Evidence of a robust SAC during MUG. (A) HeLa cells undergoing MUG were treated with the indicated drugs and monitored by time-lapse microscopy. The disappearance of the nucleolus (arrowheads) indicates entry into MUG at t = 0. In the presence of nocodazole, there is a prolonged checkpoint arrest followed by escape that is marked by formation of micronuclei and respreading. Exit from MUG is also delayed when cells are treated with monastrol or taxol. Bar, 10 μm. (B) A cell in MUG after washout (t = 0) of 1.5 μM nocodazole. A bipolar spindle forms, and kinetochores congress to the equator (36 s). Soon after, the cell exits MUG, demonstrating satisfaction of the SAC. Time is given in hours/minutes.