linguistic:voice_change, stylistic:style_transfer
Conserved function of DNA methylation

High CpG methylation in mammalian genomes has an evolutionary cost because it increases the frequency of spontaneous mutations. Loss of amino-groups occurs with a high frequency for cytosines, with different consequences depending on their methylation. Methylated C residues spontaneously deaminate to form T residues over time; hence CpG dinucleotides steadily deaminate to TpG dinucleotides, which is evidenced by the under-representation of CpG dinucleotides in the human genome (they occur at only 21% of the expected frequency). (On the other hand, spontaneous deamination of unmethylated C residues gives rise to U residues, a change that is quickly recognized and repaired by the cell.)