Non-Standard base pairing of rare tautomers

    Specificity of base pairing is determined by the number of hydrogen bonds available. Recall that the standard forms of C and G pair with three H-bonds, and the standard forms of A and T pair with two.

    The rare form of
C* has only two H-bonds, and pairs with the alternative purine, A. Likewise, the rare form of T* has three H-bonds, and pairs with the alternative pyrimidine, G. In contrast, the rare forms of G* and A* retain the correct number of H-bonds, but their modified arrangements force non-standard pairing by three and two bonds with T and C, respectively.

    The net result is that the rare tautomers pair with the alternative ("wrong") purine or pyrimidine: C* with A, T* with G, G* with T, and A* with C, respectively.

Figure 2000 by Griffiths et al. ; text 2013 by Steven M. Carr