Practice question

Visualization of specific fragments after end labeling requires selective detection of radioactivity rather than total DNA mass. Ethidium bromide intercalates into all double-stranded DNA and fluoresces under UV, revealing every fragment regardless of label. Spectrophotometry quantifies bulk nucleic acid concentration. Autoradiography exposes X-ray film only where beta particles from incorporated 32P or 35S decay, so only labeled termini appear as dark bands. Gel drying is preparative. Hence autoradiography uniquely allows ordering labeled fragments in classical restriction mapping protocols before nonradioactive fluorescent methods.