5' deoxyInosine

Historically, the first universal base employed was 2’-deoxyInosine (dI). DeoxyInosine is a naturally occurring base that, while not truly universal, is less destabilizing than mismatches involving the four standard bases. Hydrogen bond interactions between dI and dA, dG, dC and dT are weak and unequal, with the result that some base-pairing bias does exist with dI:dC > dI:dA > dI:dG > dI:dT. When present in a DNA template, deoxyInosine preferentially directs incorporation of dC in the growing nascent strand by DNA polymerase.

Structure