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.
