Biology Class 12 NCERT Solutions: Chapter 1 Reproduction in Organisms Part 3

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Q.8 Define

(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.

  • All organisms have to reach a certain stage of growth and maturity in their life, before they can reproduce sexually. This period of growth is called juvenile phase or vegetative phase (term used in plants).

(b) Reproductive phase:

  • It is the period when an individual organism reproduces sexually.

  • Reproductive stage in plants is represented by appearance of flowers

  • In animals the end of juvenile phase is indicated by many of them showing morphological and physiological changes prior to active reproductive behaviour

(c) Senescent phase:

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

  • The end of reproductive phase is one of the parameters of senescence or old stage.

  • At this stage body metabolism is slowed down and ultimately this stage leads to death

Image of Reproductive Phase of Life

Image of Reproductive Phase of Life

Q.9 Higher organisms have resorted to sexual reproduction in spite of its complexity. Why?

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.

Q.10 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.

  • Gametes are haploid cells. Gametes in all heterogametic species are of two types i.e., male and female

  • A haploid parent produces gametes by mitosis. But in majority of organisms, parent body is diploid. Thus meiosis is required by such diploid organisms, to produce haploid gametes.

  • In diploid organisms, meiocytes undergo meiosis. At the end of meiosis only one set of chromosome gets incorporated into each gamete.

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