Chromatin Architecture, Epigenetic Modifications, and Transcriptional Regulation

Comprehensive flashcards covering chromatin organization, nucleosome architecture, post-translational histone modifications, DNA methylation, transcription factors, RNA polymerase machinery, and epigenomic technologies.

What is the physical packaging challenge of human genomic DNA within a eukaryotic nucleus?

Approximately $2\,\text{meters}$ of linear double-stranded DNA must be compacted by a factor greater than $10{,}000\text{-fold}$ to fit inside a nucleus typically measuring $5\text{--}10\,\mu\text{m}$ in diameter while maintaining dynamic, functional accessibility for replication, repair, and transcription.

What progressive structural stages of chromatin organization are shown in this diagram, starting from naked DNA to a fully condensed metaphase chromosome?

The diagram illustrates: (1) $2\text{-nm}$ DNA double helix, (2) nucleosome formation, (3) nucleosome core with $1.65$ DNA turns, (4) chromatosome with linker histone H1 ($11\text{-nm}$ wide), (5) folding into a $30\text{-nm}$ fiber, (6) looping into $300\text{-nm}$ loops, (7) compression into a $250\text{-nm}$ wide fiber forming a $700\text{-nm}$ section, and (8) tight coiling to yield the $1400\text{-nm}$ mitotic chromosome.

How do mitotic chromosomes differ morphologically and functionally from interphase chromosomes?

Mitotic chromosomes are highly condensed, visually distinct, and transcriptionally inactive to facilitate chromosome segregation, whereas interphase chromosomes decondense into less distinct, functional chromosome territories within the nucleus to allow transcription and replication.

What classic example demonstrates the C-value paradox regarding chromosome number and organismal complexity in muntjac deer?

The Chinese muntjac (Muntiacus reevesi) has $2n = 46$ chromosomes, whereas the Indian muntjac (Muntiacus muntjak) has $2n = 6$ (female) or $2n = 7$ (male) chromosomes, despite both species having nearly identical total genome sizes and organismal complexity.

According to the genomic makeup table, how do the chromosome copy number, chromosome form, and genome size differ between Escherichia coli K-12 and Homo sapiens?

Escherichia coli K-12 has $1$ circular chromosome, a copy number of $1$, and a genome size of $4.6\,\text{Mb}$, whereas Homo sapiens has $22 + \text{X and Y}$ linear chromosomes, a copy number of $2$, and a genome size of $3200\,\text{Mb}$.

What proportion of the $3{,}200\,\text{Mb}$ human genome is composed of protein-coding exons versus genome-wide repeats?

Protein-coding exons constitute only about $48\,\text{Mb}$ ($\sim 1.5\%$ of the genome), whereas genome-wide repeats (transposons) account for approximately $1{,}400\,\text{Mb}$ ($\sim 44\%$), comprising LINEs ($\sim 20\%$), SINEs ($\sim 13\%$), retroviral-like elements ($\sim 8\%$), and DNA transposon fossils ($\sim 3\%$).

What does the progressive magnification of human chromosome 22 reveal about gene distribution across a chromosome arm?

Human chromosome 22 contains $48 \times 10^6\,\text{nucleotide pairs}$; a $10\%$ segment of the chromosome arm contains $\sim 40\,\text{genes}$, a $1\%$ segment contains $4\,\text{genes}$, and a single gene spans $3.4 \times 10^4\,\text{nucleotide pairs}$ broken into regulatory sequences, exons, and non-coding introns.

What are the structural features and chemical building blocks of the DNA double helix shown in this diagram?

Each nucleotide is formed from a sugar-phosphate linked to a base ($\text{A}$, $\text{T}$, $\text{G}$, or $\text{C}$), assembling into polar strands ($5'\text{-to-}3'$) that form an antiparallel double helix stabilized by complementary base pairs ($\text{G} \equiv \text{C}$ with $3$ hydrogen bonds and $\text{A} = \text{T}$ with $2$ hydrogen bonds).

What are the standard physical helical parameters of B-DNA?

B-DNA has an outer diameter of $2.0\,\text{nm}$, a helical rise of $0.34\,\text{nm}$ per base pair, approximately $10.5\,\text{bp}$ per helical turn, and a pitch of $3.4\,\text{nm}$.

