Which chromatin is transcriptionally active?
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
Ref: Lodish et al., Molecular Cell Biology, 9th ed., Chapter 8: Chromatin Structure - Euchromatin vs Heterochromatin Transcriptional Activity
