Meiosis

Abstract

Meiosis, which in the male is initiated in puberty by the secretion of FSH and LH, can be confusing. So here is a simple explanation (Figs. 6.8 and 8.1). First, each chromosome begins a mitosis, but the mitotic division does not complete because the two divided chromosomes remain held together by the centromere that does not divide. They are called chromatids. DNA content doubles but the number of chromosomes remains the same. Thus meiosis always begins with a mitosis that does not get completed. The divided chromosomes are still held together by their centromere. These cells are the primary spermatocytes; they progressively show the nuclear features that identify meiosis I stages of leptotene, zygotene, pachytene, and diplotene (Fig. 8.1). During meiosis I, after the incomplete mitosis has occurred, homologous chromosomes pair with each other in an organized way and recombine after that initial partial division. But the mitotic aspect of meiosis I is not complete, as the centromeres do not divide, and all four “chromatids” then recombine. This represents the spermatocytes all the way from leptotene to zygotene to pachytene. After the pachytene stage, then the homologous chromosomes separate from each other forming secondary spermatocytes. But the sister chromatids still have not yet separated. Secondary spermatocytes have a very brief existence as the sister chromatids then divide immediately (thus completing their initial mitotic division), and this is the second meiotic division resulting in spermatocytes and sperm.