Fig. 6. Spindle collapse occurs by sequential depolymerization of oppositely oriented microtubules. (A) A single microtubule is shown depolymerizing toward the lower spindle pole. The still images are sequential images taken at approximately 2-second intervals over 26 seconds. The arrowhead points to the plus end of the depolymerizing microtubule. Bar, 2 µm. (B) The same time-lapse movie is shown as a kymograph. The kymograph shows anaphase B spindle elongation and spindle collapse. After the spindle stops elongating (time 7 minutes), the microtubules begin to depolymerize one by one. This appears as a series of parallel lines sloping toward the spindle poles in an alternating pattern. Successive depolymerization lines are parallel, showing that rates of depolymerization are the same for each microtubule. SP, spindle pole; midzone, zone of antiparallel microtubule overlap Bar, 2 µm (see Movie 4, http://jcs.biologists.org/supplemental/).