If you want to selectively label RNA being synthesized by cells (and not DNA) what radioactive compound would you add to the medium?

a. 35s-labeled sulfate

b. 32p-labeled phosphate

c. 14c-labeled leucine

d. 3h-labeled thymidine

e. 14c-labeled uracil

The right answer is E.

Uracil is a nitrogenous base (pyrimidine) specific for RNA. The nucleoide of uracil is called uridine and nucleotide is called uridine monophosphate or uridylate. In the DNA, there is thymine instead of uracil.

So if we mark the uracils, only the RNA will be marked. The DNA will not be given that there is no uracil in it.