5-Methylcytosine 2026-11-05
5-Methylcytosine is a modified cytosine with a methyl group attached to carbon 5 of its ring. In DNA, it retains the usual pairing relationship with guanine, so DNA methylation can change the chemical information on a sequence without changing its ordinary C/G base-pair identity.
A chemical difference makes the mark experimentally readable. In conventional bisulfite sequencing, treatment converts unmethylated cytosine to a form read as thymine after amplification, while 5-Methylcytosine is relatively protected. Comparing the resulting sequences reveals methylation at individual positions. The method introduced by M. Frommer and colleagues in 1992 turned a modification invisible in a simple four-letter DNA string into a positional readout.[ref]
A genomic sequencing protocol that yields a positive display of 5-methylcytosine residues in individual DNA strands by M. Frommer and others. 1992, abstract: bisulfite conversion and strand-level sequencing
A methylation measurement must still be interpreted in its genomic and cellular context; the mark does not mean universal shutdown of transcription. Bisulfite sequencing Created 2026-10-20 Updated 2026-11-05
Bisulfite sequencing infers cytosine modification from differential chemical conversion before DNA sequencing. Under the relevant chemistry, unmodified cytosine is converted to uracil and is subsequently read as thymine, while 5-methylcytosine is retained as cytosine. Incomplete conversion, sequence coverage and strand assignment affect interpretation. Conventional bisulfite sequencing does not by itself separate 5-methylcytosine from every other conversion-resistant modification, so its signal requires a stated chemical scope. [ref]
A genomic sequencing protocol that yields a positive display of 5-methylcytosine residues in individual DNA strands by M. Frommer and others. Frommer and others, 1992, abstract: differential conversion and single-strand methylation display
M. Frommer 2026-11-05
M. Frommer led the 1992 paper establishing a sequence-based display of 5-Methylcytosine in individual DNA strands. The method exploits the different responses of ordinary cytosine and methylated cytosine to bisulfite treatment: ordinary cytosine is preferentially converted to uracil, which is read as thymine after amplification, while methylated cytosine is relatively resistant.[ref]
This connects chemical modification to an ordinary sequence readout, making bisulfite sequencing a way to map DNA methylation. The untreated reference and conversion controls matter because an observed cytosine/thymine difference could otherwise be mistaken for a sequence variant or incomplete chemical conversion.