(A) Evidence for an astral ejection force that pushes chromosomes away from the spindle poles toward the spindle equator. In this experiment, a prometaphase chromosome that is temporarily attached to a single pole by kinetochore microtubules is cut in half with a laser beam. The half that is freed from the kinetochore is pushed rapidly away from the pole, whereas the half that remains attached to the kinetochore moves toward the pole, reflecting a decreased repulsion. (B) A model of how two opposing forces may cooperate to move chromosomes to the metaphase plate. Plus-end-directed motor proteins on the chromosome arms are thought to interact with astral microtubules to generate the astral ejection force, which pushes chromosomes toward the spindle equator. Minus-end-directed motor proteins at the kinetochore are thought to interact with kinetochore microtubules to pull chromosomes toward the pole.