Practice question

Polycomb group proteins maintain developmental regulators silenced through epigenetic memory. PRC2 complex containing EZH2, EED, SUZ12 catalyzes H3K27 trimethylation, PRC1 containing BMI1, RING1B ubiquitinates histone H2A at lysine 119 and compacts chromatin, blocking SWI/SNF remodeling and preventing RNA polymerase II elongation. Targets include HOX clusters and lineage transcription factors. Compaction renders chromatin inaccessible. Polycomb loss results in ectopic gene activation and homeotic transformations. Unlike constitutive activators, Polycomb establishes facultative heterochromatin reversible during differentiation and reprogramming processes, ensuring cell identity maintenance.