How does chromatin structure affect gene expression?

Chromatin structure regulates gene accessibility. Tightly packed heterochromatin is transcriptionally silent, while open euchromatin allows active transcription. Histone modifications play key roles: acetylation (by HATs) generally activates genes by loosening chromatin, while deacetylation (by HDACs) represses. Methylation effects depend on location - H3K4me3 activates, H3K27me3 represses. Chromatin remodeling complexes (SWI/SNF, ISWI) use ATP to reposition nucleosomes, exposing or hiding regulatory sequences. DNA methylation at CpG islands typically silences genes. This epigenetic regulation is heritable and crucial for development and disease.