Figure 21. Methylation of cytosine and consequences of deamination of methyl-C.

(A) Cytosine can be methylated to generate 5-methylcytosine; this may undergo spontaneous deamination to generate thymine. The arrow indicates position of attachment to the deoxyribose ring. (B) CG dinucleotides (indicated by vertical lines) are relatively rare in the genome overall compared with the expected frequency. This is believed to be an evolutionary consequence of deamination of methylcytosine leading to conversion of many C–G base pairs within CG dinucleotides into T–A. However due to the importance of DNA methylation in transcription regulation (see Figure 25), CG dinucleotides are present at higher frequency around promoter regions (transcriptional start sites) of genes, generating so-called ‘CG islands’ (also known as CpG islands).