Prophase I is the first stage of meiosis I, it is the longest phase of meiosis. Crossing over (also known as recombination) can be defined as the process of exchange of genetic material between non-sister chromatids that occurs during Prophase I. Crossing over represents a critical source of genetic variation in offspring (the other one is the random orientation of the homologous chromosomes pairs). After crossing over, the resulting haploid (n) gametes aren't genetically identical to one another.
