Polar chromosomes pivot their way towards the spindle body

During mitosis, the cell forms a spindle that equally segregates chromosomes into two daughter cells. Soon after nuclear envelope breakdown microtubules nucleated at the centrosomes of the spindle pole capture kinetochores, protein complexes on chromosomes. Some chromosomes immediately find themselves in the area between two spindle poles, yet others positioned at the periphery first need to approach the spindle pole in order to congress, or in other words, travel to the spindle equatorial plane. The question remains how these polar chromosomes located at the back of the spindle make their way across the spindle pole and reach the spindle body, from where they can continue their congression towards the equator. By using tubulin specific dyes in RPE1 cells stably expressing CENPA-GFP and Centrin1-GFP, markers for kinetochores and centrosomes, respectively, we demonstrate that polar chromosomes, together with the microtubules they are attached to, pivot around the centrosome towards the spindle body. The angle that the polar chromosomes form with the spindle axis changed faster during the period of rapid spindle elongation, indicating a mechanism in which spindle elongation creates a hydrodynamic drag force that brings kinetochores to the spindle body. This mechanism also ensured timely mitosis, as kinetochores that failed to pivot by the end of spindle elongation significantly delayed anaphase onset. Altogether, we propose a model in which pivoting of microtubules around the spindle pole, driven by spindle elongation, promotes the movement of peripheral chromosomes towards the spindle body and consequently their proper congression to the spindle equator.