CpG doublets

It is well known that vertebrate DNAs show a characteristically low frequency of CpG doublets, whereas all other doublets are present at about the frequency expected from a random distribution of nucleotides (Josse et ah, 1961 Swartz et al., 1962). Two explanations were proposed to account for this outstanding feature of the vertebrate genome, the so-called CpG shortage. 

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Jabbari K. and Bernard G. (1998). CpG doublets, CpG islands and Alu repeats in long human DNA sequences from different isochore families. Gene 224 123-128. 

CG doublets. The only modified base commonly found in eukaryotes is 5-methylcytosine,222/223 which upon deamination becomes thymine (Eq. 27-1). Most methylation occurs when C is followed by G. Usually 60-90% of all 5 -CG sequences (CpG sequences) in eukaryotic DNA are methylated. However, the fraction of methylated cytosine varies from almost zero for Drosophilia, Caenorhabditis, and Saccharomyces to as much as 30% in higher plants.224... 

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