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Picking up where we left off in the previous section, at the end of meiosis I, we have two independent cells. One cell contains the maternal homologous pair with small segments of crossover from the paternal chromosome. The other cell contains the paternal homologous pair with small segments 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 the mitotic interphase; during meiotic interphase, chromosomes may decondense as the cell waits to proceed with meiosis. Not all cells have this rest period. Those that do, do not go through a normal interphase cycle and thus during the brief interkinesis period, no further DNA replication takes place.
Meiosis II occurs through similar phases as mitosis and meiosis I. They are called prophase II, metaphase II, anaphase II, and telophase II 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 II. As a result, prophase II is much shorter than prophase II. In fact, the two cells resulting from meiotic division I enter and proceed through each phase of meiosis II very quickly. The final stage is cytokinesis II when the cytoplasm of each of the cells divides in two.
During meiosis II, sister chromatids align at the center of the cell in metaphase II exactly the way they do in mitotic metaphase. In anaphase II, 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 meiosis II, haploid cells result.
Figure 5.05: Meiotic Division II
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 small segments of paternal DNA, another complete paternal homologue, and a final paternal homologue with small segments of maternal DNA. 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. When meiotic division does not occur correctly, and DNA is not separated in this way, nondisjunction occurs and results in gametes that are not haploid. When this happens, some gametes have more chromosomes than others. This can cause problems later during fertilization.
