Which chromatin modifications are associated with active transcription?

• A. No histone modifications.
• B. Histone acetylation (e.g., H3K27ac) and H3K4me3; DNA hypomethylation at promoters also correlates with activity.
• C. DNA hypermethylation at promoters and H3K9me3.
• D. Acetylation of DNA.

Answer: (B)

Active transcription happens when chromatin is open and accessible to the transcription machinery. Histone acetylation, such as H3K27ac, loosens the interaction between histones and DNA and helps recruit proteins needed to start transcription. H3K4me3 is a mark found at active promoters and supports the assembly of the transcription initiation complex. DNA being hypomethylated at promoters also supports activity because unmethylated cytosines allow transcription factors to bind and prevent recruitment of repressive complexes.

In contrast, marks like H3K9me3 are associated with repressive, compact chromatin, and DNA hypermethylation at promoters tends to silence genes. Acetylation of DNA isn’t a recognized chromatin modification. So the combination of histone acetylation and H3K4me3 together with promoter hypomethylation best reflects the state linked to active transcription.