NCERT Solutions For Class 12 Biology Reproduction in Organisms
QUESTIONS FROM THE TEXTBOOK SOLVED
1. Why is reproduction essential for organisms?
Ans: Reproduction is a biological process by which new individual organisms ''offspring'' are produced from their parents. Reproduction maintains the continuity of life on earth. In the absence of reproduction, no new offspring will be produced; existing species won't survive for a long time and may soon get extinct. Therefore, it is essential for organisms.
2. Which is a better mode of reproduction sexual or asexual? Why?
Ans: Sexual reproduction is a better mode of reproduction because of the following reasons:
Variation: Since fusion of gametes from different parents occurs during sexual reproduction, hence genetic recombination takes place causing variations.
Evolution: Variation being a major factor of natural selection, therefore, it plays an important role in evolution.
Adaptation: The offspring produced due to sexual reproduction adapt better to the changing environmental conditions.
Vigour and Vitality: Genetic recombination, interaction, etc. during sexual reproduction provide vigour and vitality to the offspring.
3. Why is the offspring formed by asexual reproduction referred to as clone?
Ans: Asexual reproduction is a type of reproduction in which a single individual is capable of producing offspring. A clone is a group of morphologically and genetically identical individuals. In the process of asexual reproduction, only one parent is involved and there is no fusion of the male and the female gamete. As a result, the offsprings so produced are morphologically and genetically similar to their parents and are thus, called clones.
4. Offsprings formed due to sexual reproduction have better chances of survival. Why? Is this statement always true?
Ans: Sexual reproduction involves the fusion of the male and the female gamete. This fusion allows the formation of new variants by the combination of the DNA from two (usually) different members of the species. The variations allow the individuals to adapt under varied environmental conditions for better chances of survival. However, it is not always necessary that the offspring produced due to sexual reproduction has better chances of survival.
Under some circumstances, asexual reproduction is more advantageous for certain organisms. For example, some individuals do not move from one place to another and are well settled in their environment. Also, asexual reproduction is a fast and quick mode of reproduction that does not consume much time and energy as compared to sexual reproduction.
5. How does the progeny formed from asexual reproduction differ from those formed by sexual reproduction?
Ans: The production of offspring by a single parent without the formation and fusion of gametes is called asexual reproduction. It involves only mitotic cell division that gives rise to the daughter cells which are genetically identical to the parent cell. Sexual reproduction is the production of offspring by two parents, male and female. It involves meiotic cell divisions producing haploid nuclei which on fusion produce offspring that are genetically different from their parents.
6. Distinguish between asexual and sexual reproduction. Why is vegetative reproduction also considered a type of asexual reproduction?
Ans: The difference between asexual and sexual reproduction are as follows :
7. What is vegetative propagation? Give two suitable examples.
Ans: Vegetative propagation is the formation of new plants from vegetative units. In plants, the units of vegetative propagation are runner, rhizome, sucker, tuber, offset, bulb, etc. These are capable of producing new offsprings. These structures are called v vegetative propagules.
Modified tuberous roots of sweet potato, tapioca, yam, Dahlia and Tinospora can be propagated vegetatively when planted in soil. Small plants emerging from the buds (called eyes) of the potato tuber, from the rhizomes of banana and ginger are other examples.
8. Define:
(a)Juvenile phase
(b)Reproductive phase
(c)Senescent phase.
Ans: (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 the juvenile phase. However, this phase is known as the vegetative phase in plants. This phase is of different durations in different organisms.
(b)Reproductive phase: The end of the juvenile/ vegetative phase marks the beginning of the reproductive phase. During this phase, the organisms produce offspring. In higher plants, this phase can be easily seen when they come to flower but in animals, the juvenile phase is followed by morphological and physiological changes prior to active reproductive behaviour. The reproductive phase is also of a variable period in different organisms like some plants, flower throughout the year while others show seasonal flowering. In animals like birds lay eggs seasonally “but when in captivity (as in poultry farms) can be made to lay eggs throughout the year. Placental female mammals undergo cyclical changes in reproductive organs during this phase.
(c) Senescent phase: It begins from the end of the reproductive phase. During this phase of life span, there is progressive deterioration in the body (like slowing of metabolism, etc.). Old age ultimately leads to death.
9. Higher organisms have resorted to sexual reproduction in spite of their complexity. Why?
Ans: Higher organisms have resorted to sexual reproduction in spite of its complexity because sexual reproduction results in the multiplication and perpetuation of species and also contribute to the evolution of species by introducing variation much faster than asexual reproduction in a particular population. Sexual reproduction enables higher organisms to survive during unfavourable conditions.
10. Explain why meiosis and gametogenesis are always interlinked?
Ans: Gametogenesis is the process of formation of two types of haploid gametes (male and female). In gametogenesis, gametes are haploid in number and formed by meiosis so the chromosome number is haploid. Thus gametogenesis is always linked with meiosis.
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)2n (b)2n (c)n (d)n (e)n (f)2n
12. Define external fertilization. Mention its disadvantages.
Ans: When fusion of the gametes takes place outside the body of the organisms, it is called external fertilization or external syngamy. An external medium like water is required for this form of fertilization. This form is found in many aquatic animals like fishes, amphibians, majority of algae.
In this, parents release eggs and sperms in the surrounding water, then fertilization and development of offspring occur externally. Disadvantages of external fertilization:
(i)it occurs only in aquatic medium.
(ii)A chance factor is involved requiring synchronous release of gametes nearby and absence of turbulence of the water.
(iii)There is no protection for young ones. They are vulnerable to a number of predators.
13. Differentiate between a zoospore and a zygote.
Ans: The zoospore is flagellated, motile, haploid or diploid spore formed inside a zoosporangium. It is the result of asexual reproduction.
The zygote is always diploid and formed by the fusion of gametes. It is usually non- flagellated and non-motile or motile. It is the net result of sexual reproduction.
14. Differentiate between gametogenesis
15. Describe the post-fertilization changes in a flower.
Ans: In sexual reproduction, events that occur after the formation of the zygote are called post-fertilization events. 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 ovary develops into a dying fruit that develops. a thick wall called pericarp which is a protective – function. After dispersal, seeds germinate under favourable conditions to produce new plants.
16. What is % bisexual flower? Collect five bisexual flowers from your neighbourhood and with the help of your teacher find out their common and scientific names.
Ans: Flowers in which male and female sex organs (stamens and carpels) are borne on the same flowers are called bisexual flowers. You can observe the following bisexual flowers in your kitchen and colony gardens :
(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.
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?
Ans: 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. Some unisexual plants are Papaya, Mulberry and Date-palm.
18. Why are offspring of oviparous animals at a greater risk as compared to offspring of viviparous animals?
Ans: On the basis of the development of the zygote, animals are grouped into oviparous and viviparous. The oviparous animals such as reptiles and birds lay eggs. Their fertilised eggs are covered by hard calcareous shell and are laid in a safe place in the environment. After the incubation period, young ones hatch out. In viviparous animals such as the majority of mammals including human beings, the zygote develops into a young one inside the body of the female individual. After a certain growth period, the young ones are delivered by the female individual. Due to proper care and protection, the chances of survival of young ones are more in viviparous individuals. Oviparous offsprings are at a greater risk than viviparous ones.
Intext Questions
1. Why do we say there is no natural death in single-celled organisms?
Answer.
2. Are these offspring likely to be genetically identical or different?
Answer.
3. You have learnt about vegetative reproduction in plants in Class XI. What do you think - Is vegetative reproduction also a type of asexual reproduction? Why do you say so? Is the term clone applicable to the offspring formed by vegetative reproduction?
Answer.