In this section, we will discuss the events of the first meiotic division. As we have already described, meiosis comprises two cellular divisions, meiosis I and meiosis II. We have already introduced the main event occurring in prophase I, genetic reassortment. Here we will discuss all of meiosis I, beginning with prophase I.
During prophase 1, when genetic reassortment takes place, the sister chromatids behave as a singular unit. The chromosome pair is identical except for the small region where crossover occurred. Because the very important genetic reassortment event occurs during this phase, prophase 1 can last a very long time: up to 90% of the total duration of meiosis. Other than the genetic crossover event that takes place during prophase 1, the following phases of meiosis I proceed similarly to those of mitosis.
After prophase 1, cells enter prometaphase 1. During this phase, the nuclear membrane breaks down, allowing microtubules access to the chromosomes. In this phase, the chromosomes are held through their chiasma instead of through the centromere as in mitosis. During the next phase, metaphase 1, the maternal and paternal homologous pairs of chromosomes (with crossover regions) align at the center of the cell via microtubules, as in mitotic metaphase.
In anaphase 1, a slightly different event than that of mitotic anaphase occurs. Instead of sister chromatids being pulled apart towards opposite poles as in anaphase, during anaphase 1, the entire maternal homologous pair is pulled to the opposite end as the paternal homologous pair.
During telophase 1 the chromosomes arrive at the poles, decondense, and nuclear membranes re-form around them. In the final stage, the cell physically divides, as in mitotic cytokinesis. The result of the first cell division is two independent cells. One cell contains the maternal homologous pair, or sister chromatids, with a small segment of the paternal chromosome from crossover. The other cell contains the paternal homologous pair with a small segment of the maternal chromosome. Despite the small region of crossover, the sister chromatids are still very similar and each cell at this point contains a diploid amount of DNA.
Although diploid cells result from meiosis I, these products are different from those of mitosis because both members of the diploid pair derive from either the maternal or paternal source with the exception of a small section. In mitosis, the cellular division separates sister chromatids and results in diploid cells containing one maternal and one paternal copy in each diploid pair.