Forces on new microtubule minus ends move chromosomes and maintain spindle organization. (A) Model for rapid identification and organization of new spindle microtubule minus ends. NuMA (purple) and dynein/dynactin (green) rapidly localize to new microtubule minus ends after ablation (red X). Once dynein comes into contact with neighboring microtubules, it walks processively poleward along them, pulling the new minus ends as cargo and moving the attached chromosome (dark blue chromosome). (B) Imaging and biophysical analysis suggest that poleward transport is powered by force generation at minus ends of cargo microtubules. (C) Comparing the magnitudes of spindle forces. In all spindle structures studied, the poleward transport force overpowers other forces on chromosomes and/or microtubule bundles to move them toward poles. However, the speed of poleward movement increases as opposing forces decrease. Thus, the poleward transport force dominates but is tuned to other spindle forces, allowing it to maintain pole architecture without disrupting spindle integrity.