The earliest biochemical studies employing radioactive isotopes go back to 1924, when George de Hevesy used natural radioactive lead to investigate a biological process. It was only after World War II, however, when artificially made radioactive isotopes were readily available, that the technique of using isotopic tracers became popular. In our investigations of life processes, we are especially interested in three radioactive isotopes: ³H, the hydrogen atom of mass 3; ¹⁴C, the atom of carbon with atomic weight 14; and ³²P, the atom of phosphorus with atomic weight 32. These radioactive isotopes are important because the corresponding stable isotopes of hydrogen, carbon, and phosphorus are present in practically all cellular components that are important in maintaining life. With the three radioactive isotopes, therefore, we can tag or label the molecules that participate in life processes. [Illustration: Figure 10 _A visiting scientist at an AEC laboratory uses radioactive tritium (³H) to study the effect of radiation on bean chromosomes. The famous scientist, George de Hevesy, also used beans in conducting the first biological studies ever made with radioisotopes._] Hydrogen-3 is a weak beta emitter; that is, it emits beta particles with a very low energy (0.018 Mev[6]) and therefore with a very short range. Carbon-14 is also a weak beta emitter (0.154 Mev), although the beta particles emitted by ¹⁴C have a higher energy and therefore a longer range than those emitted by ³H. The beta particles emitted by phosphorus-32 are quite energetic (1.69 Mev) and have a longer range. Radioactive Isotopes’ Value in Biological Studies To biologists, then, the essential feature in the use of radioactive isotopes is the possibility of preparing “labeled” samples of any organic molecule involved in biological processes. With labeled samples it is possible to distinguish the behavior and keep track of the course of molecules involved in a particular biological function. In this capacity the isotope may be likened to a dynamic and revolutionary type of “atomic microscope”, which can actually be incorporated into a living process or a specific cell. Just as a real microscope permits examination of the structural details of cells, isotopes permit examination of the chemical _activities_ of molecules, atoms, and ions as they react within cells. (Neither optical nor electron microscopes are powerful enough for us to see anything as small as a molecule clearly.) DNA SYNTHESIS: THE AUTOBIOGRAPHY OF CELLS _Here, surely, is the prime substance of life itself._ Isaac Asimov