Before meiosis actually begins, the DNA that is packaged into chromosomes must be fully copied. Previous to replication, a germ cell 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 and ensure 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.
Prophase I differs importantly from mitotic prophase. Whereas as in mitosis the chromosomes line up along different spindle fibers, in meiosis they line up side by side (). The chromosomes then undergo a process called genetic reassortment, 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. It is known that proteins are responsible for holding the chromosomes in their appropriate alignment, but the details of the mechanism are not yet fully understood.
The crossover of chromosomes in genetic reassortment creates new DNA combinations. In other words, the genetic reassortment gives rise to a unique genetic make-up that results in some of the genetic variation found in sexual reproduction.