Notes And Study Material for Exam Papers

Meiosis or reduction division is complicated type of cell division occurring in the reproduction of germ cells at the time of gametic formation. In animals it occurs during gametes formation. Every species has definite number of chromosomes and that this number remains constant in its somatic cells, generation after generation. To keep the number constant the chromosome number in gametes is reduced to half. When male and female gametes and their nuclei fuse, normal number of chromosomes characteristic of each species is restored. The somatic or vegetative cells having the complete number of chromosomes are called diploid (2n). While gametes containing half of the original number of chromosomes are called haploid (n) or monoploid. The events of Meiosis: Meiosis consists of two successive divisions of mother cells. First division is reduction division, during which the chromosome number (2n) in both daughter cells is reduced to half (n), the second division is simple mitotic div...

During life cycle of most animals some diploid cells undergo meiosis and haploid gametes form in a process called gametogenesis. The testes of the male, produces sperms and is called spermatogenesis, in the ovaries of female, the production of oval (eggs) is called oogenesis. Spermatogenesis: Spermatogenesis produces mature sperm as follows: (1)        Spermatogenesis begins in unspecialized germ cells called spermatogomia (singular spermatogonium). The spermatogonium enlarges and becomes primary spermatocyte. (2)        A primary spermatocyte undergoes meiosis (meiotic division I) and produces two smaller secondary spermatocytes). (3)        Both secondary spermatocytes undergo second meiotic division (meiotic division II) to form spermatids each primary spermatocyte gives rise to four spermatids, each with the haploid number of chromosomes. (4)  ...