Chromosome condensation mechanically primes the nucleus for mitosis.

Accurate transition into mitosis driven by cyclin B1-CDK1 activity is essential to avoid chromosome segregation errors and preserve genome integrity. How this activity is spatially controlled to trigger mitotic onset remains unclear. Here, we show that chromosome condensation triggers an increase in nuclear envelope (NE) tension. This increased tension is required for translocation of cyclin B1 into the nucleus and dynein loading on nuclear pore complexes (NPCs), ensuring timely mitotic entry. Micromanipulation experiments further indicate this tension-dependent mechanism requires SUN proteins on the NE. Impairment of chromosome condensation leads to the nuclear accumulation of the G2 checkpoint kinase Wee1 and an inhibition of CDK1 activity, which result in a temporary delay in mitotic entry. This delay can be overridden by increasing tension on the NE, which accelerates the nuclear translocation of cyclin B1 and dynein loading. We propose that mitotic onset is controlled by a chromosome-dependent NE tension mechanism that enables robust spatiotemporal coupling between chromosome condensation and the NE structural changes required for an efficient mitosis.