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Spec Mapping — OCR H420 Module 6.1.1 — Cellular control, content statements covering the regulation of eukaryotic transcription by transcription factors, chromatin remodelling, histone modification and DNA methylation (refer to the official OCR H420 specification document for exact wording). This lesson is the eukaryotic counterpart to the lac-operon lesson and the conceptual prerequisite for the post-transcriptional, body-plan, mutation-driven cancer, and synoptic hormonal-signalling lessons that follow.
Every cell in a multicellular organism contains the same genome, yet a muscle cell looks and behaves nothing like a neuron or a hepatocyte. The difference lies in which genes are expressed and how strongly. Regulation of gene expression at the transcriptional level is the most important and most economical point of control — it stops the cell from wasting energy making unwanted mRNA and protein. OCR A-Level Biology A specification module 6.1.1(b)(i) requires you to understand how transcription factors, chromatin remodelling and histone modification regulate eukaryotic gene expression.
The conceptual lineage matters at A-Level depth. Conrad Waddington (1942, paraphrased) coined the term epigenetics to capture the idea that the same genome could canalise development down distinct cell-fate trajectories — his metaphor of cells rolling down branching valleys of an "epigenetic landscape" is still used today. Roger Kornberg (Nobel 2006, paraphrased) reconstituted eukaryotic transcription in a test tube and showed that the basal machinery alone cannot transcribe chromatinised DNA without remodellers and modifiers. Vincent Allfrey (1964, paraphrased) showed that histone acetylation correlates with transcriptional activity. Mark Ptashne and Walter Gilbert (paraphrased) showed in bacteria that simple repressor proteins binding specific DNA sites could switch genes on or off — the principle that turned out to generalise, in much more complex form, to eukaryotes.
Key Definitions:
- Gene expression — the process by which information in a gene is used to make a functional product (usually a protein).
- Transcription factor — a protein that binds to specific DNA sequences to regulate the rate of transcription.
- Promoter — the region of DNA upstream of a gene to which RNA polymerase and general transcription factors bind.
- Enhancer — a distant regulatory sequence that increases transcription when bound by specific activators.
- Chromatin — DNA wound around histone proteins in eukaryotic nuclei.
- Histone modification — post-translational chemical changes to histones (e.g. acetylation, methylation).
- Epigenetics — heritable changes in gene expression that do not involve changes in DNA base sequence.
Gene expression can be regulated at several levels, but transcriptional control is usually the first and most decisive.
flowchart TB
A[Transcriptional control: when and how often a gene is transcribed]
A --> B[Post-transcriptional: RNA splicing, editing, stability]
B --> C[Translational: rate of ribosome binding and initiation]
C --> D[Post-translational: protein folding, modification, degradation]
Eukaryotic transcription cannot start without transcription factors (TFs). These proteins have one domain that recognises and binds a specific DNA sequence, and another that interacts with other proteins or with RNA polymerase itself.
There are two broad categories:
Oestrogen diffuses into a target cell and binds its nuclear oestrogen receptor (OR). The hormone-receptor complex is now a transcription factor: it enters the nucleus, binds to oestrogen response elements (EREs) in enhancers of target genes, and activates their transcription. This is how lipid-soluble hormones directly alter gene expression.
In eukaryotes, DNA is wound around histone octamers (2 of each of H2A, H2B, H3, H4) to form nucleosomes, which are further folded into 30 nm fibres and higher-order structures. This is called chromatin. Tightly packed chromatin (heterochromatin) is inaccessible to transcription machinery; loose, open chromatin (euchromatin) is accessible.
Chromatin-remodelling complexes are ATP-dependent machines that slide, eject or restructure nucleosomes so that the promoter becomes available for RNA polymerase and TFs. Without remodelling, transcription cannot begin — the DNA is literally hidden inside the nucleosome.
