#epigenetics

Stem Cell -ll

Which type of chromatin modification is associated with stem cell differentiation?

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.

Ref: Bernstein et al., Cell 2006; Spivakov & Fisher, Nat Rev Genet 2007: Chromatin modifications differentiation bivalency.

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Stem Cell -lll

Which of the following chromatin modifications is associated with differentiation?

Differentiation involves stable shutdown of stemness genes and activation of lineage-specific programs through coordinated chromatin remodeling processes. Histone acetylation by p300 and CBP opens embryonic enhancers to allow transcription, while subsequent deacetylation and H3K27 trimethylation via PRC2 polycomb complex closes pluripotency loci. DNA methylation by DNMT3A and DNMT3B at Oct4 and Nanog promoters locks commitment irreversibly. Heterochromatin protein 1-mediated heterochromatin formation silences repetitive elements and alternative lineages. All these modifications including acetylation changes, methylation patterning, and repressive compaction collectively drive progressive developmental restriction effectively.

Ref: Bernstein et al., Cell 2006; Meshorer & Misteli, Nat Rev Mol Cell Biol 2006: Chromatin modifications differentiation.

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Stem Cell -lll

Which of the following defines epigenetic regulation in stem cells?

Epigenetic regulation in stem cells involves heritable changes in gene activity without altering DNA sequence, primarily through chromatin remodeling, histone modifications, and DNA methylation dynamics. Bivalent domains bearing both H3K4me3 activating and H3K27me3 repressive marks keep developmental promoters poised in embryonic stem cells for rapid activation. Upon lineage commitment, Polycomb repressive complexes resolve bivalency, DNA methyltransferases silence pluripotency loci like Oct4 promoter, and histone deacetylases condense chromatin, restricting potency toward multipotent and unipotent states while preserving genomic integrity and preventing aberrant transposition.

Ref: Allis et al., Epigenetics, 2nd ed.; Gilbert, Chapter 6: Chromatin remodeling, bivalent domains and DNA methylation in stem cells.

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Pedigree Analysis

Genomic imprinting refers to

Genomic imprinting represents epigenetic phenomenon where allele expression depends on parental origin due to differential DNA methylation marks established during gametogenesis in sperm and egg. Imprinted genes carry methylation imprint silencing one parental copy permanently in somatic cells, so only maternal or paternal allele transcribes in offspring tissues like placenta and brain. This parent-specific monoallelic expression regulates growth, metabolism, and neurodevelopment. Loss of imprinting regulation leads to dosage abnormalities causing disorders like Angelman, Prader-Willi, Beckwith-Wiedemann, highlighting importance of parental-origin-dependent expression beyond Mendelian rules.

Ref: Griffiths et al., Introduction to Genetic Analysis, 12th ed., Chapter 18: Definition of Genomic Imprinting

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Eukaryotes gene regulation

Sir2 protein mainly causes:

Sir2 founding member of sirtuin family is NAD-dependent class III histone deacetylase central to mating type and telomere silencing in Saccharomyces cerevisiae. Sir2 catalyzes deacetylation of acetyl-lysine producing deacetylated histone, nicotinamide, O-acetyl-ADP-ribose using NAD as co-substrate. Preferred substrates H4K16ac, H3K9ac, H3K14ac removal creates hypoacetylated nucleosomes high-affinity for Sir3 bromo adjacent homology domain and Sir4 binding, enabling spreading of SIR complex. Fission yeast Sir2 similarly deacetylates H3K9ac facilitating Clr4 methylation. Sir2 also deacetylates non-histone proteins linking metabolic NAD levels to epigenetic state and longevity regulation. Its activity opposed by acetyltransferases Sas2, Gcn5 establishing dynamic acetylation equilibrium controlling silencing domains.

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 4: Sir2 Histone Deacetylation and Heterochromatin Spreading

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Regulation of chromatin structure

Which protein domain interacts with unmodified histone tails?

Reader domains distinguish modification state of histone tails to translate histone code into functional outcomes. SANT domains, named after Swi3, Ada2, N-Cor, TFIIIB, are structurally related to Myb DNA-binding motifs and found in co-repressor and remodeling subunits including SMRT, Ada2 and ISWI. Biochemical studies show SANT2 of SMRT and SANT of Ada2 preferentially bind unacetylated H3 and H4 tails, increasing affinity of associated deacetylase and acetyltransferase complexes for hypoacetylated substrates. Tetracetylation of H4 disrupts this interaction, illustrating sensitivity to modification status without direct recognition of methyl or acetyl marks.

