What makes the DNA fragments visible on an autoradiograph?

• A. Radioactive primer
• B. A colorimetric stain
• C. A fluorescent dye
• D. A UV-exposed dye label

Answer: (A)

Autoradiography relies on detecting radiation from a radiolabeled molecule. DNA fragments become visible because they carry a radioactive label—often introduced via a radioactive primer during synthesis or amplification. The emitted radiation from that label (for example, beta particles from a 32P label) exposes the photographic film or a phosphor screen, creating the dark bands that correspond to where the fragments are. Other detection methods use color changes or fluorescence and do not rely on radiation exposure of the film, so they wouldn’t produce an autoradiograph signal. That's why a radioactive primer is the factor that makes the DNA fragments visible.