A crossing-over is a crossing of homologous chromosomes resulting in the exchange of homologous (identical) sites between these chromosomes. During the crossing-over, the DNA double helix breaks in one maternal and one paternal chromatid. The resulting segments are re-joined between chromosomes leading to the formation of recombinant chromosomes (the process of genetic recombination). Recombination occurs in the prophase of the first division of meiosis when the two sister chromatids are closely approximated to each other. At this time, the chromatids cannot be seen separately. Later, two separate chromatids of each chromosome become visible. Now, it is clear that they are connected by their centromere and are closely approximated along the entire length. The two homologs remain connected at the points where the crossing-over between the paternal and maternal chromatids occurred. At each such site, which is called chiasma, two of the four chromatids intersect.
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Example of cytological basis of crossing over.