Tagging DNA.(a) Suppose that you want to radioactively label DNA but not RNA in dividing and growing bacterial cells. Which radioactive molecule would you add to the culture medi$\mathrm{um}?$(b) Suppose that you want to prepare DNA in which the backbone phosphorus atoms are uniformly labeled with ${}^{32}\mathrm{P}$ Which precursors should be added to a solution containing DNA polymerase and primed template DNA? Specify the position of radioactive atoms in these precursors.

Step by Step Solution: Step 1. For part (a), we need to add a radioactive molecule that will only be incorporated into DNA and not RNA. One such molecule is thymidine that specifically incorporates only into DNA. Therefore, we will add radioactive thymidine to the culture medium. Step 2. For part (b), we need to prepare DNA in which the backbone phosphorus atoms are labeled with ${}^{32}\mathrm{P}$. To do this, we need to include precursors that contain ${}^{32}\mathrm{P}$ in the backbone of DNA during its synthesis. Some such precursors are labeled deoxynucleotide triphosphates (dNTPs). Specifically, we will use ${}^{32}\mathrm{P}$-labeled dCTP (deoxycytidine triphosphate) and dATP (deoxyadenosine triphosphate) to synthesize the DNA. The ${}^{32}\mathrm{P}$ will be incorporated into the phosphate groups of the dCTP and dATP nucleotides when they are added to the DNA backbone during synthesis. Step 3. The positions of the radioactive atoms in the precursors are in the phosphate groups of the dCTP and dATP nucleotides that will be added to the DNA backbone during synthesis.