Question

A DNA molecule in which both strand have radioactive thymidine is allowed to duplicate in an environment containing non-radioactive thymidine. What will be the correct number of DNA molecules that contain some radioactive thymidine after three duplications?

• A. There will be four such molecules.
• B. There will be eight such molecules.
• C. There will be only one such molecule.
• D. There will be two such molecules.

Answer 1

Answer: (D)

There will be two such molecules.

• DNA replication is semi-conservative in nature. When the parental strand has radioactive thymine, the daughter DNA molecule will have one strand with radio active thymine whereas the other strand will have non-radioactive thymine.
• Thus, when it again replicates, one set of daughter molecules will have radioactive thymine, but the other won't have it completely and so on.
• Thus, after 3 generations, we can get 2 molecules with radioactive thymine.