Zygotene is the second stage of Prophase I where the chromosomes start pairing together. This process is called synapsis and the paired chromosomes are known as homologous chromosomes. Pachytene is the stage where bivalent chromosomes clearly appear as tetrads and is characterized by the appearance of recombination nodules which function as the sites of crossing over between non-sister chromatids of the homologous chromosomes. Crossing over is the exchange of genetic material between two homologous chromosomes and is mediated by the enzyme called recombinase. The stage where sister chromatids remain associated but homologous chromosomes separate is called Anaphase I. During Anaphase II the sister chromatids move towards the opposite poles of the cell due to the simultaneous splitting of the centromere.