Frequency in Vertebrates
CpG dinucleotides have long been observed to occur with a much lower frequency in the sequence of vertebrate genomes than would be expected due to random chance. For example, in the human genome, which has a 42% GC content, a pair of nucleotides consisting of cytosine followed by guanine would be expected to occur 0.21 * 0.21 = 4.41% of the time. The frequency of CpG dinucleotides in human genomes is 1% — less than one-quarter of the expected frequency. Scarano et al. proposed that the CpG deficiency is due to an increased vulnerability of methylcytosines to spontaneously deaminate to thymine in genomes with CpG cytosine methylation.
Read more about this topic: CpG Site
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