If we look at cells soon after they have been exposed to an RNA
precursor, we find that the radioactivity detectable by autoradiography
is only in the nuclei of the cells. No radioactivity can be detected in
the cytoplasm, although we know that the cytoplasm of living cells
contains large amounts of r-RNA and s-RNA. One or two hours later,
however, radioactive RNA appears in the cytoplasm as well as in the
nucleus. What autoradiography is telling us is that RNA is made in the
nucleus and then is slowly transferred to the cytoplasm.
Autoradiography cannot tell us whether the RNA that has been newly
synthesized in the nuclei of cells is m-RNA, s-RNA, or r-RNA. The
methods necessary to make this distinction are based on the chemical
fractionation of the tissue, isolation of RNA, determination of its
amount by quantitative analysis, and determination of the amount of
radioactivity by physical methods. Let us examine these steps
separately.
[Illustration: Figure 17 _Injecting a mouse with a radioactive
solution._]