A stable or radioactive isotope that can be used to label a metabolite and consequently follow its fate in an intact organism. Many elements found in living organisms have rare isotopes that are useful as tracers. For example, the most abundant form of carbon, carbon-12 (12
C), has six protons and six neutrons in its atomic nucleus. About 1.1% of carbon exists as the stable isotope 13
C, which has one extra neutron. 14
C, which has two extra neutrons, exists in minute amounts, and is radioactive, emitting beta rays. Both of the rarer isotopes are incorporated and function the same way as 12
C in an organism. The light-independent reactions of photosynthesis (see Calvin cycle
) were elucidated using 14
C-labelled carbon dioxide and finding the order in which substances incorporated the isotope. Tritium, a radioactive isotope of hydrogen having three neutrons instead of the usual one, has been used to label precursors of DNA, e.g. tritiated thymidine, to examine DNA synthesis.