 
      Deamination mutagenesis
(a) Loss of an amine (NH2)
          group changes C to U, which will pair with A. 
        
(b) Methylation of
          bases is one form of post-replication modification that marks
          bases as newly-replicated. Methyl C bases (mC) may undero deamination
          to T, which will pair
          with A.