Methyl group

A methyl group consists of one carbon atom that is attached to another molecule by one of its four bonds whilst its other three bonds are bonded to hydrogen atoms. It looks like the following diagram.   

       H

       l

 H - C - H 

       l   

      R'

Methyl groups can be added to DNA (Methylation) in order to modify the function of the gene associated with it, usually repressing transcription.