Reproduction in Organisms NCERT Solutions for Class 12 Biology Chapter 1 with Answers

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NCERT Solutions for Class 12 Biology Chapter 1

Page No. 17

Exercises

Q1. Why is reproduction essential for organisms?

Answer: Reproduction is a process by which an organism produces young ones of its own kind to maintain the continuity of the species. It enables the species to live generation after generation.

Q2. Which is a better mode of reproduction sexual or asexual? Why?

Answer: Sexual reproduction is a better mode of reproduction. It allows the formation of new variants by the combination of the DNA from two different individuals, typically one of each sex. It involves the fusion of the male and the female gamete to produce variants, which are not identical to their parents and to themselves. This variation allows the individual to adapt to constantly changing and challenging environments. Also, it leads to the evolution of better suited organisms which ensures greater survival of a species. On the contrary, asexual reproduction allows very little or no variation at all. As a result, the individuals produced are exact copies of their parents and themselves.

Q3. Why is the offspring formed by asexual reproduction referred to as clone?

Answer: The offsprings formed by asexual reproduction are referred to as clones because it involves only a single parent. Furthermore, there is no recombination of genes and the offsprings produced are genetically identical.

Q4. Offspring formed due to sexual reproduction have better chances of survival. Why? Is this statement always true?

Answer: The offspring that are produced by sexual reproduction are not genetically identical to their parents. They exhibit variations because they receive chromosomes from two different parents. Since they show variation, they are highly adapted to the changing environment. 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 destroy leading to extinction of species. However, this statement is not true always because of some inborn genetic disorder due to which organism have a risk in their survival, e.g., Haemophilia.

Q5. How does the progeny formed from asexual reproduction differ from those formed by sexual reproduction?

Answer:

Progeny formed from asexual reproductionProgeny formed from sexual reproduction
a) There is no fusion of gametes therefore progeny are morphologically and genetically identical to single parent.
b) Variations are absent.
c) Progeny is less adaptable to changes in environment.
a) There is fusion of gametes therefore progeny are not identical to parents.
b) Variations are present.
c) Progeny is more adaptable to changes in environment.

Q6. Distinguish between asexual and sexual reproduction. Why is vegetative reproduction also considered as a type of asexual reproduction?

Answer:

       Sexual reproduction                                   Asexual reproduction
1 It occur only in in vertebrates and lower chordates    1. It occur almost in all types of animals.
2 It is always uniparental    2. It is usually biparental
3 Gametes are not formed    3. Two types of gametes are formed
4 It involves only mitosis    4. It involves bith meiosis and mitosis
5 Daughter organisms are genetically identical to the parent    5. Daughter organisms genetically differ from their parents
6 Since there is no variation, so it does not contribute to evolutionof the species   6. Because of variations, it contributes to the evolution of species.

Q7. What is vegetative propagation? Give two suitable examples.

Answer: Vegetative propagation is a mode of asexual reproduction in which new plants are obtained from the vegetative parts of plants. It does not involve the production of seeds or spores for the propagation of new plants. Vegetative parts of plants such as runners, rhizomes, suckers, tubers, etc. can be used as propagules for raising new plants.

Examples of vegetative reproduction are:

  1. Eyes of potato:

The surface of a potato has several buds called eyes. Each of these buds when buried in soil develops into a new plant, which is identical to the parent plant.

  1. Leaf buds of Bryophyllum:

The leaves of Bryophyllum plants bear several adventitious buds on their margins. These leaf buds have the ability to grow and develop into tiny plants when the leaves get detached from the plant and come in contact with moist soil.

Q8. Define

(a) Juvenile phase,
(b) Reproductive phase,
(c) Senescent phase

Answer:

(a) The juvenile phase of an organism’s life is described as the time of growth that begins after birth and ends before it achieves reproductive maturity. It is often termed the vegetative phase.

(b) The reproductive phase is defined as the period when an individual organism is sexually active and attains reproductive maturity. In this phase, the production of gametes also takes place.

