Establishing biorientation occurs with precocious separation of the sisterkinetochores, but not the arms, in the early spindle of budding yeast

Sister kinetochores are bioriented toward the spindle poles in higher eukar yotic prometaphase before chromosome segregation. We show that, in budding yeast, the sister kinetochores are separated in the very early spindle, whi le the sister arms remain associated. Biorientation of the separated kineto chores is achieved already after replication. Mtw1p, a homolog of fission y east Mis12 required for biorientation, locates at the centromeres in an Ndc 10p-dependent manner. Mtw1p and the sequences 1.8 and 3.8 kb from CEN3 and CEN15, respectively, behave like the precociously separated kinetochores, w hereas the sequences 23 and 35 kb distant from CEN3 and CEN5 previously use d as the centromere markers behave like a part of the arm. Mtw1p and Ndc10p are identically located except for additional spindle localization of Ndc1 0p. A model explaining small centromeres and early spindle formation in bud ding yeast is proposed.