Explanation

Autoradiography is the study of labelled precursors like 3 H by knowing the movement of radioactivity with the help of photographic films and emulsions at short intervals. Radioactive material like tritiated thymidine which is formed by replacing normal hydrogen of thymidine with H₃ (heavy isotope of hydrogen). Thymidine only is used for this purpose because RNA will not be labelled by this.