Figure 3.

Reorientation of unreplicated centromeres between the new and the old SPBs. The behaviours of unreplicated centromeres (four representative ones are illustrated) in IPL1+ and ipl1 mutant cells (Tanaka et al. 2002) are shown. To inhibit DNA replication (tension is not applied on unreplicated centromeres), we depleted Cdc6 that is required for DNA replication initiation. The old SPB, inherited from the previous cell cycle, remained intact during SPB duplication (Adams & Kilmartin 2000) and always entered the bud during anaphase (Pereira et al. 2001). The new SPB was formed in the vicinity of the old SPB during S phase and remained in the mother cell during anaphase. In yeast, centromeres are tethered to the old SPB by microtubules in G1 phase (Winey & O'Toole 2001; Tanaka et al. 2002). During anaphase of Cdc6-depleted IPL1+ cells, unreplicated centromeres segregated with the old and new SPBs with equal frequency (Piatti et al. 1995; Stern & Murray 2001). This suggests that, after the new SPB had been formed, kinetochores had detached from the old SPB and reoriented between the old and new SPBs. By contrast, in Cdc6-depleted ipl1 mutant cells, unreplicated centromeres predominantly segregated with the old SPB during anaphase (Tanaka et al. 2002). This implies that the frequency of reorientation from the old SPB to the new SPB is considerably reduced when Ipl1 is defective.