Why does sequence-specific protein binding occur predominantly in the major groove of B-DNA rather than the minor groove?

The major groove is wide and deep, displaying unique chemical recognition codes (hydrogen bond donors, acceptors, nonpolar hydrogens, and methyl groups) that distinguish all four base pairs without requiring unwinding of the helix.

What is the structural composition and symmetry of the nucleosome core particle?

The nucleosome core particle consists of an octamer containing two copies each of histones H2A, H2B, H3, and H4, around which $146\text{--}147\,\text{bp}$ of DNA is wrapped in $1.65$ left-handed superhelical turns, organized around a pseudo-twofold dyad symmetry axis.

What are the molecular weights, basic residue enrichments, and stoichiometric ratios of the core and linker histones in chromatin?

Linker H1: $23{,}000\,\text{Da}$, lysine-rich, molar ratio $1$. Core histones: H2A ($13{,}960\,\text{Da}$, slightly lysine-rich, ratio $2$), H2B ($13{,}744\,\text{Da}$, slightly lysine-rich, ratio $2$), H3 ($15{,}342\,\text{Da}$, arginine-rich, ratio $2$), and H4 ($11{,}282\,\text{Da}$, arginine-rich, ratio $2$).

How conserved are core histones across eukaryotic evolution, as exemplified by histone H4?

Core histones are among the most evolutionarily conserved proteins known; calf thymus H4 and garden pea H4 differ by only two conservative amino acid substitutions out of $102\,\text{residues}$ across more than a billion years of evolutionary divergence.

What is the structural domain conserved among all four core histones, and how does it mediate dimerization?

The histone fold, consisting of three $\alpha$-helices ($\alpha1$, $\alpha2$, $\alpha3$) separated by two short loop segments ($L1$, $L2$), mediates dimerization via a reciprocal head-to-tail "handshake" motif.

What is the stepwise in vitro assembly pathway of the nucleosome octamer onto genomic DNA?

First, an $(\text{H}3 \cdot \text{H}4)_2$ tetramer binds DNA to form a tetrasome intermediate; second, two independent $\text{H}2\text{A} \cdot \text{H}2\text{B}$ heterodimers bind sequentially to form a hexasome and then the complete octasome; finally, linker histone H1 binds the entry and exit DNA sites.

Which dinucleotide sequences favor positioning where the minor groove faces inward toward the histone octamer?

$\text{AA}$, $\text{TT}$, and $\text{TA}$ dinucleotides bend easily toward the minor groove and show a $10\text{--}11\,\text{bp}$ periodicity localized where the minor groove faces inward toward the histone core.

What are the specific histone chaperone binding partners and functions of NAP1 and CAF1?

NAP1 binds $\text{H}2\text{A}/\text{H}2\text{B}$ heterodimers to mediate histone shuttling and nucleosome assembly, whereas CAF1 coordinates replication-coupled deposition of canonical H3.1/H3.2 and H4 behind DNA replication forks.

What does micrococcal nuclease (MNase) digestion reveal when analyzed by sucrose gradient centrifugation and agarose gel electrophoresis?

Partial digestion generates an ordered nucleosomal ladder at integer multiples of $\sim 200\,\text{bp}$ ($200$, $400$, $600$, $800\,\text{bp}$) corresponding to sedimentation peaks D (mono-), C (di-), B (tri-), and A (tetra-nucleosomes), while complete digestion trims linker DNA to the core limit of $146\text{--}147\,\text{bp}$.

What is nucleosome breathing, and what is its equilibrium frequency?

Nucleosome breathing is the spontaneous, transient unwrapping of nucleosomal DNA from the histone core, which occurs at an equilibrium frequency of approximately $\sim 1/20\text{th}$ of the time to allow brief access to DNA-binding proteins.

What are the four major families of ATP-dependent chromatin remodeling complexes and their primary functions?

SWI/SNF (promotes nucleosome sliding and octamer ejection), ISWI (optimizes regular nucleosome spacing for repression), NuRD (combines remodeling with HDAC1/2-mediated deacetylation), and INO80/SWR1 (catalyzes histone variant exchange, such as depositing H2A.Z).

