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NCERT Class 12 Biology Solutions: Chapter 1- Reproduction in Organisms

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Why is reproduction essential for organisms?

Solution: Reproduction is essential for organisms as:

(i) It is a biological process through which living organisms produce offspring’s similar to them.

(ii) 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?

Solution: Sexual reproduction is a better mode of reproduction than asexual reproduction because:

(i) It allows the formation of new variants which are not identical to their parents and to themselves by the combination of the DNA from two different individuals. This variation allows the individual to adapt to different environments. On the contrary, asexual reproduction allows very little or no variation at all.

(ii) Also, it leads to the evolution of better-suited organisms which ensures greater survival of a species.

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

Solution: In the process of asexual reproduction there is no fusion of male and female gamete as only one parent is involved. 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?

Solution: Sexual reproduction involves the fusion of male and female gamete. This fusion allows the formation of new variants by the combination of the DNA from two different individuals. The variations allow the individuals to adapt to changing environmental conditions this gives them a better chance of survival.

However, it is not always true as:
(i) Sometimes 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.

(ii) It is a fast and quick mode of reproduction which does not consume much time and energy.

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

Solution:

 Sl. No. Progeny from asexual reproduction Progeny from sexual reproduction
1.  Asexual reproduction does not involve the fusion of the male and the female gamete.   Sexual reproduction involves the fusion of the male and the female gamete of two individuals, typically one of each sex. 
2.  Offspring are morphologically and genetically identical to parents.   Offspring are not identical to parents. 
3.  Offspring thus produced do not show variations and are called clones.   Offspring thus produced show variations from each other and their parents. 

 

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

Solution:

 Sl. No. Sexual reproduction Asexual reproduction
1.  It involves the fusion of the male and female gamete.   It does not involve the fusion of the male and female gamete. 
2.  It usually requires two different individuals.   It requires only one individual. 
3.  The individuals produced are not identical to their parents and show variations.   The individuals produced are identical to the parent and are hence, called clones. 
4.  Most animals reproduce sexually.   It is common in organisms having simple organizations such as algae and fungi. 
5.  It is a slow process.   It is a fast process. 

 

What is vegetative propagation? Give two suitable examples.

Solution: 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.

Examples of vegetative reproduction are:

Eyes of potato:

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

Leaf buds of Bryophyllum:

The leaf buds of Bryophyllum 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.

Define

  • Juvenile phase,
  • Reproductive phase,
  • Senescent phase.

Solution: (a) Juvenile phase:

It is the period of growth in an individual organism after its birth and before it reaches reproductive maturity.

(b) Reproductive phase:

It is the entire period when an organism is reproductively active.

(c) Senescent phase:

It is the period when an organism grows old and loses the ability to reproduce.

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

Solution: Although sexual reproduction involves more time and energy, higher organisms have resorted to sexual reproduction in spite of its complexity. This is because:

  1. This mode of reproduction introduces new variations by the combination of the DNA from two different individuals. These variations allow the individual to cope better with various environmental conditions and thus, makes the organism better suited for the environment.
  2.  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 variations in the individuals produced.’

Explain why meiosis and gametogenesis are always interlinked?

Solution: Meiosis is a process of reductional division in which the amount of genetic material is reduced to half. Gametogenesis is the process of the formation of gametes. Gametes produced by organisms are haploids, 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 reductional division to produce haploid gametes. Hence, meiosis and gametogenesis are always interlinked.

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 ———————————

Solution:

(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)

Define external fertilization. Mention its disadvantages

Solution: External fertilization is the process in which the fusion of the male and the female gamete takes place outside the female’s body in an external medium, generally in water.

For example, fertilization in fishes, frogs, starfish, etc.

Disadvantages of external fertilization:

  1.  In external fertilization, chances of fertilization of the egg is less. This can lead to the wastage of a large number of eggs produced during the process.
  2. There is an absence of proper parental care to the offspring, which results in their low rate of survival.

Differentiate between a zoospore and a zygote

Solution:

 Sl. No. Zoospore Zygote
1.  A zoospore is a motile asexual spore that uses the flagella for movement.    A zygote is a non – motile diploid cell formed as a result of fertilization.   
2.  It is an asexual reproductive structure.   It is formed as a result of sexual reproduction.  

Differentiate between gametogenesis from embryogenesis.

Solution:

Gametogenesis Embryogenesis
 It is the process of the formation of haploid male and female gametes from diploid meiocytes through the process of meiosis     It is the process of the development of the embryo from diploid zygote by repeated mitotic divisions of it.   


Describe the post-fertilization changes in a flower.

Solution: Events occurring after fertilization is called post fertilization changes. After fertilization, the zygote 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.

 

Related: NCERT Class 12 Biology Solutions – PDF Notes | Download for Free

 

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

Solution: 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

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?

Solution: Cucurbit plant bears unisexual flowers. The staminate flowers bear bright, yellow colored petals along with stamens that represent the male reproductive structure. On the other hand, the pistillate flowers bear only the pistil that represents the female reproductive structure.

Other examples of plants that bear unisexual flowers are corn, papaya, cucumber, etc.

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

Solution: The offspring of an oviparous animal is at greater risk as compared to the offspring of a viviparous animal as oviparous animals lay eggs outside their body due to which 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 female’s body.

 

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