A midzone-based ruler adjusts chromosome compaction to anaphase spindle length.

Partitioning of chromatids during mitosis requires that chromosome compaction and spindle length scale appropriately with each other. However, it is not clear whether chromosome condensation and spindle elongation are linked. Here, we find that yeast cells could cope with a 45% increase in the length of their longest chromosome arm by increasing its condensation. The spindle midzone, aurora/Ipl1 activity, and Ser10 of histone H3 mediated this response. Thus, the anaphase spindle may function as a ruler to adapt the condensation of chromatids, promoting their segregation regardless of chromosome or spindle length.