What is the effect of histone acetylation on chromatin and transcription?

• A. Histone acetylation methylates histones to enhance repression.
• B. Histone acetylation replaces acetyl groups with methyl groups.
• C. Histone acetylation tightens DNA-histone interactions and reduces transcription.
• D. Histone acetylation neutralizes positive charges, loosening DNA-histone interactions, resulting in open chromatin and increased transcription.

Answer: (D)

Adding acetyl groups to histone tails changes how tightly DNA is packaged. The acetyl groups neutralize the positive charges on lysine residues, which weakens the attraction between histones and the negatively charged DNA. That loosens the nucleosome structure, making the chromatin more open (euchromatin) and accessible to transcription factors and RNA polymerase, so transcription increases. Enzymes called histone acetyltransferases carry out this modification, while histone deacetylases remove acetyl groups, tightening DNA-histone interactions and reducing transcription. This acetylation effect is distinct from methylation, which is a different modification with its own regulatory outcomes.