Practice question

Maxam-Gilbert sequencing, also called chemical sequencing, identifies sequence not by polymerase extension but by base-specific chemical modification and cleavage of existing DNA. End-labeled DNA is divided into reactions treated with reagents preferential for G, A+G, C+T, C alone. For example, dimethyl sulfate methylates G, formic acid depurinates A and G, hydrazine attacks pyrimidines. Piperidine then cleaves backbone at modified sites, electrophoresis resolves resulting fragments. Pyrosequencing, Illumina SBS and Ion Torrent all synthesize DNA; only Maxam-Gilbert directly cleaves DNA at specific bases chemically.