RNA synthesis is investigated with radioactive tracers in the same way
as DNA synthesis. If we can mark, with a radioactive atom, a small
molecule that is incorporated into newly formed RNA, we can then trace
the course of the labeled RNA molecule with a radiation-detection
device. DNA had one advantage in this regard—the fact that one compound,
thymidine, was a precursor of DNA, a specific material that could be
incorporated only into DNA. We do not know similar specific precursors
of RNA. But we know several precursors that are predominantly
incorporated into RNA; the most common of these are the nucleosides
adenine, cytidine, and uridine, and the smaller molecule, orotic acid.
All these precursors can be labeled with either ³H or ¹⁴C, and their
incorporation into RNA can be measured.