#chromatin

Basics of Cell Biology

Which of the following is a characteristic of heterochromatin?

Heterochromatin denotes highly condensed chromatin domains resistant to nuclease digestion, visible as electron-dense clumps along nuclear periphery associated with lamina and around nucleolus, comprising 10 to 20 percent mammalian genome depending on cell type. Enriched in repetitive elements including LINE-1, satellite repeats at centromeres and telomeres, and developmentally silenced genes, it carries repressive histone modifications such as H3K9 trimethylation deposited by SUV39H1 and SUV39H2, recognized and bound by hetero protein HP1 alpha that drives liquid-liquid phase separation and chromatin compaction through oligomerization, and H3K27 trimethylation for facultative heterochromatin marking developmental regulators and inactive X chromosome coated by Xist long noncoding RNA recruiting PRC2 complex with EZH2 catalytic subunit. DNA methylation by DNMT3A/B at CpG dinucleotides further locks silent state. Functionally transcription is minimal due to exclusion of RNA polymerase II and transcriptional activators, replication occurs late in S phase, high nucleosome density, suppressing transposon mobilization and illegitimate recombination preserving genome integrity during cell division. Euchromatin contrasts as gene-rich, loosely packed, DNase hypersensitive, early replicating. Transcriptionally inactive nature is defining characteristic distinguishing constitutive versus facultative heterochromatin types regulated developmentally and environmentally.

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 4: Heterochromatin and Transcriptional Silencing.

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Cell Organelles

Which type of chromatin is transcriptionally active?

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.

Ref: Kouzarides Cell; euchromatin acetyl H3K27ac open active, heterochromatin repressed, chromatin states.

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DNA Structure Double Helix and Packaging - Nucleosome