Intrinsic checkpoint deficiency during cell cycle re-entry from quiescence

Jacob Peter Matson; Amy M. House; Gavin D. Grant; Huaitong Wu; Joanna Perez; Jeanette Gowen Cook

Strikingly, the amount of loaded MCM at the G1/S transition was markedly different between the first and second cell cycles after G0. Cells in the first cell cycle progressed into S phase (orange cells) with substantially less MCM loaded than cells in the second cell cycle. By the second cell cycle, G1 cells progressed into S phase as a tight group with relatively high amounts of loaded MCM (transition from blue to orange). In contrast, many cells in the first cell cycle entered S phase with low amounts of loaded MCM. This behavior creates the filled orange triangle on flow cytometry plots of the first S phase instead of the normally clear region under a high arc characteristic of the second S phase. We also observed decreased loaded MCM in the first cycle using biochemical chromatin fractionation and immunoblotting (Fig. 1D) .