Ref: Guo et al., Nature Scientific Reports, EZH2 SANT1 Domain Reads Unmodified H4 Tail, Structural Basis

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Regulation of chromatin structure

Which histone tail modification is mainly associated with chromatin opening?

Chromatin accessibility regulated by chemical state of histone tails altering net charge. Among modifications, lysine acetylation catalyzed by HAT families GCN5, p300, Myst neutralizes epsilon-amino positive charge, weakening histone-DNA electrostatic contacts and disrupting internucleosomal H4 tail-acidic patch interaction essential for 30 nanometer fiber condensation, thereby promoting open euchromatic state. Methylation retains positive charge and can recruit repressors or activators depending context, phosphorylation adds negative charge mainly signaling mitosis or damage rather than general opening, ubiquitination adds bulky 76 amino acid protein altering stability. Therefore acetylation most directly linked to chromatin opening.

Ref: Allfrey et al., 1964; Lodish et al., Molecular Cell Biology, 9th ed., Histone Tail Modification Mainly Acetylation Opening

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Eukaryotic chromatin and chromosome

Histone acetylation causes:

Acetylation of epsilon amino groups on histone tail lysines by HAT complexes like p300, CBP and GCN5 neutralizes positive charge, weakening interaction with negatively charged phosphate backbone and loosening nucleosome wrapping around DNA gyres. Bromodomain-containing proteins specifically recognize acetyllysine and recruit ATP remodelers and polymerase associated factors. Acetylation at H4 K16 directly disrupts interaction with acidic patch on neighboring H2A-H2B dimer required for 30 nanometer fiber formation. Consequently acetylation produces open relaxed chromatin permissive for transcription, replication and repair, opposing condensation and methylation-driven silencing pathways effectively.

Ref: Allfrey et al., 1964; Alberts et al., Molecular Biology of the Cell, Chapter 4: Histone Acetylation Causes Relaxation

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Eukaryotic chromatin and chromosome

DNA methylation usually causes:

DNA methylation at 5-methylcytosine within CpG context catalyzed by DNMT1 maintenance during replication and DNMT3A/3B de novo enzymes generally correlates with stable gene silencing. Methyl-CpG recruits methyl-binding domain proteins MeCP2, MBD1, MBD2 which recruit histone deacetylase complexes and H3K9 methyltransferases, generating compact repressive chromatin impeding transcription factor access. Promoter methylation blocks initiation and maintains X inactivation and genomic imprinting. Removal via TET-mediated hydroxymethylation restores activity, confirming methylation predominantly represses transcription rather than activating replication or recombination alone in differentiated tissues.

Ref: Bird and Wolffe 1999 Methylation Mechanisms; Lodish et al., Chapter 8: DNA Methylation Usually Causes Repression

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Eukaryotic chromatin and chromosome

Polycomb proteins cause:

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.

Ref: Lewis Polycomb 1978; Lodish et al., Molecular Cell Biology, Chapter 8: Polycomb Proteins Cause Gene Repression

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Eukaryotic chromatin and chromosome

Which histone modification activates transcription?

Active promoter epigenetic signature includes specific histone methylation. H3 lysine 4 trimethylation deposited by SET1 and MLL complexes at transcription start sites recruits chromatin remodelers, histone acetyltransferases and general transcription factor TFIID via TAF3 PHD finger recognition, reducing nucleosome stability and stimulating preinitiation complex assembly and pause release. Conversely H3K9 trimethylation and H3K27 trimethylation deposited by SUV39H and Polycomb EZH2 cause repression. H4K20 trimethylation marks pericentric heterochromatin. Thus H3K4 trimethylation functions as universal epigenetic hallmark of actively transcribed euchromatic genes across eukaryotes.

Ref: Santos-Rosa et al., Nature 2002; Alberts et al., Molecular Biology of the Cell, Chapter 4: H3K4me3 Active Transcription

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Eukaryotic chromatin and chromosome

H3K9 methylation marks:

Histone code marks specific chromatin states. H3 lysine 9 di and trimethylation catalyzed by SUV39H1 and SUV39H2 methyltransferases creates high affinity binding site for heterochromatin protein 1 alpha via its chromodomain, inducing spreading, compaction and transcriptional repression. H3K9 trimethylation enriched at pericentric satellite repeats, subtelomeric repeats and retrotransposons characterizes constitutive heterochromatin, contrasting with H3K4 trimethylation at active promoters. HP1 oligomerization and interaction with DNMTs maintain domain stability. Disruption causes position effect variegation, ectopic transcription and chromosome missegregation defects compromising genome stability severely.

Ref: Bannister et al., Nature 2001; Lodish et al., Molecular Cell Biology, 9th ed., Chapter 8: H3K9 Methylation Heterochromatin Mark

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