What is DNA methylation?

When I was doing my Ph.d, I took a course "Molecular Biology of The Cell". DNA methylation is stated as one of the epigenetic mechanisms that cells use to regulate gene expression. Typically, it is caused by adding a methyl (CH3) group to DNA and commonly observed in eukaryotic organisms like plants and animals. Based on the previous studies, DNA methylation is highly associated with several human diseases. 

(DNA methylation pathways, refer to Moore et. al Neuropsychopharmacology. 2013 Jan; 38(1): 23–38. doi: 10.1038/npp.2012.112)