The distribution of polar ejection forces determines the amplitude of chromosome directional instability.

BACKGROUND: Polar ejection forces have often been hypothesized to guide directional instability of mitotic chromosomes, but a direct link has never been established. This has led, in part, to the resurgence of alternative theories. By taking advantage of extremely precise femtosecond pulsed laser microsurgery, we abruptly alter the magnitude of polar ejection forces by severing vertebrate chromosome arms. RESULTS: Reduction of polar ejection forces increases the amplitude of directional instability without altering other characteristics, thus establishing a direct link between polar ejection forces and the direction of chromosome movements. We find that polar ejection forces limit the range of chromosome oscillations by increasing the probability that motors at a leading kinetochore abruptly disengage or turn off, leading to a direction reversal. CONCLUSIONS: From the relation between the change in oscillation amplitude and the amount a chromosome arm is shortened, we are able to map the distribution of polar ejection forces across the spindle, which is surprisingly different from previously assumed distributions. These results allow us to differentiate between the mechanisms proposed to underlie the directional instability of chromosomes.