Isocytosine

CAS 108-53-2

DNA normally uses four letters, but isocytosine helped researchers test an additional pair. It has the same molecular formula as cytosine, with functional groups rearranged to change its hydrogen-bonding pattern. In landmark experiments reported in 1989 and 1990, appropriately constructed isocytosine-containing nucleic-acid components helped demonstrate polymerase recognition of a nonstandard pair with isoguanine. The free base itself is not a nucleotide: sugars, phosphates and their geometry matter. So do alternative molecular forms that can expose different recognition patterns. A new pair must fit a helix, select its intended partner and work with copying enzymes. Isocytosine shows how moving familiar chemical groups can turn a question about life's alphabet into an experiment, while leaving the harder problem of reliable repeated copying to be tested.