[Illustration: Figure 13 _The DNA molecule and how it replicates. (a) The constituent submolecules. (b) Assembly of subunits in complete DNA molecule. (c) “Unzipping” of the double nucleotide strand. (d) and (e) The forming of a new strand by each individual strand. (f) DNA molecule in twisted double-strand configuration._ Adapted from _Viruses and the Nature of Life_, Wendell M. Stanley and Evans C. Valens, E. P. Dutton & Co., Inc., 1961, with permission.] Labeling DNA with a Radioactive Isotope Of the four bases in DNA, three are also found in the other nucleic acid, RNA; but the fourth, thymine, is found only in DNA. Therefore, if thymine could be labeled and introduced into a number of cells, including a cell in which DNA is being formed, we would specifically label the newly synthesized DNA, since neither the old DNA nor the RNA would make use of the thymine. We could in this way mark cells preparing to divide. (Actually, thymine itself is not taken up in mammalian cells, but its nucleoside is. A nucleoside is the base plus the sugar, or, in other words, the nucleotide minus the phosphoric acid.) The nucleoside of thymine is called thymidine, and we say that thymidine is a specific component of DNA and can be used, both in laboratory studies and in living organisms, for labeling DNA. Thymidine labeled with radioactive compounds is available as ¹⁴C-thymidine (thymidine with a stable carbon atom replaced by a radioactive carbon atom) and as ³H-thymidine (thymidine in which a stable hydrogen atom has been replaced by tritium). Thus, when cells actively making DNA are exposed to radioactive thymidine, they incorporate it, and the DNA becomes radioactive. We have thus found a way to complete the first part of the task, the labeling of new DNA. We still must find out how to distinguish labeled DNA among the many components of the cell. We might do it with a system based on measuring the amount of radioactivity incorporated into the DNA of cells exposed to radioactive thymidine, as an approximation of the frequency of cell division in the group of cells. However, a better method for studying cells synthesizing DNA, and thus preparing to divide, is the use of high-resolution autoradiography. Detecting DNA with Autoradiography