To maintain genome stability, cells need to replicate their DNA before dividing. The kinases CDK1 and PLK1 drive mitotic entry and become active when bulk DNA synthesis is completed at the S/G2 transition. Here, we have tested the hypothesis that DNA replication controls activation of mitotic kinases. Using an optimized double-degron system, we find that human cells unable to initiate DNA replication in S-phase promptly activate CDK1 and PLK1 and prematurely enter mitosis. In the presence of DNA replication, inhibition of CHK1 and p38 leads to premature activation of CDK1 and PLK1. While CDK2 activity promotes DNA replication, activation of CDK1 in S-phase induces severe replication stress. We propose that mitotic kinase activation is governed by a CDK2- and DNA replication-dependent feed-forward loop that ensures timely cell division while preserving genome stability. DNA replication thus functions as a break that coordinates cell cycle activities and determines cell cycle duration
