THE CHECKPOINT CONTROL FOR ANAPHASE ONSET DOES NOT MONITOR EXCESS NUMBERS OF SPINDLE POLES OR BIPOLAR SPINDLE SYMMETRY

Exit from mitosis in animal cells is substantially delayed when spindl e assembly is inhibited, spindle bipolarity is disrupted, or when a mo nopolar spindle is formed, These observations have led to the proposal that animal cells have a 'spindle assembly' checkpoint for the metaph ase-anaphase transition that monitors bipolar spindle organization, Ho wever, the existence of such a checkpoint is uncertain because perturb ations in spindle organization can produce unattached kinetochores, wh ich by themselves are known to delay anaphase onset, In this study we have tested if cells monitor bipolar spindle organization, independent of kinetochore attachment, by analyzing the duration of mitosis in se a urchin zygotes and vertebrate somatic cells containing multipolar sp indles in which all kinetochores are attached to spindle poles, We fou nd that sea urchin zygotes containing tripolar or tetrapolar spindles progressed from nuclear envelope breakdown to anaphase onset with norm al timing, We also found that the presence of supernumerary, unpaired spindle poles did not greatly prolong mitosis, Observation of untreate d PtK1 cells that formed tripolar or tetrapolar spindles revealed that they progressed through mitosis, on average, at the normal rate. More importantly, the interval between the bipolar attachment of the last monooriented chromosome and anaphase onset was normal, Thus, neither o f these cell types can detect the presence of gross aberrations in spi ndle architecture that inevitably lead to aneuploidy, We conclude that animal cells do not have a checkpoint for the metaphase-anaphase tran sition that monitors defects in spindle architecture independent of th e checkpoint that monitors kinetochore attachment to the spindle, For dividing cells in which spindle microtubule assembly is not experiment ally compromised, we propose that the completion of kinetochore attach ment is the event which limits the time of the metaphase-anaphase tran sition.