Histone modification that loosens chromatin structure and enhances transcription is histone acetylation.

• A. RNA polymerase activation
• B. Histone deacetylation
• C. DNA methylation
• D. Histone acetylation

Answer: (D)

Histone acetylation loosens chromatin and promotes transcription because adding acetyl groups to lysine residues on histone tails neutralizes their positive charge. This weakens the interaction between histones and the negatively charged DNA, making the chromatin structure more open (euchromatin) and accessible to transcription factors and RNA polymerase. When chromatin is open, genes can be more readily transcribed.

Histone deacetylation would remove those acetyl groups, tightening the DNA-histone interaction and repressing transcription. DNA methylation also tends to repress transcription by promoting a closed chromatin state. RNA polymerase activation isn’t a histone modification itself; it’s the enzyme that transcribes DNA, whose access is regulated by histone modifications.