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Genetic Rearrangement in Meiosis

Each gamete is genetically unique because of two characteristics of meiosis I: crossing-over and random assortment of paternal and maternal homologs (Figure 1a).

Figure 1 a shows the different stages of meiosis. The cell starts the process at Interphase, where DNA is condensed, followed by Prophase one, where chromosomes are formed duplicated. Then it moves to Meiosis one that consists of Prometaphase one, where spindle fiber microtubules are attached to the centres of the chromosomes, Metaphase one, where chromosomes are arranged in the centre of the cell, Anaphase one, where chromosomes are pulled apart, and Telophase one, where chromosomes arrive at opposite poles of the cell. Meiosis one is finished with cytokinesis, resulting in two haploid cells. In Meiosis two, each cell goes through the same phases as in the case of meiosis one, but this time sister chromatids are separated in Anaphase two, and the whole process results in forming four unique haploid cells. Figure 1 b shows examples of possible arrangements of chromosomes. Example one shows an arrangement where the maternal and paternal chromosomes separate independently, illustrated with red in one daughter cell and blue in the other. This leads to two different arrangements. The second example shows an arrangement where the maternal and paternal chromosomes are mixed, which leads to two other types of arrangements in the daughter cells, seen as mixed with both blue and red chromosomes. Therefore this illustration shows four different arrangements.

Figure 1: a) Meiosis including crossing-over (prophase I) and random assortment (metaphase I). b) Random segregation of chromosomes in meiosis.

Crossing-over

In meiosis 1 the chromosomes replicate, just like in mitosis. Unlike mitosis, the paternal and maternal version of each chromosome, the homologous pair, then pair up (Figure 1a). This arrangement allows for genetic recombination to occur. During this crossing over process, equivalent, but not identical, portions of the chromosomes are exchanged. For example, the maternal gene for eye color may be exchanged with the paternal gene for eye color. As a result, the paternal chromosome contains a few genes from the maternal chromosome and vice versa. The new chromosomes are genetically unique, observe how in Figure 1a small portions of the red and blue homologs have been exchanged.

Random Assortment

In late meiosis II, prior to cytokinesis, the homologous pairs separate and one homolog is pulled into each daughter cell. The arrangement of the chromosomes is random so that each daughter cell might inherit the paternal chromosome, but the maternal homolog from another chromosome. Observe how in Figure 1a each daughter cell contains some red and some blue chromosomes.

Because both crossing-over and random assortment apply to all 23 pairs of chromosomes, there number of possible variations are huge. Individual humans and other organisms that reproduce sexually are therefore all genetically unique. Let’s use an example image with two chromosomes (n=2) undergoing independent assortment at metaphase I. In this case, there are two possible arrangements, the paternal and maternal chromosomes could separate independently as seen in the upper panel of figure 1b, or the maternal and paternal chromosomes could mix as shown in the lower panel of figure 1b. This ultimately means that there are four possible arrangements. With more chromosomes, the number of possible arrangements increases dramatically.