Practice question

End-labeling strategies label only the terminus where radioactive nucleotide incorporation occurs, creating a single reference point for mapping. After partial digestion, many fragments contain the labeled end plus internal segments of varying lengths, while fragments derived purely from interior regions lacking the original terminus remain unlabeled. Autoradiography detects only species retaining the labeled end, so internal fragments generated by two internal cuts without involving labeled terminus stay invisible on film. Therefore unlabeled class corresponds to internal fragments, which become visible only with universal staining like ethidium bromide.