(c) The senescence phase is the time after the reproductive phase when a cell loses its ability to reproduce. Here the cell can grow old and stop dividing, but it does not die. This is also known as cell ageing.

Q9. Higher organisms have resorted to sexual reproduction in spite of its complexity. Why?

Answer: 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 than asexual reproduction in a particular population. Sexual reproduction enables higher organisms to survive during unfavourable conditions.

Q10. Explain why meiosis and gametogenesis are always interlinked?

Answer: Meiosis is the process that occurs during gametogenesis. Gametogenesis refers to the process of how gametes are formed. Meiosis is necessary for the formation of gametes. Hence, both the processes are said to be interlinked.

Q11. 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 _______

Answer:

(a) Diploid (2n)
(b) Diploid (2n)
(c) Haploid (n)
(d) Haploid (n)
(e) Haploid (n)
(f) Diploid (2n)

Q12. Define external fertilization. Mention its disadvantages.

Answer: External fertilisation is a type of fertilization that occurs outside the female body, in external medium, such as water.
Disadvantages of external fertilixation are

  • Very less chance of fusion of male and female gametes.
  • Offspring are vulnerable to predators because of no parental care.

Q13. Differentiate between a zoospore and a zygote.

Answer:

ZoosporeZygote
These are formed inside the zoosporangium.
Result of asexual reproduction.
Flagellated and motile spore.
Can be haploid or diploid.
Participates in dispersal.
It is found in algae, fungi and protozoans.
These are formed by the fusion of male and female gametes.
Result of sexual reproduction.
Non-motile.
Diploid.
Does not participate in dispersal.
Found in higher organisms.

Q14. Differentiate between gametogenesis from embryogenesis.

Answer:

GametogenesisEmbryogenesis
a) It is the process of generation of haploid gametes.
b) It involves meiosis.
c) Further transformation may be required to make male gametes motile
a) It is the process of formation of embryo.
b) It involves mitosis.
c) Further cell differentiation required for the formation of specialised tissues and organs.

Q15. Describe the post-fertilization changes in a flower.

Answer: The post-fertilization changes that take place in a flower are as follows:

  • The formation of zygote which later develops into an embryo and a primary endosperm cell which develops into an endosperm takes place.
  • While the sepals, petals and stamens are shed, the pistil remains intact.
  • The fertilized ovule develops into seeds.
  • The ovary matures into a fruit that later develops a thick protective wall, called the pericarp.
  • Seeds after dispersal germinate under favourable conditions which later develop into a new plant.

Q16. 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.

Answer: A flower that contains both the male and female reproductive structure (stamen and pistil) is called a bisexual flower. Examples of plants bearing bisexual flowers are:

(1) Water lily ( Nymphaea odorata)
(2) Rose (Rosa multiflora )
(3) Hibiscus (Hibiscus Rosa-sinensis )
(4) Mustard ( Brassica nigra)
(5) Petunia (Petunia hybrida)

Q17. 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?

Answer: Flowers that bear stamens are called staminate flowers while the flowers that bear pistil are known as pistillate flowers. Cucurbit plants bear unisexual flowers, i.e., both the male and female reproductive structures are on the same plant.

The staminate flowers of cucurbit have petals that are coloured bright yellow. It also has stamens which function as the male reproductive structures of a flower. The female reproductive structures of a flower are the pistils, which is present on the pistillate flowers.

Papaya is another plant that bears unisexual flowers.

Q18. Why are offspring of oviparous animals at a greater risk as compared to offspring of viviparous animals?

Answer: Oviparous animals lay eggs outside their body. As a result, the eggs of these animals are under continuous threat from various environmental factors. On the other hand, in viviparous animals, the development of the egg takes place inside the body of the female. Hence, the offspring of an egg-laying or oviparous animal is at greater risk as compared to the offspring of a viviparous animal, which gives birth to its young ones.

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