Genome Maintenance and Regulation: Transcription Factors, Chromatin Organisation, and Developmental Cascades

Comprehensive vocabulary flashcards covering general transcription factors, RNA polymerase II initiation, chromatin and nucleosome architecture, histone post-translational modifications, and morphogen-driven developmental cascades.

Totipotent Stem Cell

A fertilized cell capable of giving rise to all differentiated cell types and lineages in an organism, establishing the embryo and extraembryonic tissues.

TATA Box

A conserved core promoter DNA sequence located approximately $25\,\text{bps}$ upstream of the transcription initiation site that is recognized and bound by TBP.

TFIID

A general transcription factor complex composed of TATA-binding protein (TBP) and TBP-associated factors (TAFs) that induces an approximate $90^\circ$ bend in DNA upon binding.

TFIIF

A general transcription factor that binds RNA polymerase II, prevents it from binding non-specifically or randomly across the genome, and recruits TFIIE and TFIIH to the pre-initiation complex.

TFIIH

A multi-subunit general transcription factor that unwinds promoter DNA and phosphorylates serine at position 5 of the RNA polymerase II CTD heptapeptide repeat to stimulate promoter clearance.

C-Terminal Domain (CTD)

A repeating 7-amino-acid sequence ($\text{Tyr-Ser-Pro-Thr-Ser-Pro-Ser}$ or $\text{YSPTSPS}$) on the largest subunit of RNA polymerase II whose phosphorylation at serine 5 by TFIIH activates transcription and mediates promoter clearance.

Repairosome

A multiprotein nucleotide excision repair complex formed when repair proteins associate with TFIIH; inherited molecular defects in this machinery cause Xeroderma Pigmentosum.

Zinc Finger Domain

A DNA-binding structural domain stabilized by zinc ions, grouped into Class 1 ($\text{C}_2\text{-H}_2$, e.g., TFIIIA), Class 2 ($\text{C}_2\text{-C}_2$ or $\text{C}_4$, e.g., steroid receptors), and Class 3 ($\text{C}_6$, e.g., GAL4).

bHLH (Basic Helix-Loop-Helix)

A dimeric transcription factor motif featuring basic DNA-contact residues and two amphipathic $\alpha$-helices separated by a loop; binds the canonical E-box consensus sequence $\text{5'-CAnnTG-3'}$.

E-box

The specific hexanucleotide DNA target sequence $\text{5'-CAnnTG-3'}$ recognized by basic helix-loop-helix (bHLH) transcription factors such as MyoD-E47 and Atoh1-E47 heterodimers.

Transcriptional Activation Domains

Modular protein regions of transcription factors that interact with general transcription factors or TAFs, categorized by sequence composition into acidic (Asp [D] and Glu [E] rich), glutamine-rich (Gln [Q]), and proline-rich (Pro [P]) classes.

Nucleosome Core Particle

The fundamental structural subunit of chromatin, comprising 2 turns of DNA wrapped around an octameric histone core made of an $\text{H3-H4}$ tetramer and two $\text{H2A-H2B}$ heterodimers.

Histone H1

A linker histone that binds to DNA on the exterior of the nucleosome core particle, stabilizing nucleosome packaging and facilitating folding into the $30\,\text{nm}$ chromatin fiber.

Histone Fold Domain

A conserved tertiary protein motif present in core histones consisting of three $\alpha$-helices linked by two loops, mediating the heterodimerization of histone pairs.

Histone Acetyltransferase (HAT)

An enzyme (e.g., CLOCK, CBP, p300, PCAF) that transfers acetyl groups to lysine residues on histone tails ($\text{NH}_3^+ \rightarrow \text{NHCOCH}_3$), neutralizing positive charges to relax chromatin and stimulate transcription.

Bromodomain

A specialized protein recognition motif (found in regulatory proteins such as PCAF and hTAF1) that specifically binds acetylated lysine residues on histone tails.

Chromodomain

A protein structural module (found in factors like Heterochromatin Protein 1 [HP1]) that binds methylated histone tails (e.g., H3K9me2/3 and H3K27me2/3) to mediate transcriptional silencing and heterochromatin packaging.

PHD Finger Domain

A structural protein motif (found in regulatory chromatin proteins like ING2 and BPTF) that specifically binds methyl-lysine marks on histone tails, such as H3K4me3 and H3K9me3.

FACT Complex (Facilitates Chromatin Transcription)

A histone chaperone complex that displaces histone dimers ahead of advancing RNA polymerase II and reassembles them onto the DNA behind the transcription machinery.

Transcription Factory

A discrete subnuclear compartment containing 5 to 10 active RNA polymerase II complexes where multiple actively transcribed genes and shared signaling pathways converge.

Chromosome Territories (CTs)

Distinct, organized 3D spatial regions occupied by individual chromosomes inside the mammalian nucleus, polarized so that euchromatin and heterochromatin occupy discrete domains.

Digital Regulation

An all-or-none transcriptional mechanism in which a master transcription factor exceeding a strict concentration threshold (e.g., $>25\,\text{nM}$) triggers a cell fate switch, often locked in by positive auto-regulation.

Analogue Regulation

A concentration-graded transcriptional response where varying master transcription factor levels above a baseline threshold (e.g., $25\,\text{nM}$, $30\,\text{nM}$, and $35\,\text{nM}$) sequentially activate distinct sets of target genes based on enhancer binding affinities.

Transcription Cascade

A developmental regulatory hierarchy in which an upstream master transcription factor directly activates primary target genes, some of which encode second-tier transcription factors that activate secondary and tertiary target genes.

Cellular Blastoderm

The early Drosophila developmental stage formed when synchronously dividing cleavage nuclei migrate to the embryo's periphery and are enclosed by invaginating cell membranes.

Pair-Rule Genes

Drosophila segmentation genes (such as even-skipped and fushi-tarazu) regulated by maternal and gap gene proteins that are transcribed in alternating transverse stripes across the 14 body segments.

Hox Genes

A master set of homeotic transcription factor genes, discovered by Edward Lewis, that determine positional identity and direct the anatomical specialization of specific body segments along the anterior-posterior axis.