Researchers pulsed rapidly dividing cultured cells with radioactive thymidine for 30 minutes. The cells were then exposed to a solution containing non-radiolabeled thymidine. Cells were analyzed at 2-hour intervals. At the 2-hour time point, no cells appeared to be dividing. Only after 4 hours did some labeled cells appear to be in M phase. This result can be explained in the following way:
a. The cells were arrested in a non-dividing state because of the treatment and could not enter M phase until several hours after the label was removed.
b. The synthesis (S) phase is lengthy, about 12 hours in most cell types, and the radioactive thymidine was not present long enough for most cells to be labeled.
c. Radio-labeled compounds are somewhat cytotoxic, and cell division was initially inhibited.
d. There seems to be a gap or a lag in the cell cycle, between the synthesis of DNA and cell division.