Lack of tension at kinetochores activates the spindle checkpoint in budding yeast

The spindle checkpoint delays the onset of anaphase until all pairs of sist er chromatids are attached to the mitotic spindle. The checkpoint could mon itor the attachment of microtubules to kinetochores, the tension that resul ts from the two sister chromatids attaching to opposite spindle poles, or b oth. We tested the role of tension by allowing cells to enter mitosis witho ut a prior round of DNA replication. The unreplicated chromatids are attach ed to spindle microtubules but are not under tension since they lack a sist er chromatid that could attach to the opposite pole. Because the spindle ch eckpoint is activated in these cells, we conclude that the absence of tensi on at the yeast kinetochore is sufficient to activate the spindle checkpoin t in mitosis.