Class 12 Biology Reproduction In Organisms Pre- fertilization

Pre- fertilization

  • The events of sexual reproduction which occurs before fertilization are called pre-fertilization events.
  • Pre- fertilization events are a) gametogenesis and b) gamete transfer.
  1. Gametogenesis-
  • The process of formation of gametes is called gametogenesis.
  • Gametes are the specialized haploid cells and are of two types- male and female gametes.

Fig.male gametes, sperms are surrounding the female gametes, egg.

  • When both the gametes are similar in appearance that these can be distinguished, these gametes are called homogametes or isogametes. Example- algae.

fig. homogametes of algae

  • When both the gametes are dissimilar in appearance, the gametes are called heterogametes or anisogametes and the male gamete is called antherozoid or sperm and the female gamete is called egg or ovum. Example- human, higher plants.

Fig. heterogametes

 

  • Gametes are haploid even though the parent body is diploid.
  • If the parent is haploid, the parents produce gamete by mitosis. Example- monera, algae.
  • In diploid parents, the parents produce gametes by meiosis, the reduction division to produce haploid gametes. Example- human beings, higher plants.
  • In diploid organisms, the gamete mother cells which undergo meiosis to produce gametes are called meiocytes, as a result at the end of meiosis only one set of chromosome gets incorporated into each gamete hence, the gamete becomes haploid.
  • Human beings are diploid organisms and have 46 chromosomes and so the gametes contain 23, half of the number of chromosome by reduction division.
  1. Gamete transfer-
  • The process of bringing together of male and female gamete for fertilization is called gamete transfer.

Fig. transfer of gametes

 

  • In majority of organisms male gametes are motile and female gametes are non-motile while in algae and fungi both the types of gametes are motile.
  • In bryophytes and pteridophytes, gamete is transferred through water.
  • In seed producing plants, pollens are transferred to the stigma by pollination.
  • In bisexual self-fertilizing plants, anthers and stigma are located close to each other and so transfer of pollen grains is easy. Example- pea.
  • In cross-pollinating plants, the agents like wind, insects moving for collecting nectar from the flowers help in pollination.

Fig. pollination by insect

 

 

Fig. pollination

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