NCERT Solutions for Class 12 Biology Reproduction in Organisms [Old Book]

NCERT
SOLUTIONS
CLASS - 12th

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

Q1 Why is reproduction essential for organisms?

Answer. Reproduction is a fundamental feature of all living organisms. It is a biological process
through which living organisms produce offspring 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.

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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.

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

Answer. 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.

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

Answer. 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 (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.

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

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

Vegetative propagation is a process in which new plants are obtained without the production of
seeds or spores. It involves the propagation of plants through certain vegetative parts such as the
rhizome, sucker, tuber, bulb, etc. It does not involve the fusion of the male and the female gamete
and requires only one parent. Hence, vegetative reproduction is considered as a type of asexual
reproduction.

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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.

2. 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 De ne :
(a) Juvenile phase,
(b) Reproductive phase,
(c) Senescent phase.

Answer. (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 period when an individual organism reproduces sexually.
(c) Senescent phase:
It is the period when an organism grows old and loses the ability to reproduce.

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

Answer. 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.

Q10 Explain why meiosis and gametogenesis are always interlinked?

Answer. 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.

Q11 Identify each part in a owering 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) Ovary Diploid (2n)
(b) Anther Diploid (2n)
(c) Egg Haploid (n)
(d) Pollen Haploid (n)
(e) Male gamete Haploid (n)
(f) Zygote Diploid (2n)

Q12 De ne external fertilization. Mention its disadvantages.

Answer. 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, star sh 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.

Q13 Differentiate between a zoospore and a zygote.

Q14 Differentiate between gametogenesis from embryogenesis.

Q15 Describe the post-fertilization changes in a ower.

Answer. 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 oral 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.

Q16 What is a bisexual ower? Collect feve bisexual owers from your neighborhood and with the
help of your teacher nd out their common and scienti c names.

Answer. A ower that contains both the male and female reproductive structure (stamen and
pistil) is caned a bisexual ower. Examples of plants bearing bisexual owers are:
(1) Water lily (Nymphaea odorata)
(2) Rose (Rosa multi ora)
(3) Hibiscus (Hibiscus Rosa-sinensis)
(4) Mustard (Brassica nigra)
(5) Petunia (Petunia hybrida)

Q17 Examine a few owers of any cucurbit plant and try to identify the staminate and pistillate
owers. Do you know any other plant that bears unisexual owers?

Cucurbit plant bears unisexual owers as these owers have either the stamen or the pistil. The
staminate owers bear bright, yellow coloured petals along with stamens that represent the male
reproductive structure. On the other hand, the pistillate owers bear only the pistil that
represents the female reproductive structure. Other examples of plants that bear unisexual owers
are corn, papaya, cucumber, etc.

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.