The centromere bottlebrush requires a multi-microtubule attachment

Area of Science:

• Cell Biology
• Genetics
• Biophysics

Background:

• The pericentromeric region forms a DNA bottlebrush structure crucial for chromosome segregation.
• This structure relies on cohesin enrichment and organized DNA loops relative to the mitotic spindle.
• Faithful chromosome segregation depends on tension-sensing mechanisms involving this DNA spring.

Purpose of the Study:

• To investigate the minimum number of centromeres required for pericentric bottlebrush formation.
• To understand the role of centromere number in cohesin loading and DNA loop organization.
• To analyze the impact of altered bottlebrush structure on mitotic spindle dynamics and chromosome segregation.

Main Methods:

• Utilized haploid yeast strains engineered with two chromosomes instead of the typical 16.
• Observed and compared mitotic spindle length and centromere clustering in wild-type and engineered yeast.
• Analyzed cohesin distribution and chromatin loop organization in relation to the mitotic spindle.

Main Results:

• Engineered two-chromosome yeast exhibited a disrupted cohesin barrel and a significantly longer mitotic spindle (∼2.4 µm vs. 1.5 µm).
• Kinetochores became declustered in the two-chromosome strain, indicating loss of centromere organization.
• Coordination between chromosome movement (anaphase A) and spindle elongation (anaphase B) was abrogated, causing a mid-anaphase pause.

Conclusions:

• A minimum number of centromeres is essential for forming a functional pericentric DNA bottlebrush and proper cohesin loading.
• Disruption of the bottlebrush structure compromises spindle tension, leading to increased spindle length and impaired chromosome segregation.
• The release and expansion of confined DNA loops appear critical for synchronous chromosome segregation during anaphase.