Cell cycle inertia underlies a bifurcation in cell fates after DNA damage

The G(1)-S checkpoint is thought to prevent cells with damaged DNA from entering S phase and replicating their DNA and efficiently arrests cells at the G(1)-S transition. Here, using time-lapse imaging and single-cell tracking, we instead find that DNA damage leads to highly variable and divergent fate outcomes. Contrary to the textbook model that cells arrest at the G(1)-S transition, cells triggering the DNA damage checkpoint in G(1) phase route back to quiescence, and this cellular rerouting can be initiated at any point in G(1) phase. Furthermore, we find that most of the cells receiving damage in G(1) phase actually fail to arrest and proceed through the G(1)-S transition due to persistent cyclin-dependent kinase (CDK) activity in the interval between DNA damage and induction of the CDK inhibitor p21. These observations necessitate a revised model of DNA damage response in G(1) phase and indicate that cells have a G(1) checkpoint.