Figure 5.

Orientation of the initial kinetochore-to-pole microtubule connections. The ipl1 mutant cells give important clues to early kinetochore–microtubule interactions that have happened before reorientation of kinetochore-to-pole connections is initiated, because the reorientation is blocked in ipl1 mutant cells. The initial microtubule connections of early- and late-replicating centromeres (Tanaka et al. 2002) are illustrated. Thin and thick lines represent microtubules and chromosomes, respectively. The actual experiments were done using circular minichromosomes. Here, to simplify the model, linear chromosomes are shown where replication is initiated in the vicinity of centromeres. When centromeres replicate in early S phase, mono-orientation of sister kinetochores is predominantly found at the old SPB in ipl1 mutant cells. By contrast, when centromeres replicate in late S phase, mono-orientation is found equally at the old and new SPBs. These results address whether centromere DNA replication abolishes pre-existing attachment to microtubules or not (note that centromeres are tethered to the SPB in G1 phase). Formation of mono-orientation to the new SPB suggests that DNA replication indeed transiently disrupts kinetochore–microtubule interactions (see rectangles).