How does chemical acetylation of a lysine residue alter the electrostatic properties of histone tails?

Histone acetyltransferases transfer an acetyl group from Acetyl-CoA to the $\epsilon$-amino group of lysine, transforming a positively charged amine ($\text{NH}_3^+$) into an uncharged amide, which neutralizes the positive charge and weakens electrostatic attraction to DNA.

What structural motif characterizes a bromodomain, and what post-translational modification does it recognize?

A bromodomain is an antiparallel four-helix bundle ($\alpha Z$, $\alpha A$, $\alpha B$, $\alpha C$) that specifically recognizes and binds acetylated lysine residues on histone tails.

How does the small-molecule inhibitor JQ1 suppress oncogene expression in cancer cells?

JQ1 competitively binds the acetyl-lysine binding pocket of bromodomain proteins like BRD4, displacing them from hyperacetylated super-enhancers and promoter regions to downregulate oncogenes such as MYC.

What metabolic cycle provides the universal methyl donor for histone and DNA methyltransferases?

The methionine cycle, wherein methionine adenosyltransferase converts methionine to S-adenosylmethionine (SAM); methyltransferases transfer the methyl group to acceptors (generating S-adenosylhomocysteine, SAH), which is recycled via homocysteine back to methionine.

Which histone methyltransferase is the primary exception to the conserved SET domain family?

DOT1L, which targets H3K79 within the nucleosome globular core, is a non-SET domain methyltransferase.

What are the cofactor requirements and enzymatic limitations of the histone demethylase LSD1 (KDM1A)?

LSD1 requires flavin adenine dinucleotide (FAD) as an electron acceptor; it demethylates mono- and dimethylated H3K4 and H3K9 via an amine oxidation reaction, but cannot demethylate trimethylated lysines due to the absence of an unshared electron pair.

What catalytic cofactors are required by Jumonji C (JmjC) domain-containing demethylases?

JmjC domain demethylases require $\text{Fe}(\text{II})$ and $\alpha$-ketoglutarate ($\alpha\text{-KG}$) to demethylate mono-, di-, and trimethylated lysines via radical-mediated hydroxylation.

What is the trans-histone cross-talk mechanism linking H2B ubiquitination to H3 methylation?

Mono-ubiquitination of H2B at Lys120 ($\text{H2BK120ub1}$) by Rad6/Bre1 acts as an obligate prerequisite that allosterically activates Set1/COMPASS to methylate H3K4 and DOT1L to methylate H3K79; subsequent deubiquitination by Ubp8/USP22 is required for transcript elongation.

How do chromodomains and bromodomains differentially affect chromatin state in response to histone H3 modifications?

Chromodomains (CD) bind methylated lysines (e.g., HP1 binding H3K9me) to direct chromatin compaction and gene silencing, whereas bromodomains (BD) bind acetylated lysines to maintain open, transcriptionally accessible chromatin.

What is the "methyl-phospho switch" mechanism during mitosis involving H3S10 phosphorylation?

Aurora B kinase phosphorylates Ser10 on histone H3 ($\text{H3S10ph}$), which introduces bulk and negative charge next to Lys9, directly evicting Heterochromatin Protein 1 (HP1) from neighboring $\text{H3K9me3}$ during mitotic chromosome condensation.

What domain architecture is shared among Aurora kinases A, B, and C shown in this diagram?

All three Aurora kinases share an $\text{N}$-terminal domain containing a KEN motif and a D-box activating motif (absent or truncated in Aurora C), a central catalytic kinase domain, and a $\text{C}$-terminal domain carrying a destruction box (D-box with consensus $\text{PxxRxxL}$).

How does phosphorylation of linker histone H1 affect chromatin structure according to this pathway?

Phosphorylation ($\text{Ph}$) of histone H1 disrupts its interaction with Heterochromatin Protein 1 (HP1), causing HP1 release and driving the transition from closed chromatin to open, transcriptionally competent chromatin.

What modification occurs on histone variant H2AX in response to DNA double-strand breaks?

ATM and ATR kinases rapidly phosphorylate H2AX at Ser139 within its $\text{C}$-terminal SQEY motif to produce $\gamma\text{-H2AX}$, which directly recruits DNA repair enzymes (DRE) and checkpoint proteins to the damage site.

