Nucleotides  can  exist  in tautomeric forms (structural isomers) by a
   position change of a proton (tautomeric shift).
   The  less  common,  transient  tautomers  can form hydrogen bonds with
   noncomplementary bases.
   The  anomalous pairing is always between a pyrimidine and a purine, as
   shown in the T - G and C - A pairs.