Researchers pulsed rapidly dividing cultured cells for 30 minutes with radioactive thymidine. The cells were then exposed to a solution containing non-radiolabeled thymidine. Cells were analyzed at 2-hour intervals. At the 2-hourtime point, no cells appeared to be dividing. Only after 4 hours did some labeled cells appear to be in M phase. Thisresult can be explained in the following way:
A) Radiolabeled compounds are somewhat cytotoxic, and so cell division was initially inhibited.
B) The cells were arrested in a nondividing state because of the treatment and could not enter M phase until several hours after the label was removed.
C) 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.
D) There seems to be a gap or a lag in the cell cycle, between the synthesis of DNA and cell division

Pulse chase experiments were used to study the proteins transport and its movement in the different cell organelle. The radioactive molecule is used during the pulse chase experiment.

The cell cycle has been studied in the research by using the radioactive labeled thymidine. It has been found that cell found in M phase after 4 hours. This indicates that there may be some lag or gap phase between cell division and DNA synthesis. This phase checks the checkpoint of the cell and prevent the mutation.

Thus, the correct answer is option (D).