Pb

▲ FIGURE 20-37 Model of the forces stabilizing metaphase chromosomes at the equatorial plate.

(a) Cytoplasmic dynein, a (—) end-directed motor (light green) at the kinetochore and a (+) end-directed motor (pink) at the spindle pole pull chromosomes toward the pole. Chromokinesin, a nonkinetochore (+) end-directed motor on the chromatid arms, exerts an opposite force on polar microtubules, pushing chromosomes away from the pole. CENP-E, a kinesin that does not mediate movement, keeps the kinetochore tethered to the kinetochore microtubule. (b) Treadmilling of tubulin subunits briefly stabilizes the lengths of spindle microtubules at metaphase by balancing assembly at the kinetochore with disassembly at the poles.

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