Meiosis

Picking up where we left off in the previous section, at the end of meiotic division I we have two independent cells. One cell contains the maternal homologous pair with a small segment of crossover from the paternal chromosome. The other cell contains the paternal homologous pair with a small segment of crossover from the maternal chromosome. Once the nuclear envelope has re-formed after the first meiotic division, the cell enters a short interphase. This interphase is not as specific as mitotic interphase; during meiotic interphase, chromosomes may decondense as the cell waits to proceed with meiosis.

Meiotic division II occurs through the similar phases as mitosis and meiotic division I. They are called prophase 2, prometaphase 2, metaphase 2, anaphase 2, and telophase 2 to distinguish them from the first round of meiotic division. One very important difference between the events of meiosis I and II is that no further genetic reassortment takes place during prophase 2. As a result, prophase 2 is much shorter than prophase 1. In fact, all of the phases of meiosis 2 proceed very quickly. One other point to remember is that the two cells that result after meiotic division I enter meiosis II very quickly. During the brief interphase period, no further DNA replication takes place!

During meiosis II, chromosomes align at the center of the cell in metaphase 2 exactly the way they do in mitotic metaphase. In anaphase 2, the sister chromatids are separated, again, in the same fashion as in mitotic anaphase. The only difference is that since there was no second round of DNA replication, only one set of chromosomes exist. Thus, when the cells split at the end of meioisis II, haploid cells result.

The result of meiotic division II is four haploid cells. One cell is composed completely of a maternal homologue, another of a maternal homologue with a small segment of paternal DNA, another complete paternal homologue, and a final paternal homologue with a small segment of maternal DNA. At this point, we have created germ cells. Other processes occur to mature these cells into gametes which, in higher organisms, then go on to work together in sexual reproduction to create new individuals.