DNA that has been liberally labeled with radioactive 14C is used as the template for replication.Replication is carried out in a medium containing only unlabeled nucleotides.After two rounds of replication,what percent of DNA molecules are still labeled?
A)25%
B)50%
C)75%
D)100%

Question

Quizplus · Accepted Answer

After the first round of replication, each new DNA molecule consists of one old (labeled) strand and one new (unlabeled) strand, so all DNA molecules are still labeled. After the second round, each of the original labeled strands is distributed into a separate DNA molecule, while the rest of the DNA (including all newly synthesized strands) is unlabeled. Since the DNA content doubles with each replication, after two rounds, only 1/4 of the DNA molecules contain the original labeled template strand, making the correct answer 25%.

DNA That Has Been Liberally Labeled with Radioactive 14C Is