Practice question

Transcriptional competence directly depends on chromatin accessibility. Euchromatin appears lightly stained, decondensed, internal nuclear localization and early replicating in S phase. Its open nucleosome arrays allow RNA polymerase II, general transcription factors and ATP-dependent remodelers like SWI/SNF to access promoters and enhancers. Heterochromatin including centromeric satellite blocks and inactive X Barr bodies remains highly condensed with H3K9 trimethylation, HP1 binding and DNA methylation blocking initiation. Therefore euchromatin maintains active transcription through histone acetylation and H3K4 trimethylation at promoter regions. Open state enables rapid induction.