are the structures that give rise to new organism whereas zygote is formed after fertilisation which develops into a mature organism.
Question 14. Differentiate between gametogenesis from embryogenesis.
Sol. Differences between gametogenesis and embryogenesis
Question 15. Describe the post-fertilisation changes in a flower.
Sol. After fertilisation the zygote is formed. The zygote then undergoes many divisions by the process of mitosis, to form an embryo by a process called embryogenesis. In sexual reproduction, events that occur after the formation of zygote are called post-fertilisation events.
The Post-fertilisation changes in a flower are:
- In flowering plants, the zygote is formed inside the ovule.
- After fertilization the sepals, petals and stamens of the flower wither and fall off. But the pistil remains attached to the plant.
- The zygote develops into the embryo and the ovules develop into the seed. The seed contains an embryo, enclosed in a protective covering, called the seed coat. As the seed grows, further, other floral parts wither and fall off. This lead to the growth of the ovary, which enlarges and ripens to become a fruit with a thick wall called the pericarp.
Question 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.
Sol.• Flowers in which male and female sex organs (stamens and carpels) are borne on the same flowers are called bisexual flowers.
• Examples of plants bearing bisexual flowers are as follows:
(i) Brassica (sarson) – Brassica campestris
(ii) Onion – Allium cepa
(iii) Garden pea (Edible pea)-Pisum sativum
(iv) Petunia – Petunia hybrida
(v) China rose (shoe flower) -Hibiscus rosa-sinensis.
Question 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?
Sol. • By studying a flowering plant of family cucurbitaceous it is noted that cucurbit plant bears unisexual flowers as these flowers have either the stamen or the pistil. Male flowers are present at the summit peduncles and female flowers are solitary.
• The male or staminate flowers of cucurbits bear bright coloured petals and a prominent group of stamens. Male plants or staminate flowers do not bear fruits.
• The female or pistillate flowers bear fruits. In a fertilised young pistillate flower very small fruit is visible below petals and sepals.
• Other examples of plants that bear unisexual flowers are: Papaya, Mulberry and Date-palm.
Question 18. Why are offspring of oviparous animals at a greater risk as compared to offspring of viviparous animals?
Sol. Animal shows different methods of production of young ones. Oviparous and viviparous are such two methods.
Offsprings of oviparous animals are at greater risk as compared to offspring of viviparous animals because,
• In oviparous animals, the development of the zygote takes place outside the body of the female parent whereas in viviparous animals, it takes place inside the body of the female.
• In oviparous animals, the fertilised eggs are laid in the open environment where they are not protected from the predators and their whole embryonic development is exposed to harmful environmental and biological factors.Thus, the survial rate of oviparous animals is very less as compared to offspring of viviparous animals.
- NCERT Solutions for Class 12 (All Subjects)
- NCERT Solutions for Class 12 Biology
- Reproduction in Organisms Class 12 Notes Biology Chapter 1