crossing over is the process involving the exchange of genetic material between two homologous chromosomes . This event occurs specifically during the pachytene stage of Prophase I in meiosis . The sources define the complex formed by a pair of synapsed homologous chromosomes as a bivalent or tetrad . Crossing over is an enzyme-mediated process (facilitated by the enzyme recombinase) that takes place at recombination nodules between the non-sister chromatids of these bivalents . This leads to genetic recombination, which is essential for variation and evolution . Mastering these meiotic definitions is a high-priority strategy, as NCERT textbooks are the primary source for approximately 95% of NEET Biology questions .