NCERT Solutions for Class 12-science Biology Chapter 1 - Reproduction in Organisms

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Chapter 1 - Reproduction in Organisms Exercise 17

Solution 1

Reproduction is essential for organisms 

  • For multiplication and maintenance of the identity of a species
  • To introduce variation among the individuals of a species
  • To maintain and inherit the genetic constitution of a species
  • To ensure continuity of the thread of life of a species
Solution 2

Asexual reproduction generates little or no variation at all. As a result, the offspring produced are identical to their parents.

Sexual reproduction is a better mode of reproduction as compared to asexual reproduction because

  • It involves two parents which contribute to genetic variation because of crossing over and recombination which occurs during gamete formation by meiosis.
  • Because of the combination of new characters, better offspring are produced.
  • The variations enable the offspring to adjust according to the changes in the environment.
  • They are essential for evolution and survival of species under unfavourable conditions.
Solution 3
  • A clone is a group of genetically and morphologically identical individuals.
  • In the process of asexual reproduction, only one parent is involved, so the fusion of gametes does not occur. 
  • Therefore, there is no variation and the offspring formed are not only exact copies of their parents but also morphologically and genetically identical to each other.
  • Hence, they are called clones.
Solution 4

Offspring formed due to sexual reproduction have better chances of survival because

  • Sexual reproduction introduces genetic variation in the offspring which is essential for evolution and survival of the species.
  • Offspring exhibit heterosis or hybrid vigour which enables them to adapt better in the changing environment.
  • The enhanced traits allow the offspring to be tolerant and survive under adverse conditions.

However, during sexual reproduction, there is crossing over and random selection due to which the offspring produced may have traits which are inferior to the parents.

Hence, it is not always true that the offspring formed by sexual reproduction have better chances of survival.

Solution 5

Progeny formed from asexual reproduction

Progeny formed from sexual reproduction

  • They are formed from a single parent without the formation of gametes.
  • They are formed from two parents with the formation and fusion of gametes followed by their fertilisation.
  • They are not only identical to each other but also exact copies of their parents.
  • They are different from each other and from their parents.
  • They do not show genetic variation. Variations may occur only due to mutation.
  • They show genetic variation due to mutation, crossing over and recombination.
  • They are less adapted to the changing environment due to the absence of variations.
  • They are very well adapted to the changing environment due to the presence of variations.
Solution 6

Differences between asexual and sexual reproduction:


 

Characteristics

Asexual Reproduction

Sexual Reproduction

Number of parents

A single parent is involved.

Generally, two parents are involved.

Sex organs

Does not require the production of sex organs.

Requires the production of sex organs.

Nature of cells

Somatic cells are involved.

Germ cells are involved.

Types of divisions

Involves mitosis only.

Involves both mitosis and meiosis.

Fusion of gametes

Does not involve the fusion of cells or gametes.

Involves the fusion of cells or gametes.

Genetic constitution

Offspring are genetically similar to the parent.

Offspring are genetically different from either of the parents.

Variations

Variations are not introduced and so does not have evolutionary importance.

Variations are introduced and so has evolutionary importance.

Rate of reproduction

Rapid method of multiplication.

Slower method of multiplication.

Occurrence

Found in lower invertebrates and lower chordates.

Found in higher plants and animals, and human beings.


Vegetative reproduction involves the following characteristics:

  • It involves only a single parent.
  • It does not involve gametes.
  • It involves vegetative propagules which are somatic cells.
  • It involves only mitotic cell division and no meiosis is involved.
  • It produces offspring which are genetically identical to the parent.
  • It does not introduce variations, and so, the parental characters are preserved.

Because vegetative reproduction has characteristics which are similar to asexual reproduction, vegetative reproduction is also considered a type of asexual reproduction.

Chapter 1 - Reproduction in Organisms Exercise 18

Solution 1
  • Meiosis is the process of reduction division in which the amount of genetic material is reduced to half.
  • Gametogenesis is the process of formation of male and female gametes.
  • The gametes produced by gametogenesis are always haploid, while the body of an organism is usually diploid.
  • Therefore, gametogenesis is always interlinked with meiosis because the formation of haploid gametes with half the number of chromosomes than that in the parent is possible only through meiosis.
  • In diploid organisms, meiocytes undergo meiosis. At the end of meiosis, only one set of chromosome gets incorporated into each gamete.
Solution 2

Plant part

Ploidy

Ovary

Diploid (2n)

Anther

Diploid (2n)

Egg

Haploid (n)

Pollen

Haploid (n)

Male gamete

Haploid (n)

Zygote

Diploid (2n)

Solution 3

In most aquatic organisms such as algae, fish and amphibians, syngamy or fusion of compatible gametes occurs outside the body of an organism in the external medium (water). This type of gametic fusion is called external fertilisation.

Disadvantages of external fertilisation:

  • Requires a medium for the fusion of gametes.
  • Involves a chance factor for the synchronous release of gametes nearby and the absence of turbulent water.
  • Offspring easily become vulnerable to the attack of predators because of their large number.
  • They are not protected from predators, and their survival is threatened up to adulthood.
Solution 4

Differences between a zoospore and a zygote:

 

Characteristics

Zoospore

Zygote

Formation

It is formed during asexual reproduction.

