Chromosome size affects alignment efficiency in mammalian mitosis.

Abstract

The kinetochore connects chromosomes to spindle microtubules during cell division. Accurate chromosome segregation requires sister kinetochores to biorient, attaching to opposite spindle poles. To achieve biorientation, the kinetochore destabilizes incorrect attachments and stabilizes correct ones. How it discriminates correct from incorrect attachments is not clear. Here, we test the model that tension serves as the stabilizing cue at correct attachments and how that model affects chromosomes of different sizes. Live imaging of mitotic PtK2 cells reveals long chromosomes align at the metaphase plate more slowly than short chromosomes. Using laser ablation to shorten long chromosome arms—reducing polar ejection forces on them—we show that chromosomes align faster after ablation, indicating chromosome size affects alignment efficiency. Finally, artificially enriching for incorrect attachments using STLC washouts and imaging error correction live, we show that long chromosomes exhibit a delay in correcting errors, rather than simply in attachment formation. We propose a model where increased polar ejection forces on long chromosomes stabilize not only correct but also incorrect attachments, delaying their biorientation and alignment. As such, long chromosomes may experience more challenges correcting errors and, as a result, higher missegregation rates.