Where are histone variants H2A.Z and macroH2A localized within eukaryotic genomes, and what are their functional roles?

H2A.Z is enriched at $-1$ and $+1$ nucleosomes flanking active promoters to facilitate Pol II passage, whereas macroH2A carries a large non-histone domain ($30\,\text{kDa}$) and is enriched on the inactive X chromosome ($\text{X}_i$) in females to maintain stable silencing.

What are the chaperone requirements and genomic locations of the H3 variants H3.3 and CENP-A?

H3.3 is deposited replication-independently by HIRA at active gene bodies/promoters and by DAXX/ATRX at telomeres and pericentric heterochromatin; CENP-A is deposited replication-independently by HJURP exclusively at centromeres to assemble kinetochores.

What combination of histone modifications defines a bivalent promoter in embryonic stem cells?

The simultaneous presence of the activating mark $\text{H3K4me3}$ and the repressive Polycomb mark $\text{H3K27me3}$, which keeps developmental genes silent but poised for rapid activation upon differentiation.

What are the two competing biophysical models for the 30-nm chromatin fiber?

The One-Start Solenoid model (a continuous, hollow cylindrical helix containing $\sim 6\,\text{nucleosomes/turn}$ with an $\sim 11\,\text{nm}$ central lumen) and the Two-Start Zigzag model (nucleosomes alternate in a straight-linker configuration where alternate nucleosomes $n$ and $n+2$ make closer contact than adjacent ones).

What electron microscopy experiment demonstrated salt-dependent superstructures of chromatin in vitro?

Thoma, Koller, and Klug showed that increasing monovalent or divalent salt concentrations shields negative DNA backbone charges, transitioning chromatin from an open "beads-on-a-string" configuration into a compacted $30\text{-nm}$ fiber dependent on histone H1.

How is heterochromatic H3K9me propagated and maintained following DNA replication?

Parental nucleosomes carrying $\text{H3K9me}$ partition to daughter DNA strands; Heterochromatin Protein 1 (HP1) binds $\text{H3K9me}$ via its chromodomain and recruits histone methyltransferase (HMT/SUV39H1) via its chromoshadow domain, which methylates adjacent newly deposited nucleosomes.

How are chromosome territories arranged within the interphase eukaryotic nucleus?

Individual chromosomes do not intermingle randomly; each occupies an anatomically distinct, discrete territory, with euchromatic active compartments (A) residing toward the nuclear interior and heterochromatic inactive compartments (B, LADs) localized at the nuclear periphery.

How do CTCF and the Cohesin complex establish Topologically Associating Domains (TADs)?

Cohesin actively extrudes DNA loops bidirectionally until it stalls upon encountering two CTCF proteins bound to DNA motifs in a convergent orientation ($\rightarrow \dots \leftarrow$), forming a stable structural boundary.

Why is the vertebrate genome depleted of approximately 75% of its expected non-island CpG dinucleotides?

Spontaneous hydrolytic deamination of unmethylated cytidine yields uridine, which is recognized and excised by uracil DNA glycosylase (UDG), whereas deamination of 5-methylcytidine yields thymidine, forming a $G \cdot T$ mismatch that is poorly repaired, driving irreversible $C \rightarrow T$ transition mutations over evolutionary time.

What is the biochemical distinction between de novo and maintenance DNA methyltransferases?

DNMT3A and DNMT3B are de novo methyltransferases that methylate previously unmodified CpG dinucleotides during development, whereas DNMT1 is a maintenance methyltransferase that specifically targets hemi-methylated CpG sites at replication forks to copy methylation to daughter strands.

What are the sequential oxidation steps catalyzed by TET dioxygenases during active DNA demethylation?

TET enzymes iteratively oxidize 5-methylcytosine ($\text{5mC}$) into 5-hydroxymethylcytosine ($\text{5hmC}$), then 5-formylcytosine ($\text{5fC}$), and finally 5-carboxylcytosine ($\text{5caC}$); $\text{5fC}$ and $\text{5caC}$ are subsequently excised by Thymine DNA Glycosylase (TDG) and repaired with unmodified cytosine via base excision repair.

How do histone H3K9 methylation and DNA methylation reinforce one another to produce closed chromatin?