It is formed during sexual reproduction.

Motility 

It is motile because of the presence of flagella.

It is non-motile because of the absence of flagella.

Genetic constitution

It can be haploid or diploid.

It is always diploid.

Fate

On germination, it gives rise to new plants.

It undergoes the process of embryogenesis.

Significance

It takes part in dispersal.

It does not have a significant role in dispersal.

Occurrence

It is produced in lower plants such as algae and fungi.

It is produced in higher plants and animals.

Solution 5

Differences between gametogenesis and embryogenesis:

Characteristics

Gametogenesis

Embryogenesis

Function 

It involves the formation of haploid male and female gametes.

It involves the formation and development of a multicellular embryo from a unicellular zygote.

Cell division

It involves meiosis.

It involves mitosis.

Genetic constitution

Haploid gametes

Diploid embryo

Processes

It is of two types—spermatogenesis and oogenesis.

It involves cell division, cell growth and cell differentiation.

Fate

It leads to fertilisation.

It leads to organogenesis.

Solution 6

Fertilisation is the process of the fusion of male and female gametes to form a diploid zygote. After fertilisation, the following changes occur in a flower:

  • The zygote is formed inside the ovule and develops into the embryo.
  • The primary endosperm cell is formed which later develops into the endosperm.
  • The sepals, petals and stamens wither and fall off. The pistil remains attached to the flower.
  • The fertilised ovule develops into the seed.
  • The ovary develops into the fruit.
  • The ovary wall is transformed into a protective layer called the pericarp.
  • After dispersal, the seeds germinate under favourable conditions and later on give rise to new plants.
Solution 7

A flower bearing both male and female sex organs (stamens and carpels) is called a bisexual flower.

Examples of plants bearing bisexual flowers:

S. No.

Common name

Scientific name

1.      

Water lily

Nymphaea odorata

2.      

Rose

Rosa multiflora

3.      

Hibiscus

Hibiscus rosa-sinensis

4.      

Mustard

Brassica nigra

5.      

Petunia

Petunia hybrida

 

Solution 8

In staminate (male) flowers, stamens (male reproductive structures) are present and carpels or pistils (female reproductive structures) are absent. They do not develop fruits. In pistillate (female) flowers, carpels or pistils are present and stamens are absent. They develop fruits. 

Cucumber is a good example of Cucurbita. Cucumber plants have separate male and female flowers. A pistillate flower is distinct by appearance of a baby fruit at the base of the flower, while a male or staminate flower does not possess such a baby fruit.

Another example of a unisexual flower is the flower of maize. The male flower in maize can be distinguished from the female as it grows higher up the stem, while the female flower remains on the lower part of the stem.

Date palm and papaya are some other examples of plants bearing unisexual flowers.

Solution 9

In viviparous animals, the development of the young one occurs inside the body of the female. As a result, the young one gets better protection and nourishment for proper development.

Oviparous animals lay eggs outside their body and their whole process of embryonic development occurs inside the calcareous shell outside the body of the female. Therefore, the young ones are not effectively protected and nourished and are under continuous threat from various environmental factors and predators. Thus, the offspring of oviparous animals are at a greater risk as compared to the offspring of viviparous animals.

Solution 10

Vegetative propagation is a type of asexual reproduction in plants in which vegetative propagules such as suckers, runners and rhizomes reproduce and give rise to new plants.  It occurs in lower and higher plants.

Examples of vegetative propagation:

  • Vegetative propagation by roots: Tap roots, fleshy roots and adventitious roots take part in vegetative propagation. Examples: Sweet potato, guava
  • Vegetative propagation by leaves: Adventitious buds present on the leaves of several plants help in vegetative propagation. Examples: Begonia, Bryophyllum
Solution 11

(a) Juvenile phase: All organisms have to attain a certain stage of physical growth and maturity in their lives before they start reproducing sexually. This pre-reproductive phase in the life cycle is called the juvenile phase. In humans, the juvenile phase lasts up to 12−13 years of age.

(b) Reproductive phase: When the reproductive system is mature enough to produce either male or female gametes, the organisms are ready to mate and reproduce sexually. This phase in the life cycle is called the reproductive phase. Some major hormonal changes mark this phase. In humans, the reproductive phase starts at about 12−13 years and lasts up to the age of 55−60 years.

(c) Senescent phase: After the reproductive phase, the organisms slowly lose their rate of metabolism and reproductive potential and show deterioration of physiological activities of their bodies. This post-reproductive phase in the life cycle is called the senescent phase.

Solution 12

Higher organisms have resorted to sexual reproduction in spite of their complexity because sexual reproduction

  • Involves the fusion of gametes resulting in the pooling of resources and the combination of genetic potentialities.
  • Restores a high gene pool in a population.
  • Enables them to tide over unfavourable conditions.
  • Induces genetic variability or adaptations which make the offspring better equipped for the struggle of existence and evolution.
  • Helps restore vigour and vitality of the race.
  • Involves nurturing and proper parental care during the vulnerable stages of life.