Practice question

Mammalian nucleus partitions chromatin into active and inactive states with distinct cytology and epigenetic marks. Euchromatin appears lightly stained, decondensed, nuclease hypersensitive and enriched in histone acetylation, H3K4 trimethylation, H3K36 methylation associated with ongoing transcription and replication early S phase, located interiorly. Heterochromatin appears electron dense, concentrates at periphery adjacent to nuclear lamina, around nucleolus and at centromeres, enriched in H3K9 trimethylation, H4K20 trimethylation, HP1 binding and DNA methylation repressing transcription by limiting accessibility of RNA polymerase II and transcription factors. Facultative heterochromatin such as inactive X chromosome marked by Xist and H3K27 trimethylation can switch to euchromatin under developmental cues, while constitutive heterochromatin containing repetitive satellites remains silent. Nucleoplasm describes soluble nuclear compartment, lamins are intermediate filament proteins forming structural mesh not chromatin type. ATAC-seq and DNase-seq distinguish open euchromatin from closed heterochromatin genome-wide. Integration with cell cycle kinases, calcium signaling and mechanical cues ensures coordinated remodeling during growth, migration and differentiation.