The centromere bottlebrush requires a multi-microtubule attachment

Pericentromeric bottlebrush converts DNA into a stiff spring through density and organization of loops relative to the mitotic spindle axis. This spring is integral to tension-sensing mechanisms required for faithful chromosome segregation. Cohesin enrichment is a hallmark of yeast pericentric loops. We used haploid yeasts engineered to contain two instead of the normal 16 chromosomes to determine the number of centromeres required for cohesin loading to form a pericentric bottlebrush. In wild-type yeasts, the mitotic spindle is 1.5 µm long and 16 centromeres appear in tight clusters. Cohesin surrounds the metaphase spindle forming a cylindrical barrel and cross-linking the radial array of chromatin loops. In the two-chromosome strain, our findings show a disrupted cohesin barrel and a longer spindle (∼2.4 µm). The reduction in spring stiffness would lead to the increase in spindle length necessary to achieve a force balance with spindle microtubules. In the two-chromosome strain kinetochores are declustered. Additionally, coordination between the clusters moving toward the poles (anaphase A) and spindle elongation (anaphase B) is abrogated resulting in a mid-anaphase pause. The lack of anaphase A suggests that release and expansion of hitherto confined DNA loops contributes to synchronous chromosome segregation in anaphase.