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DNA Replication
Before meiosis actually begins, the DNA that is packaged into chromosomes must be fully copied. Prior to replication, a gamete contains two copies of each chromosome: a maternal copy and a paternal copy. The maternal and paternal homologues represent the maternal and paternal genetic contributions to each cell, ensuring that each cell has a 50% genetic make-up coming from each parent. These two chromosomes are independent but very similar and are termed homologues of each other.
DNA replication occurs in the same fashion as it does during mitosis. After replication, the homologues are doubled, and each chromosome now has a homologous pair. After replication, the maternal and paternal homologues remain closely associated with their copy as sister chromatids.
Figure 5.02: DNA replication
Prophase I: Genetic Reassortment
Prophase I differs importantly from mitotic prophase. Whereas in mitosis the chromosomes line up in a line, in meiosis, they line up, side by side (figure 5.3). In both cases, spindle fibers that are attached to kinetochores bring the chromosomes to the correct location. Kinetochores are protein structures near the center of the chromosomes. The chromosomes then undergo a process called crossing over, in which a piece of chromosome from the maternal homologous pair crosses over with a piece of chromosome from the paternal homologous pair. This crossover occurs after the two sets of homologous pairs (maternal and paternal) become physically linked through the formation of a chiasma.
Figure 5.03: Genetic reassortment
The crossover of chromosomes in genetic reassortment creates new DNA combinations. In other words, genetic reassortment gives rise to a unique genetic make-up that results in some of the genetic variation found in sexual reproduction.