Histones have long "tails" that stick out from the nucleosome core. These tails can be chemically modified at specific residues, and different modifications have different meanings. OCR requires you to know two important types:
| Modification | Effect |
|---|---|
| Histone acetylation (e.g. H3K9ac) | Opens chromatin; activates transcription |
| Histone deacetylation | Closes chromatin; silences transcription |
| H3K4 methylation | Activates transcription |
| H3K9 or H3K27 methylation | Silences transcription |
| DNA methylation (at CpG islands) | Silences transcription |
Methylation of the cytosine bases in CpG islands (clusters of C-G dinucleotides often in promoters) recruits methyl-CpG-binding proteins that further compact chromatin and block transcription factor binding. DNA methylation is a form of epigenetic inheritance: patterns can be copied from mother to daughter cells during DNA replication and, in some cases, passed to offspring. It is the mechanism behind genomic imprinting and X-chromosome inactivation.
Epigenetic modifications (histone modifications and DNA methylation) are heritable changes in gene expression that do not change the DNA sequence itself. They explain:
flowchart TB
S[Signal: hormone, stress, developmental cue] --> TF[Activated transcription factor]
TF --> EN[Binds enhancer]
EN --> CR[Recruits chromatin remodellers and HATs]
CR --> OP[Chromatin opens, nucleosomes shift]
OP --> POL[RNA polymerase II loads on promoter]
POL --> TR[Transcription begins]
When you answer "describe how transcription is controlled" questions, structure your response in a clear sequence: signal → transcription factor → enhancer/promoter binding → chromatin change → polymerase loading → transcription. Mention named molecules where you can (HATs, HDACs, mediator complex, TFIID) to show precision.
A typical RNA Pol II promoter has several elements arrayed upstream of the transcription start site (TSS):
| Element | Position | Function |
|---|---|---|
| Enhancer | distal — can be kb away | Activator-binding; loops to contact promoter |
| Silencer | distal | Repressor-binding; reduces transcription |
| Proximal elements | ~−40 to −200 bp | Bind specific transcription factors |
| TATA box | ~−25 bp | Anchors TFIID/TBP, positions Pol II |
| +1 (TSS) | start | Where transcription begins |
The geometry is fundamentally three-dimensional: an enhancer 50 kb upstream is brought into contact with the promoter by DNA looping, mediated by the mediator complex. Without this loop, the activator cannot reach the pre-initiation complex.
Eukaryotic transcription assembles in a defined sequence:
flowchart TB
A[Signal e.g. hormone] --> B[Activated specific TF binds enhancer]
B --> C[Mediator + chromatin remodellers recruited]
C --> D[Histone acetyltransferases HATs open chromatin]
D --> E[TFIID binds TATA box via TBP subunit]
E --> F[TFIIA, TFIIB, TFIIF assemble]
F --> G[RNA Pol II recruited]
G --> H[TFIIE, TFIIH join]
H --> I[TFIIH helicase melts DNA + phosphorylates Pol II CTD]
I --> J[Promoter clearance and elongation begin]
Phosphorylation of the C-terminal domain (CTD) of RNA Pol II on Ser5 by TFIIH is the trigger for promoter clearance — a beautiful example of post-translational modification regulating a regulator.
Most signalling pathways converge on transcription factors. The general logic:
flowchart LR
H[Hormone or growth factor] --> R[Receptor]
R --> CK[Cytoplasmic kinase cascade]
CK --> TFP[Phosphorylates transcription factor]
TFP --> NL[Nuclear translocation]
NL --> EN[Binds enhancer / response element]
EN --> GA[Recruits Pol II and remodellers]
GA --> EX[Gene transcribed]
Worked example: growth factor → receptor tyrosine kinase → RAS → RAF → MEK → ERK → CREB phosphorylation → CRE binding → c-fos transcription. Five sequential phosphorylation events deliver a signal from cell surface to gene, amplified at each step. This cascade is also the canonical example of an oncogenic pathway: gain-of-function mutations in RAS (e.g. KRAS G12D in colorectal and pancreatic cancers) lock the cascade ON regardless of upstream signal, driving uncontrolled proliferation. Failure of gene regulation is one of the molecular roots of cancer.
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