Practice question

Non-isotopic labeling is divided into direct and indirect systems based on detection. Indirect labels such as biotin and digoxigenin are haptens requiring secondary reagents like streptavidin or antibody conjugates for visualization. Direct labeling attaches a reporter immediately detectable without additional steps. Fluorescein, a fluorescent dye, exemplifies direct non-isotopic labeling because its emission is observed directly under excitation. Autoradiography is not a label but a detection method for radioactive isotopes. Therefore fluorescein enables rapid one-step detection, widely used in fluorescent in situ hybridization and direct probe imaging.