Which process involves the addition of a methyl group to DNA?

The addition of a methyl group to DNA is known as methylation. This process plays a critical role in the regulation of gene expression and can affect how genes are turned on or off. Specifically, when a methyl group is added to the cytosine bases in DNA, it can significantly influence the accessibility of the DNA to transcription machinery, thereby impacting whether a gene is expressed.

Methylation is part of a broader field called epigenetics, which studies changes in gene activity that do not involve alterations to the underlying DNA sequence. These modifications can be inherited and can affect an organism's phenotype without changing its genotype.

Understanding the context of methylation is essential, as it differs from other processes like deacetylation, which involves the removal of acetyl groups, or phosphorylation, which is the addition of phosphate groups to molecules, typically proteins. Amino acid modification refers to the chemical changes that occur to amino acids in proteins, which is distinct from the epigenetic modifications of DNA. These distinctions highlight the specificity of methylation in the context of DNA modification and its implications in biological processes.