methyl

Meaning: A methyl group is one carbon bonded to three hydrogens and attached to the rest of a molecule by a single bond (–CH₃). C–H bonds are nearly nonpolar, so the group is hydrophobic. Alanine's side chain is a methyl group, and methyl groups are also part of the side chains of valine, leucine, isoleucine and methionine. Methylation is the transfer of a methyl group onto another molecule, with S-adenosylmethionine (SAM) as the usual donor. In eukaryotes, methylation of cytosine in DNA (often at CpG sites) is typically associated with lower gene expression. Histone methylation can raise or lower expression depending on which residue is modified.

On the MCAT: A passage describes a gene whose promoter is hypermethylated, and you predict reduced transcription. Questions may also ask why a methyl-containing amino acid sits in a protein's hydrophobic core, or how a methyl substituent changes solubility. Methylation is the standard example of an epigenetic change: it alters expression, can persist through cell division, and leaves the DNA sequence intact. Bacteria also methylate their own DNA to protect it from their restriction enzymes.

Common confusion: DNA methylation is not a mutation, because the bases still read the same in sequence. Also, "methyl" names a substituent, not a free molecule; methane (CH₄) and methanol (CH₃OH) are different compounds.