Practice question

Interphase chromatin organization reflects transcriptional competence. Lightly staining euchromatin appears dispersed in electron micrographs, shows high sensitivity to DNase I and micrococcal nuclease, replicates early S phase, enriched in acetylated histones H3K27ac, H3K9ac, H3K4 trimethylation marking active promoters, H3K36 trimethylation in gene bodies, and less condensation allowing enhancer promoter looping mediated by Mediator and cohesin. RNA polymerase II and bromodomain proteins readily access DNA. Heterochromatin contrasts as dark dense clusters peripherally near lamina and around nucleolus, replicates late, enriched in H3K9 trimethylation, H4K20 trimethylation, HP1 alpha binding and DNA methylation repressing transcription by compact fiber folding and phase separation excluding activators. Nuclear lamina is protein scaffold not chromatin state. ATAC sequencing reveals euchromatin as open peaks. Developmental regulation illustrates facultative heterochromatin such as inactive X converting to euchromatin upon reprogramming, demonstrating reversible epigenetic switch that controls genome expression capacity. Integration with cell cycle kinases, calcium signaling and mechanical cues ensures coordinated remodeling during growth, migration and differentiation.