Which base does DMS specifically modify in Maxam-Gilbert method?
Maxam-Gilbert sequencing is a chemical method where base-specific reagents modify nucleotides before piperidine cleavage. Dimethyl sulfate, DMS, methylates guanine at the N7 position, making the glycosidic bond labile and susceptible to breakage. Upon hot piperidine treatment, the strand cleaves at methylated guanines, producing fragments terminating at G positions specifically. Under different conditions DMS also methylates adenine, but classical G reaction uses low temperature to favor guanine selectivity. Thymine and cytosine are targeted by hydrazine instead, adenine by formic acid chemistry.
Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.