Which describes how chromatin structure is modified to regulate transcription?

• A. Histone acetylation loosens chromatin and promotes transcription; DNA methylation can repress or, in some contexts, activate transcription; ATP-dependent chromatin remodelers reposition nucleosomes.
• B. DNA methylation always represses transcription; histone acetylation represses transcription; ATP-dependent remodelers degrade nucleosomes.
• C. Histone acetylation tightens chromatin and represses transcription; DNA methylation has no effect; ATP-dependent chromatin remodelers degrade nucleosomes.
• D. DNA methylation always activates transcription; histone acetylation has no effect; histone methylation is the only remodeling.

Answer: (A)

Chromatin structure regulates transcription by controlling how accessible DNA is to the transcription machinery. Histone acetylation adds acetyl groups to histone tails, neutralizing positive charges and loosening the interaction between histones and DNA. This relaxation creates more open chromatin, allowing transcription factors and RNA polymerase II to access promoters and enhancers and drive transcription.

DNA methylation at CpG sites typically acts as a repressive signal, often by recruiting methyl-binding proteins that bring in corepressors and histone deacetylases, which lead to a more compact chromatin state and reduced transcription. In some contexts, methylation can influence activation, such as by affecting the binding of certain regulatory proteins or altering enhancer activity, but the general trend at promoters is repression.

ATP-dependent chromatin remodelers use energy from ATP to reposition, slide, evict, or replace nucleosomes, changing which DNA regions are exposed or hidden. This remodeling can promote or hinder transcription factor binding and polymerase access, finely tuning transcription.

This combination—acetylation opening chromatin, methylation providing context-dependent regulation, and remodelers altering nucleosome placement—best describes how chromatin is modified to regulate transcription.