Synapsis and cross over

The crossing over happens in the Prophase I1 of meiosis.

The tight pairing of the homologous chromosomes is called synapsis. In synapsis, the genes on the chromatids of the homologous chromosomes are aligned precisely with each other. The synaptonemal complex supports the exchange of chromosomal segments between non-sister homologous chromatids, a process called crossing over. Crossing over can be observed visually after the exchange as chiasmata (singular = chiasma).

One blue and one red X shape structures, called homologous chromosomes, are placed next to each other. Each structure has two yellow spheres in its crossing center. The Grey arrow to the right points towards the same structures, which now overlap one of their lower legs with each other, and this view is called chromatid crossover. Grey arrow points down towards the same structures, but now the lower right leg of the blue X-shaped structure is red, and the lower-left leg of the red X-shaped structure is blue, as they exchange those fragments between themselves. The structures are now called recombinant chromatids.

Crossover occurs between non-sister chromatids of homologous chromosomes. The result is an exchange of genetic material between homologous chromosomes.

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