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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.
Figure %: Events of Meiotic Anaphase 1 versus Mitotic Anaphase.
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.
