Sunday, July 26, 2020

Reproduction in Organisms - Notes | Class 12 | Part 2: Sexual Reproduction

1. REPRODUCTION IN ORGANISMS
SEXUAL REPRODUCTION
  • It is the reproduction that involves formation of male and female gametes, either by the same individual or by different individuals of the opposite sex.
  • It results in offspring that are not identical to the parents or amongst themselves.
  • It is an elaborate, complex and slow process as compared to asexual reproduction.
  • The period of growth to reach in maturity for sexual reproduction is called the juvenile phase. In plants, it is known as vegetative phase.
  • In higher plants, the flowering indicates the end of vegetative phase (beginning of reproductive phase).
  • Annual & biennial plants show clear cut vegetative, reproductive & senescent phases. In perennial plants, these phases are very difficult to identify.
  • Some plants exhibit unusual flowering. E.g.
  • Bamboo species flower only once in their lifetime (after 50-100 years), produce large number of fruits and die.
  • Strobilanthus kunthiana flowers once in 12 years.
  • In animals, juvenile phase is followed by morphological & physiological changes prior to reproductive behaviour.
  • Birds living in nature lay eggs only seasonally. However, birds in captivity (e.g. poultry) can be made to lay eggs throughout the year.
  • The females of placental mammals exhibit cyclical changes in the ovaries, accessory ducts and hormones during the reproductive phase. It is called oestrus cycle in non-primates (cows, sheep, rat, deer, dog, tiger etc.) and menstrual cycle in primates (monkeys, apes & humans).
  • Based on breeding season, mammals are 2 types:
a.  Seasonal breeders: The mammals (living in natural conditions) exhibiting reproductive cycles only during favourable seasons.
b.  Continuous breeders: They are reproductively active throughout their reproductive phase.

Senescence (old age):
  • It is the last phase of lifespan and end of reproductive phase.
  • During this, concomitant changes occur in the body. E.g. slowing of metabolism etc. It ultimately leads to death.
  • In plants & animals, hormones cause transition between juvenile, reproductive & senescence phases. Interaction between hormones and environmental factors regulate the reproductive processes and the associated behavioural expressions of organisms.
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