As in DNA synthesis, we can use autoradiography to follow the
incorporation of precursors into RNA. By proper treatment of the
tissues, we can make sure that all the radioactivity visible by
autoradiography is due to labeled RNA, even though some of the precursor
also enters DNA molecules. Even so, the kind of information obtained
from autoradiographs of tissues exposed to RNA precursors is different
from that obtained with DNA precursors. The advantage of high-resolution
autoradiography in DNA studies is the possibility of identifying
particular cells that are synthesizing nucleic acid. This advantage is
apparently lost in the case of RNA. The reason is that, at any given
time, only a few cells are making DNA, whereas practically all cells are
synthesizing RNA constantly. The only exceptions are cells in the
midpoint of mitosis. At the beginning (prophase) and at the end of cell
division (telophase), RNA is synthesized. If we want a quantitative
measurement of RNA synthesis, other methods, to be examined presently,
are considerably more precise. But autoradiography can still give us
valuable information.