If a cell is grown in a medium containing radioactively labeled thymidine--which is one of the nucleotides needed to synthesize DNA--after the first round of replication, all of the DNA molecules will have one labeled strand and one unlabeled strand. Why? Because the radioactively labeled thymidine will be incorporated into the newly synthesized strands. If the cell undergoes ANOTHER round of replication in the same medium, the same sequence of events will occur.