NCERT Solutions for Class 12 Biology Chapter 1

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Class:12
Subject:Biology
Chapter 1:Reproduction in Organisms

NCERT Solutions for class 12 Biology Chapter 1

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Solutions for 12 Biology Chapter 1

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NCERT Solutions for class 12 Biology Chapter 1 Reproduction in Organisms

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Important Terms on Reproduction in Organisms
  • Monoecious Plants: Plants having both male and female flowers on same plant. e.g. cucurbits and coconut. The term “homothallic” is used in Fungi for same condition.
  • Dioecious Plants: Plants having male and female flowers on separate plant. e.g. Papaya and date palm. The term “heterothallic” is used in fungi for the same condition.
  • Oestrus Cycle: The reproductive cycle in non-primate mammals like cows, sheep, rats, deer, dogs and tigers etc. The sexually active females referred to as being in ‘heat’ at a specific time of Oestrus cycle. They reabsorbs the endometrium if conception does not occur.



Important Questions
  1. There are 380 chromosomes in meiocytes of a butterfly. How many chromosomes does male gamete of butterfly have?
  2. Which characteristic property of Bryophyllum is exploited by gardeners?
  3. Mention the unique flowering phenomenon exhibited by strobilanthus kunthiana (Neelakuranji).
  4. Mention the unique feature with respect to flowering and fruiting in bamboo species.



Answers:

1. 190 chromosomes.
2. Adventitious bud arising from margin of the leaf.
3. Flower one in 12 years.
4. Flower once in their life time after 50-100 years, produce large no. of fruits and die.

Questions

5. Higher organisms have resorted to sexual reproduction in spite of its complexity. Why?
6. Bryophytes and Pteridophytes produce a large number of male gametes but relatively very few female gametes. Why?
7. (a) Why is vegetative propagation also considered as a type of asexual reproduction?
(b) Which is better mode of reproduction: Sexual or Asexual? Why?


Answers

5. Because of variations, gene pool, vigour and vitality and parental care.
6. Because male gamete need medium (water) to reach egg/female gamete. A large number of the male gametes fail to reach the female gamete. It increases the probability of fertilisation.
7. (a) Vegetative propagation takes place when new individuals arise from vegetative part of parent and have characters similar to that of parent plant.
(b) Sexual reproduction, it introduces variations in offsprings and has evolutionary significance. It helps offsprings to adjust according to the changes in environment. It produces better off springs due to character combination.

Why is reproduction essential for organisms?

Reproduction is a fundamental feature of all living organisms. It is a biological process through which living organisms produce offspring’s similar to them. Reproduction ensures the continuance of various species on the Earth. In the absence of reproduction, the species will not be able to exist for a long time and may soon get extinct.

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

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.

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

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.

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

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 who do not move from one place to another and are well settled in their environment. Also, asexual reproduction is a fast and a quick mode of reproduction which does not consume much time and energy as compared to sexual reproduction.

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

Although sexual reproduction involves more time and energy, higher organisms have resorted to sexual reproduction in spite of its complexity. This is because this mode of reproduction helps introduce new variations in progenies through the combination of the DNA from two (usually) different individuals. These variations allow the individual to cope with various environmental conditions and thus, make the organism better suited for the environment. Variations also lead to the evolution of better organisms and therefore, provide better chances of survival. On the other hand, asexual reproduction does not provide genetic differences in the individuals produced.

Explain why meiosis and gametogenesis are always interlinked?

Meiosis is a process of reductional division in which the amount of genetic material is reduced. Gametogenesis is the process of the formation of gametes. Gametes produced by organisms are haploids (containing only one set of chromosomes), while the body of an organism is diploid. Therefore, for producing haploid gametes (gametogenesis), the germ cells of an organism undergo meiosis. During the process, the meiocytes of an organism undergo two successive nuclear and cell divisions with a single cycle of DNA replication to form the haploid gametes.

Define external fertilization. Mention its disadvantages.

External fertilization is the process in which the fusion of the male and the female gamete takes place outside the female body in an external medium, generally water.
Fish, frog, starfish are some organisms that exhibit external fertilization.

Disadvantages of external fertilization:
In external fertilization, eggs have less chances of fertilization. This can lead to the wastage of a large number of eggs produced during the process.
Further, there is an absence of proper parental care to the offspring, which results in a low rate of survival in the progenies.

Describe the post-fertilization changes in a flower.

Fertilization is the process of the fusion of the male and the female gamete to form a diploid zygote. After fertilization, the zygote divides several times to form an embryo. The fertilized ovule forms a 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 leads to the growth of the ovary, which enlarges and ripens to become a fruit with a thick wall called the pericarp.

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

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.