As cells frequently divide to replace dead and damaged cells, checkpoints and regulatory cyclin-dependent enzymes in the cell cycle work to destroy cancer cells (see Figure 1). While kinase enzymes help healthy cells divide, kinase inhibitors block cell division of damaged cells. The Aurora and PCNA kinase inhibitors, in particular, trigger apoptosis at different parts of the cell cycle, in coordination with other inhibitors and specific tumor-suppressing proteins.
Another kinase inhibitor, SD-208, was found to arrest the cell cycle at the G2/M checkpoint. Figure 2 shows the cell count for control and SD-208 treated cells during a five-day period.
Based on the information in Figures 1 and 2, what is the primary role of protein kinases in the cell cycle?