Background: Chromosomes must biorient on the mitotic spindle, with the two sisters attached to opposite spindle poles. The spindle checkpoint detects unattached chromosomes and monitors biorientation by detecting the lack of tension between two sisters attached to the same pole. After the spindle has been depolymerized and allowed to reform, budding yeast sgo1 mutants fail to biorient their sister chromatids and die as cells divide.
Results: In sgo1 mutants, chromosomes attach to microtubules normally but cannot reorient if both sisters attach to the same pole. The mutants' fate depends on the position of the spindle poles when the chromosomes attach to microtubules. If the poles have separated, sister chromatids biorient, but if the poles are still close, sister chromatids often attach to the same pole, missegregate, and cause cell death.
Conclusions: These observations argue that budding yeast mitotic chromosomes have an intrinsic, geometric bias to biorient on the spindle. When the poles have already separated, attaching one kinetochore to one pole predisposes its sister to attach to the opposite pole, allowing the cells to segregate the chromosomes correctly. When the poles have not separated, the second kinetochore eventually attaches to either of the two poles randomly, causing orientation errors that are corrected in the wild-type but not in sgo1 mutants. In the absence of spindle damage, sgo1 cells divide successfully, suggesting that kinetochores only make stable attachments to microtubules after the cells have entered mitosis and separated their spindle poles.