Methylation of H3K9 creates a binding site for HP1, which directly recruits DNA methyltransferase (Dnmt); Dnmt methylates flanking CpG dinucleotides, establishing a self-reinforcing repressive loop that drives higher-order chromatin coiling.

What are the structural correspondences between the eukaryotic RNA polymerases (Pol I, Pol II, Pol III) and the prokaryotic core RNA polymerase?

The largest eukaryotic subunits (such as Rpb1 and Rpb2 in Pol II) correspond to bacterial $\beta'$ and $\beta$; Rpb3 and Rpb11 correspond to the bacterial $\alpha$ dimer ($\alpha_2$); and Rpb6 corresponds to the bacterial $\omega$ subunit.

What are the differential sensitivities of eukaryotic RNA Polymerases I, II, and III to alpha-amanitin?

RNA Polymerase I is completely insensitive to $\alpha$-amanitin; RNA Polymerase II is highly sensitive ($\text{IC}_{50} \approx 0.02\,\mu\text{g/mL}$); and RNA Polymerase III is moderately sensitive ($\text{IC}_{50} \approx 10\text{--}20\,\mu\text{g/mL}$ in mammals).

What mechanical functions do the Clamp and Wall domains perform in RNA Polymerase II?

The Clamp swings shut over downstream double-stranded DNA to lock the template strand inside the active cleft, while the rigid Wall forces the DNA template strand to bend sharply by roughly $90^\circ$, positioning the template base into the catalytic center.

What are the physical dimensions of the transcription bubble and DNA-RNA hybrid within elongating RNA Polymerase II?

The transcription bubble consists of $13\text{--}14\,\text{bp}$ of melted DNA, within which an $8\text{--}9\,\text{bp}$ DNA-RNA hybrid is maintained before the nascent RNA transcript is separated and extruded through the RNA exit channel.

What is the consensus heptapeptide repeat of the Pol II CTD, and how many repeats exist in yeast versus mammals?

The consensus repeat is $\text{Tyr}_1\text{-Ser}_2\text{-Pro}_3\text{-Thr}_4\text{-Ser}_5\text{-Pro}_6\text{-Ser}_7$ ($\text{YSPTSPS}$); yeast Rpb1 carries $26\,\text{repeats}$, whereas mammalian Rpb1 carries $52\,\text{repeats}$.

How does TATA-binding protein (TBP) function as a universal transcription factor across all three nuclear RNA polymerases?

TBP acts as the central initiation organizing subunit by assembling into distinct multi-protein initiation complexes: SL1 for RNA Polymerase I, TFIID for RNA Polymerase II, and TFIIIB for RNA Polymerase III.

How do the phosphorylation states of Ser5 and Ser2 on the Pol II CTD regulate the transcription cycle?

Recruitment of TFIIH drives Ser5 phosphorylation ($\text{S5ph}$) by CDK7 to trigger transcriptional initiation, promoter escape, and $5'$ capping; after promoter pausing, recruitment of P-TEFb drives Ser2 phosphorylation ($\text{S2ph}$) by CDK9, stimulating transcriptional elongation, splicing factor recruitment, and Set2 binding.

What are the sequence consensus and relative promoter positions of the TATA box, Initiator (Inr), and DPE?

TATA box: $\text{TATAWAAR}$ ($\text{W} = \text{A/T}$, $\text{R} = \text{A/G}$), located at positions $-30$ to $-25$; Initiator (Inr): $\text{YYANWYY}$ ($\text{Y} = \text{C/T}$), overlapping the $+1$ start site; Downstream Promoter Element (DPE): located at positions $+28$ to $+32$ relative to $+1$.

What structural role does the TFIIB "B-finger" play during pre-initiation complex assembly?

The B-finger of TFIIB inserts directly into the active-site cleft of RNA Polymerase II, contacting the template strand and positioning it precisely over the catalytic magnesium ion for accurate start-site selection.

What are the subunits and kinase targets of the P-TEFb complex during pause release?

P-TEFb consists of CDK9 and Cyclin T1; it phosphorylates Ser2 of the Pol II CTD, NELF (triggering its dissociation), and SPT5 of DSIF (at Ser666 and Thr806), converting DSIF into a positive elongation factor.

