During meiosis, crossing over occurs during prophase I. It is the exchange of genetic material between homologous chromosomes that results in recombinant chromosomes, which contribute to genetic diversity.
During prophase I, each of the homologous pairs of chromosomes can be seen as bivalent or as a grouping of two chromosomes said to be sister chromatids. The site at which the crossing over occurs on the sister chromatids is known as the chiasma. Prophase I is the longest phase during the meiosis process and takes up to 90 percent of the total time of cell division. Chromosomes also appear in this phase, the nucleus disappears, and the meiotic spindle appears in the cytoplasm.