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 2 instead of the normal 16 chromosomes to determine the number of centromeres required for cohesin loading to form a pericentric bottlebrush. In wildtype yeasts the mitotic spindle is 1.5 microns long and 16 centromeres appear in tight clusters. Cohesin surrounds the metaphase spindle forming a cylindrical barrel and crosslinking the radial array of chromatin loops. In the two-chromosome strain, our findings show a disrupted cohesin barrel and a longer spindle (∼2.4 microns). 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.

• PMID: 40266738
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Journal Article

Authors

Kolbin D, Stanton J, Kokkanti A, Yeh E, Bloom K

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