How does the FACT complex overcome the nucleosomal barrier during transcription elongation?

FACT (Spt16 and SSRP1) temporarily dislodges an $\text{H}2\text{A} \cdot \text{H}2\text{B}$ heterodimer from the nucleosome octamer ahead of the advancing Pol II, forming a hexasome, and redeposits the dimer immediately behind the transcribing polymerase.

How does the INTAC (Integrator-PP2A) complex negatively regulate transcriptional elongation?

INTS6 and INTS8 recruit the PP2A catalytic core to dephosphorylate Ser2 of the Pol II CTD and Thr806/Ser666 of SPT5, directly counteracting P-TEFb activity and triggering termination of non-productive transcription complexes.

How does the allosteric model explain transcription termination by RNA Polymerase II?

Upon transcribing the polyadenylation signal ($\text{AAUAAA}$), Pol II transfers polyadenylation factors (AT) to the nascent RNA, inducing a conformational change in the polymerase active site that drastically reduces processivity and drives dissociation from DNA.

How does the torpedo model describe Pol II termination following cleavage at the poly(A) site?

Following endonucleolytic cleavage at the poly(A) site, the uncapped $5'$ end of the remaining nascent transcript is attacked by the $5'\text{-to-}3'$ exonuclease XRN2 (Rat1), which degrades the RNA faster than Pol II transcribes, collides with the polymerase, and evicts it from the DNA template.

How does liquid-liquid phase separation (LLPS) govern the transition of Pol II from initiation to elongation?

Unphosphorylated Pol II partitions into dynamic promoter condensates formed by Mediator, p300/CBP, and transcription factors; sequential CTD phosphorylation at Ser5 and Ser2 changes its physical properties, evicting Pol II from promoter condensates and translocating it into gene-body condensates containing splicing and elongation machinery.

What structural and operational criteria define super-enhancers?

Super-enhancers are broad clusters of individual enhancers spanning large genomic intervals ($> 12.5\,\text{kb}$, often $> 50\,\text{kb}$) characterized by exceptionally high densities of master transcription factors, Mediator (MED1), BRD4, and $\text{H3K27ac}$, driving high expression of cell identity genes.

What two distinct activities are executed by insulator elements?

Enhancer-blocking activity (preventing an enhancer from activating a promoter when placed between them) and barrier activity (preventing the linear spread of heterochromatin into euchromatic regions).

How does the human beta-globin Locus Control Region (LCR) activate developmental globin gene expression?

Located $10\text{--}50\,\text{kb}$ upstream, the LCR (containing 5'HS1 to 5'HS5) loops across distance to activate individual globin genes sequentially during erythropoiesis ($\epsilon$ in embryo, $\text{G}\gamma$ and $\text{A}\gamma$ in fetus, $\delta$ and $\beta$ in adult), while flanking insulator elements (I) isolate neighboring folate and odorant receptor genes.

How do glucose and lactose levels combinatorially regulate the lac operon via LacI and CAP?

In the presence of both lactose and glucose, allolactose evicts LacI, but low cAMP prevents CAP binding, resulting in low/basal transcription; in the presence of lactose and absence of glucose, elevated cAMP enables the cAMP-CAP dimer to bind upstream of the promoter, driving high-level transcription.

What does this sequence logo illustrate about the binding motif preferences of the Nanog transcription factor?

It illustrates the information content (in bits) across the Nanog cis-regulatory sequence, showing invariant high conservation for an adenine doublet ($\text{AA}$) flanked by thymine ($\text{T}$) and guanine/cytosine bases.

How does the Helix-Turn-Helix (HTH) structural motif mediate sequence-specific DNA binding?

The HTH motif contains two $\alpha$-helices separated by a fixed turn; the $\text{C}$-terminal recognition helix inserts directly into the major groove of B-DNA, typically binding as a dimer spaced $3.4\,\text{nm}$ apart (one helical turn).

What is the consensus zinc-coordinating sequence motif of classical Cys2His2 zinc finger proteins?

$\text{C-X}_2\text{-C-X}_{12}\text{-H-X}_3\text{-H}$, wherein two cysteines in a $\beta$-sheet and two histidines in an $\alpha$-helix tetrahedrally coordinate a single $\text{Zn}^{2+}$ ion.

