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.
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