Stages of meiosis, chromosomal events, genetic recombination

Meiosis is a unique form of cell division restricted to gametes (spermatocytes and oocytes). There are two primary purposes of meiosis:

1) To have a reduction division.

2) To provide genetic variation by exchanging segments of homologous chromosomes.

Spermatogonia divide by mitosis and differentiate until they become primary spermatocytes. They will then stay in the primary spermatocyte stage until puberty. At puberty, primary spermatocytes begin to divide by meiosis, which consists of two divisions. In the first meiotic divisions, the prophase is long and involves so many specific nuclear events, that it is subdivided into five parts: leptotene, zygotene, pachytene, diplotene and diakinesis.

In the leptotene stage, the chromosomes are evident as thin, delicate filaments which attach themselves to the nuclear envelope (Fig. 1). Each chromosome is composed of two sister chromatids. In the subsequent zygotene stage, there is intimate pairing of homologous chromosomes; in the human, 23 homologous chromosomes pair and form a trilaminar structure called the synaptonemal complex. In the pachytene stage, there is exchange of genetic material between homologous chromosomes that is mediated by the synaptonemal complex and a large recombinant nodule. This stage lasts 16 days in the human. In the diplotene stage, desynapsis occurs and the areas where there was exchange of genetic material are clearly seen at connecting sites called chiasmata. In the final stage, diakinesis, the chromosomes condense.

The cells then proceed with metaphase where the paired chromosomes align at the equatorial plate. Chiasmata separate and the homologous chromosomes move to opposite poles of the cell during anaphase. This division is in distinction to mitosis, during which each pair of sister chromatids separates and moves to opposite poles. In telophase, cytokinesis occurs and two separate daughter cells result. At the end of this first meiotic division, the cells have differentiated to become secondary spermatocytes.

There is a very short interphase between the first and second meiotic divisions, and no DNA synthesis occurs during this interphase. Almost immediately, the second division begins with the secondary spermatocyte progressing from prophase through metaphase, anaphase and telophase. The second division closely resembles mitosis where there is separation of the sister chromatids along the centromere. At the end of the second division, the secondary spermatocyte has become a spermatid.

In summary (Fig. 1), at the end of the first meiotic division, one primary spermatocyte has divided into two secondary spermatocytes. At the end of the second meiotic division, each secondary spermatocyte has divided into two spermatids so that there are a total of four spermatids that were derived from the primary spermatocyte (Table 1). Interestingly, cytokinesis in both the first and second divisions is incomplete so that very small intercellular bridges form between the cells. This bridge is termed a ``syncytium'' and allows for simultaneous communication amongst the cells.

Table 1. Diploid Versus Haploid: The 4N, 2N, 1N Quandary ``N'' can refer to either the number of chromosomes or the amount of DNA in a cell.

A. If N refers to the number of chromosomes, 2N is diploid and 1N is haploid. Since all somatic cells have 46 chromosomes or 23 homologous pairs (one paternal and one maternal), somatic cells are diploid (2N). Likewise, spermatogonia are also diploid (2N). At the end of the first meiotic division, the secondary spermatocytes have 23 double-stranded (containing a pair of daughter chromatids) unpaired chromosomes and are, therefore, haploid (1N). At the end of the second meiotic division, there are 23 single-stranded chromosomes and these are also considered haploid (1N).

B. N can also refer to the amount of DNA in a cell. In this particular case, N refers to the minimal amount of chromosomal material which contains all of the genes. Therefore, spermatogonia prior to the S-phase are diploid and have 2N amount of DNA. After the S-phase (at the beginning of meiosis) the primary spermatocytes have doubled their amount of DNA and, therefore, are 4N. At the end of the first meiotic division, the secondary spermatocytes are haploid, but contain 2N amount of DNA since they are double-stranded chromosomes (sister chromatids). At the end of the second meiotic division, the spermatids are haploid and contain 1N amount of DNA.