Final answer:
14C-labelled uracil triphosphate would label RNA in cell cultures because uracil is unique to RNA. When adding synthetic cytosine RNA to a biochemical mixture, a polypeptide chain consisting of proline would be synthesized. Radioactive phosphorous labels DNA due to its presence in DNA's structure.
Step-by-step explanation:
If you were growing cells in the lab and decided to add 14C-labelled uracil triphosphate to the growth medium, the macromolecule that would be labeled is RNA. Uracil is one of the four nitrogenous bases used in the synthesis of RNA, and since DNA uses thymine instead of uracil, only RNA will incorporate the labeled uracil triphosphates.
Considering an experiment similar to that of Nirenberg and Matthaei, if you added synthetic RNA containing only the base cytosine (C) to a mixture, the polypeptide chain produced would be one consisting of the amino acid proline. This is because the codon CCC in the genetic code corresponds to the amino acid proline.
As for labeling experiments, when radioactive phosphorous is used to label molecules, it will tag DNA since phosphorous is a component of the phosphate group in DNA's backbone structure.
Radioactive sulfur, on the other hand, would typically not be used to label nucleic acids, as sulfur is found in some amino acids like cysteine or methionine, so it would be more commonly used in probing proteins.