DNA methylation is a chemical modification where a methyl group is added to a cytosine base in DNA, typically occurring at CpG sites.

“so so DNA methylation is a chemical modification of the D DNA so in particular at the C in the ATC code the C stands for AC cytosine so that's a particular um nucleotide or or base in in DNA um uh the the um Maman biology can methylate it means that it can add a methyl group but a methyl group is just a a single carbon atom right um with with three hydrogens and then bonded to that cytosine um and so that's what DNA methylation is so to say a cytosine is methylated it means that it has that um you know single you know methyl group bonded to it um uh turns out that there's about 28 million positions in the human genome um that can be methylated it usually occurs at um what's called cpg sites which is um if you if you go along one strand of DNA this is not pairing of the DNA but one strand um where a c um or a a g uh follows a c so that's what a cpg is it's a c with a phosphate bond to a g um and so at those positions in the genome there are enzymes that can methylate the cytosine and demethylated um and there's again about”