Deamination of cytosine yields uracil, which is thought to be why DNA has evolved to contain methyl uracil (thymine), to enable the correction of this chemically inevitable corruption of the coded message. However, there are regions of the genome that contain significantly higher proportions of methyl cytosine, often preceding a G in the sequence. These regions are associated with gene silencing in promoter regions, rather than protein-encoding portions of the DNA. Deamination of methyl cytosine yields thymine that has the effect of increasing the proportion of TG dinucleotides and reducing the abundance of CG dinucleotides; this outcome is seen in dinucleotide analysis of eukaryotic genomes. The most commonly occurring dinucleotide pair is TG, and the least common is CG; the incidence of CG is significantly lower than for any of the other dinucleotide pairs.
