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stable hydrogen atom has been replaced by tritium).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.