The process of gene exchange takes place in the prophase I of meiosis. In the beginning of prophase I, chromosomes appear as long thread like structure and show bead-like thickenings at regular intervals. The homologous chromosomes form pairs. This phenomenon of chromosomal pairing is called synapsis of the chromosomes. The paired chromosomes are identical, one coming from the father, through his sperm and the other from mother, through her ovum. Pairing is remarkably exact and specific. It takes place to point to point. The two homologues do not fuse during pairing but remain separated. The paired chromosomes become short and thick and contain four chromatids.
Each chromatid at this stage has its own centromere, resulting in four centromeres. These chromatids are called sister chromatids. Now, two of the chromatids (one paternal and one maternal) of the homologous, exchange segments. The process is called crossing over.
