Practice question

Stem cell differentiation is accompanied by dramatic widespread chromatin reorganization from open to restrictive configuration. Pluripotent cells exhibit globally open euchromatin enriched in activating H3K9 acetylation and H3K4 trimethylation marks, maintaining accessibility of self-renewal genes like Nanog. Upon lineage commitment, histone deacetylases remove acetyl groups, H3K27 methyltransferase EZH2 component of PRC2 deposits repressive marks, DNA methyltransferases methylate CpG islands of Oct4 and Nanog promoters, and heterochromatin protein 1 consolidates constitutive heterochromatin foci at repeats. These coordinated modifications collectively silence pluripotency program and permanently lock lineage-specific expression patterns essential for stability.