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.
