Now that you have established the exact time course of cell cycle progression in synchronized cultures of mouse fibroblasts, you are ready to determine whether substance X arrests cells at the R point or at the G1/S transition point. You perform the experiment illustrated here. Scroll down to see the results.

TREATMENT	G1	S	G2/M
No exposure to substance X	51	41	8
Exposure to substance X during t=0-8	55	35	10
Exposure to substance X during t=8-16	88	7	5

8. You may draw the following conclusions (TRUE or FALSE):
A. Exposure to substance X during G0 arrests the cells in G0.
B. Exposure to substance X after the cells have crossed the R point arrests the cells before S phase.
C. Cells are accumulated before S phase, because the FACS analysis was performed too soon and the cells did not have enough time to enter S phase.

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