Sexual reproduction involves the steps: gametogenesis, fertilization and development of zygote. The process of gametogenesis and fertilization are closely related and form the foundation of sexual reproduction. Together, they ensure the combination of genetic material from two parents, resulting in offspring with genetic diversity.
6.2.1 Gametogenesis
Gametogenesis is the first step in the sexual reproduction in which the specialized sex cells, called gametes, are produced in the reproductive organs. This process involves meiosis, a type of cell division that reduces the chromosome number by half.
In males, this process is called spermatogenesis, where sperm cells are produced in the testes.
1) Spermatogenesis
Spermatogenesis is a continuous event that starts from puberty and remains continue throughout the life simultaneously in both testes. During spermatogenesis, some cells in the walls of the seminiferous tubules in the testes keep dividing through mitosis to create many diploid cells called spermatogonia. Some of these spermatogonia develop into primary spermatocytes. Each primary spermatocyte undergoes the first division of meiosis (meiosis I) to produce two haploid cells known as secondary spermatocytes. These secondary spermatocytes then go through the second division of meiosis (meiosis II), resulting in four haploid cells called spermatids. Spermatids are non- motile, and they go through changes to become motile. Their nuclei shrink, and structures such as the acrosome, a tail, and a mitochondrial ring are formed. After these changes, the spermatids become sperms.
ii) Oogenesis
Oogenesis is a cyclic event that generally occurs only once in the period of one reproductive cycle. It starts before birth of an individual but arrests in prophase I. At puberty, it restarts and remain continue alternatively in both ovaries till specific age in the life. During oogenesis, some cells in the ovary form vesicular structures called follicles, each contains one diploid cell called oogonium (plural oogonia). Some of these oogonia develop into diploid primary oocytes which acts as egg mother cell. One primary oocyte undergoes the first stage of meiosis (meiosis I), producing two haploid cells. The smaller cell is called the first polar body, while the larger one is the secondary oocyte. The secondary oocyte then completes the second stage of meiosis (meiosis II), producing two haploid cells: the second polar body and an egg cell. In oogenesis, the meiosis II generally occurs after ovulation if the sperms are available in female reproductive tract.