Homologous chromosomes during prophase I - Animation

The chromosomes were duplicated during interphase and the sister chromatids are now in thin threadlike form. Each chromosome becomes zippered to its homologue, so that all four chromatids are closely aligned. The chromosomes are tightly aligned, but we will show them as separate so that you can visualize what's happening. Intimate contact between the chromosomes allows crossing over. Nonsister chromatids break at the same places along their length and exchange corresponding segments. As prophase I ends, the chromosomes unzipper from one another, except for at the places where they physically cross. These places are called chiasmata. What is the function of crossing over? It breaks up old combinations of alleles and puts new ones together in pairs of homologous chromosomes.

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