What spacing rule dictates hormone response element (HRE) binding by nuclear receptor heterodimers?

Direct repeats of $\text{AGGTCA}$ separated by $1\,\text{bp}$ (DR1) bind RXR/RXR homodimers; $2\,\text{bp}$ (DR2) and $5\,\text{bp}$ (DR5) bind RAR/RXR; $3\,\text{bp}$ (DR3) binds VDR/RXR; and $4\,\text{bp}$ (DR4) binds TR/RXR.

What enables pioneer transcription factors like FOXA1 and SOX2 to bind condensed heterochromatin?

Pioneer factors possess structures that mimic linker histones or bind exposed outer DNA faces of nucleosomes without requiring unwrapping, establishing high-affinity contacts with histone surfaces, displacing linker H1, and recruiting SWI/SNF or p300/CBP to open chromatin.

How does the long non-coding RNA HOTAIR function as a molecular scaffold for epigenetic silencing?

Transcribed from the HOXC cluster, HOTAIR uses its $5'$ domain to bind PRC2 (which deposits repressive $\text{H3K27me3}$) and its $3'$ domain to bind the LSD1/CoREST complex (which demethylates active $\text{H3K4}$), bridging the two complexes to silence the distant HOXD cluster in trans.

What genetic lesions in the TFIIH complex cause Trichothiodystrophy (TTD) and Xeroderma Pigmentosum (XP)?

Point mutations in the XPB and XPD ATP-dependent DNA helicase subunits of TFIIH, which impair both nucleotide excision repair and Pol II transcription initiation.

What is the molecular pathology underlying Rett Syndrome?

Loss-of-function mutations in the gene encoding MECP2 (methyl-CpG-binding protein 2), preventing normal binding to methylated DNA and recruitment of HDAC-containing corepressor complexes in neurons.

What enzymatic steps comprise the CUT&RUN methodology for mapping in vivo protein-DNA interactions?

Nuclei immobilized on Concanavalin A beads are incubated with a primary antibody, followed by a Protein A-micrococcal nuclease (pA-MNase) fusion; addition of $\text{Ca}^{2+}$ activates MNase to cleave chromatin specifically around target protein sites, releasing protected fragments into the supernatant for sequencing.

What is the biochemical principle of MeDIP-seq shown in this workflow?

Sonicated single-stranded genomic DNA fragments are incubated with a monoclonal antibody specifically recognizing 5-methylcytosine ($\text{5mC}$); the bound methylated fragments are selectively immunoprecipitated and sequenced by high-throughput DNA sequencing.

What happens to unmodified cytosine versus 5-methylcytosine during sodium bisulfite genomic sequencing?

Sodium bisulfite treatment deaminates unmodified cytosine to uracil (which amplifies as thymine, $\text{T}$, in PCR), whereas 5-methylcytosine ($\text{5mC}$) and 5-hydroxymethylcytosine ($\text{5hmC}$) are resistant to deamination and amplify as cytosine ($\text{C}$).

How does the isoschizomer pair HpaII and MspI distinguish DNA methylation at the sequence 5'-CCGG-3'?

Both enzymes recognize $\text{5'-CCGG-3'}$, but HpaII is methylation-sensitive and blocked when the internal cytosine is methylated ($\text{5'-C(5mC)GG-3'}$), whereas MspI cleaves regardless of cytosine methylation.

What is the difference in genomic resolution and throughput between 3C, 4C, 5C, and Hi-C technologies?

3C tests a single "one-to-one" pairwise interaction using specific PCR primers; 4C tests "one-to-all" genome-wide interactions from a single viewpoint via inverse PCR; 5C tests "many-to-many" pairwise contacts within a locus ($< 1\text{--}2\,\text{Mb}$); and Hi-C maps "all-to-all" interactions genome-wide using biotinylation and high-throughput sequencing.

How does an Electrophoretic Mobility Shift Assay (EMSA) verify sequence-specific protein binding via a supershift?

Protein-DNA complexes migrate more slowly through a native polyacrylamide gel than free probe; adding an antibody against the specific transcription factor creates an even higher molecular weight complex that migrates slower still (a "supershift"), confirming factor identity.