The introduction of an acetyl group (CH3CO–) into a compound. The acetylation of coenzyme A to acetyl coenzyme A is an important stage in the Krebs cycle: the acetyl group is derived from pyruvate after the removal of a molecule of carbon dioxide and two hydrogen atoms. Acetylation of histone proteins in chromatin loosens the tightly packed nucleosomes, thereby making the DNA of the chromosomes accessible for gene transcription. See chromatin silencing.