non-heritable (but gene-based)                            

 Nucleotide interchanges are of two types
        transitions   - alternative pyrimidines [ CT ]  or purines [ AG ]
             Most mutations are transitions: interchanges of bases of same shape
        transversions -  purine  pyrimidine [C / T A / G]
            depurination - misrepair of base may produce transversion
                loss of purine (A or G) base in intact polynucleotide
                     produces apurinic  site
                     common form of damage in 'ancient' DNA
                apyrimidinic sites similar: loss of C or T

Spontaneous mutation
       tautomeric shift - spontaneous, transient rearrangement to alternative form
               keto (standard)  enol  (rare) forms of G & T
               amino (standard)  imino (rare) forms of A & C
               Both involve shift of H proton

       Non-standard bases have altered pairing rules :
               modified purine' bonds with alternative pyrimidine
               modified pyrimidine' bonds with alternative purine

                T' (enol)    pairs with G (keto)
                C' (imino)  pairs with A (amino)
                G' (enol)    pairs with T (keto)
                A' (imino)  pairs with C (amino)

        A tautomeric shift produces a transition mutation in the complementary strand [see example]

Homework: Based on this diagram, show that tautomeric shifts of the C, G, & T bases also produce transition mutations.