ABERRANTLY SEGREGATING CENTROMERES ACTIVATE THE SPINDLE ASSEMBLY CHECKPOINT IN BUDDING YEAST

The spindle assembly checkpoint is the mechanism or set of mechanisms that prevents cells with defects in chromosome alignment or spindle as sembly from passing through mitosis. We have investigated the effects of mini-chromosomes on this check-point in budding yeast by performing pedigree analysis. This method allowed us to observe the frequency an d duration of cell cycle delays in individual cells. Short, centromeri c linear mini-chromosomes, which have a low fidelity of segregation, c ause frequent delays in mitosis. Their circular counterparts and longe r linear mini-chromosomes, which segregate more efficiently, show a mu ch lower frequency of mitotic delays, but these delays occur much more frequently in divisions where the mini-chromosome segregates to only one of the two daughter cells. Using a conditional centromere to incre ase the copy number of a circular mini-chromosome greatly increases th e frequency of delayed divisions. In all cases the division delays are completely abolished by the mad mutants that inactivate the spindle a ssembly checkpoint, demonstrating that the Mad gene products are requi red to detect the subtle defects in chromosome behavior that have been observed to arrest higher eukaryotic cells in mitosis.