Crossing over is an essential element of meiosis occurring during prophase, when non-sister chromatids of homologous chromosomes exchange portion of DNA strands.
Chromosomes become thick and short during prophase and formation of mitotic spindle takes place.
Pairing of chromosome occurs in prophase-I of meiosis-I. With the genes of the chromosomes thus aligned, a DNA strand of one homologous can pair with the complementary DNA stand of the other. The process of pairing is called synapse.
