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Crossing over occurs at which stage(two or four strand stage of chromosome)?
Crossing over or recombination actually results from exchange of chromosome segments which is believed to take place during prophase I of meiosis. During prophase I of meiosis, homologous chromosomes are brought together during synapsis at zygotene. Each chromosome also divides longitudinally into two chromatids at this stage. At pachytene, chromatids exchange segments.
Crossing over occurs at the four strand stage, when two homologous chromosomes, each consisting of a sister chromatids are paired.


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