The movement phases of chromosomes are designated pro-metaphase, metaphase, and anaphase. During pro-metaphase, the nuclear lamina disintegrates and the nuclear envelope breaks into small vesicles permitting the fibers of the spindle to “invade” the nuclear region. The spindle microtubules then associate with kinetochores. These are called kinetochore microtubules. The microtubules from one pole associate with the kinetochore of one of the members of a pair of chromatids. Microtubules from the other pole associate with the kinetochore of the other member. Repulsive forces from the poles push chromosomes toward the center, or equatorial plate, in a rather aimless back and forth motion. The two chromatids are held together, presumably by proteins called cohesins. During metaphase, the kinetochores arrive at the equatorial plate. Chromosomes are fully condensed and have distinguishable shapes. Cohesins break down. DNA topoisomerase II unravels the interconnected DNA molecules at the centromere, and all the chromatids separate simultaneously. Anaphase begins when the centromeres separate. The process takes 10 to 60 minutes for the chromosomes to move to opposite poles. Molecular motors at the kinetochores move the chromosomes toward the poles, accounting for about 75% of the motion. About 25% of the motion comes from shortening of the microtubules at the poles. Additional distance is gained by the separating of the mitotic centers. This increase in distance between the poles is done by the polar microtubules, which have motor proteins associated in the overlapping regions. By this process the distance between the poles doubles.
