Figure 2.

Linkage between homologous chromosomes through chiasma and cohesion. A, Pairs of homologous telocentric chromosomes are described. When no chiasma is present, all cohesins on the chromosomes mediate sister chromatid cohesion. Once a chiasma is formed, cohesins localizing on chromosome arms distal to chiasma (in relation to centromeres) become involved in linkage between homologous chromosomes. When two chiasmata are formed, only the cohesins localizing on chromosome arms between two chiasmata become responsible for the homologous chromosomes linkage. When three chiasmata are formed, the cohesin localizing both between the first and second chiasmata and distal to the third chiasma become responsible for the homologous chromosomes linkage. B, A pair of homologous metacentric chromosomes is described. When two chiasmata are formed on both sides of chromosome arms in relation to centromeres, cohesins localizing distal to chiasmata are responsible for linkage between homologous chromosomes. C, When a chiasma is formed near the edge of telocentric chromosomes, the cohesins involved in linkage between homologous chromosomes are fewer than when a chiasma is formed at the middle of chromosome arms as described in A