Euchromatin and Heterochromatin: Implications for DNA Accessibility and Transcription

The organization and dynamics of chromatin play a fundamental role in transcriptional regulation. In higher eukaryotic cells, genomic DNA is wrapped around histones to form nucleosomes, which are irregularly folded into condensed chromatin domains ranging from euchromatin to heterochromatin. These domains have distinct functions and physical differences that impact chromatin accessibility. Recently, replication-dependent histone (Repli-Histo) labeling was developed to specifically mark nucleosomes in euchromatin and heterochromatin based on DNA replication timing. Using this technique, nucleosome behavior was revealed across four chromatin classes, from euchromatin to heterochromatin. Interestingly, nucleosome motion is progressively constrained along this spectrum. Nucleosomes in euchromatin domains fluctuate more like a liquid, whereas those in heterochromatin are highly constrained, resembling a gel-like state. How these physical properties relate to DNA/chromatin accessibility and genome function remains an important question. In this review, we discuss not only the physical differences between euchromatin and heterochromatin but also how nucleosome behavior influences the physical access of proteins to their DNA targets within chromatin, which underlies the initiation of essential DNA transaction processes, including transcription.