Origin of DNA Methylation
In prokaryotes, DNA methylation occurs at 5 prime region of the cytosine pyrimidine ring and the number 6 nitrogen of the adenine purine ring. However, in case of eukaryotes, DNA methylation occurs only at the 5th carbon of cytosine pyrimidine ring. And in mammalians, DNA methylation occurs at the 5th carbon of cytosine of CpG dinucleotide, however, there is only 1 percent of CpG present in human genome. Almost 70 to 80 percent of CpGs are methylated, however, unmethylated CpGs are found in clusters called CpG islands. It has been found that aberrant DNA methylation at the chromosomal regions known, as CpG islands are a crucial step in carcinogenesis.
In case of E.coli, wherever, there is a GATC sequence in the DNA, an enzyme called Dam methylase methylates the adenine of this sequence. This does not affect base pairing or DNA structure. It takes time after its synthesis for the new strand to be methylated and so for this brief period, it is unmethylated. Thus the parental strand is methylated but the new strand is not.
Mutations in of the DNA methylation machinery highlight several inherited syndromes such as immunodeficiency, centromere instability, facial abnormalities, and Rett syndrome, etc. There are other syndromes that are caused due to the disturbances in DNA methylation.
It is therefore, important for the cellular mechanism to work properly and DNA methylation is indeed one of the most important cellular machineries that ensure base pairing during DNA replication.