Why metaphase chromosomes wander

Civelekoglu-Scholey et al. explain why some metaphase chromosomes shift positions.

After chromosomes line up at the metaphase plate, sister chromatid pairs often jiggle, sliding a short distance toward one cell pole and then back toward the other. But in some cells, the chromosomes remain stationary until they’re ready to separate. Civelekoglu-Scholey et al. focused on PtK1 cells, an unusual case in which chromosomes at the edge of the metaphase plate are immobile, whereas the ones in the central region oscillate.

To explore this difference, the team created mathematical models of chromosome maneuvers in these cells. A model in which molecular motors at the ends of microtubules provide the power couldn’t fully explain PtK1 cell chromosome movements. Replacing the molecular motors with dynamic and stretchy connections between microtubules and kinetochores could account for chromosome behavior, however. The researchers assumed that the Ndc80 complex, which couples kinetochores to microtubules, showed so-called bi-phasic detachment, meaning it was more likely to remain connected if it was under moderate tension.

The new model suggests that chromosomes at different locations on the metaphase plate behave differently because of variations in the microtubule-dependent forces that push chromosomes toward the cell equator. The strength of these polar ejection forces increases faster at the edge of the cell than in the middle. This disparity could explain the chromosome movements, the researchers say, because other work has determined that weaker polar ejection forces result in greater oscillations.