All species mark their DNA with methyl groups. This is done to regulate gene expression, distinguish indigenous DNA from foreign DNA, or to mark old DNA strands during replication. Methylation is carried out by certain enzymes called methyltransferases, which decorate DNA with methyl groups in certain patterns to create an epigenetic layer on top of DNA.
Click here for original story, Scientists uncover mystery of DNA methylation
Source: Phys.org