Transcription Factors and Gene Regulation

Every nucleated cell in a human body contains the same ~20,000 protein-coding genes, yet a hepatocyte expresses albumin and cytochrome P450s while a B-lymphocyte expresses immunoglobulin heavy and light chains. The mechanism that resolves this paradox is regulated transcription: the selective activation or repression of particular genes by sequence-specific DNA-binding proteins called transcription factors. This lesson develops the molecular logic of transcriptional control in eukaryotes, contrasts it with the simpler operon framework first established in bacteria, and uses the oestrogen receptor as a canonical example of hormone-mediated gene regulation.

Spec mapping: This lesson sits in AQA 7402 Section 3.8.2 — Most of a cell's DNA is not translated. The specification expects candidates to describe how transcription of target genes can be stimulated or inhibited by specific transcription factors, to explain the role of oestrogen in initiating transcription via binding to a nuclear receptor, and to recognise that epigenetic modifications act in concert with transcription factors to determine which genes are expressed. (Refer to the official AQA specification document for exact wording.)

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Why Gene Regulation Matters

Differential gene expression is the basis of cellular differentiation, developmental patterning, the response to hormones, the immune response, the cell cycle, and most physiological adaptation. A few quantitative anchors set the scale of the problem:

• A typical mammalian cell expresses 10,000–15,000 distinct genes at any one time, out of a total of ~20,000.
• Of these, a small subset (~5%) are differentially expressed between cell types — yet that small subset accounts for the entire phenotypic distinction between, for example, a neurone and a renal tubule cell.
• The expression level of a single gene can vary across 6 orders of magnitude between tissues (e.g. β-globin is expressed at ~10⁵ molecules of mRNA per erythroblast and ~1 molecule per neurone).

The cell achieves this regulation primarily — though not exclusively — at the level of transcription initiation.

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Prokaryotic vs Eukaryotic Gene Regulation

Bacterial and eukaryotic cells solve the regulation problem with related but architecturally different machinery. Understanding the contrast is essential for A* answers.

Feature	Prokaryotes (e.g. E. coli)	Eukaryotes (e.g. human)
Genome organisation	Single circular chromosome; genes often clustered into operons (functionally related genes sharing one promoter)	Linear chromosomes packaged into chromatin; each gene typically has its own promoter
Regulatory range	Single RNA polymerase; ~3,000 transcription factors of relatively few classes	Three RNA polymerases (I, II, III); ~1,500 sequence-specific transcription factors plus general transcription factors
Compartmentation	Transcription and translation are coupled (no nuclear envelope)	Transcription occurs in the nucleus; mRNA is processed, exported, then translated
Regulatory elements	Promoter, operator (negative control), CAP site (positive control), all within ~100 bp	Promoter, plus distant enhancers and silencers that can act over tens or hundreds of kilobases
Chromatin	No nucleosomes; DNA is broadly accessible	DNA wrapped on histones; accessibility itself is regulated (see Lesson 1: Epigenetics)
Co-regulators	None required — repressors and activators bind DNA directly	Activators recruit coactivators and the mediator complex to bridge to RNA polymerase II

A key consequence: in prokaryotes a transcription factor often acts as the sole switch for a gene. In eukaryotes a typical promoter integrates inputs from dozens of transcription factors and coactivators, producing a combinatorial code that allows nuanced, context-dependent expression.

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The Eukaryotic Promoter and RNA Polymerase II

Most protein-coding genes in eukaryotes are transcribed by RNA polymerase II (Pol II). Pol II cannot recognise promoters on its own — it depends on general transcription factors (GTFs) to position it at the transcription start site.

Core promoter elements

• TATA box — a short A/T-rich sequence (consensus: TATAAA) located ~25–30 bp upstream of the transcription start site in many (though not all) Pol II genes. Bound by the TATA-binding protein (TBP), a subunit of the general transcription factor TFIID.
• Initiator element (Inr) — surrounds the transcription start site.
• Downstream promoter element (DPE) — present in TATA-less promoters.

The pre-initiation complex (PIC)

Assembly of the transcription machinery proceeds in steps:

1. TFIID (via TBP) recognises the TATA box.
2. TFIIA and TFIIB stabilise TBP-DNA binding.
3. Pol II is recruited along with TFIIF.
4. TFIIE and TFIIH complete the pre-initiation complex.
5. TFIIH has both a helicase activity (opening the DNA duplex) and a kinase activity (phosphorylating the C-terminal domain of Pol II), which together release Pol II from the promoter and allow elongation to begin.

This assembly is sufficient for low-level basal transcription. To produce the high transcription rates needed for biologically meaningful expression, additional sequence-specific transcription factors are required.

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Transcription Factor Classes: Activators vs Repressors

Key Definition: A transcription factor is a protein that binds to a specific DNA sequence and thereby regulates the rate of transcription of nearby genes. Transcription factors are classified as activators (which increase the rate of transcription) or repressors (which decrease the rate).

Structural features

Most transcription factors have at least two functional domains:

• A DNA-binding domain that recognises a specific short sequence (typically 6–12 bp). Common structural motifs include the helix-turn-helix, zinc finger, leucine zipper, and helix-loop-helix.
• A transactivation domain (for activators) or repression domain (for repressors) that interacts with other proteins of the transcription machinery — coactivators, corepressors, the mediator complex, or directly with the general transcription factors.

Activators

• Bind to enhancer sequences (which may lie hundreds of kilobases from the gene they regulate, either upstream, downstream, or within introns).
• Recruit coactivators (including histone-modifying enzymes such as histone acetyltransferases) that loosen chromatin and increase promoter accessibility.
• Recruit the mediator complex — a large multi-subunit bridge between the activator and Pol II that stabilises pre-initiation complex assembly.
• The activator–enhancer–mediator interaction is brought into physical contact with the promoter by DNA looping, which folds the chromatin so that distant elements are juxtaposed.

Repressors

• Bind to silencer sequences (or to operator-like sites overlapping the promoter).
• Recruit corepressors (including histone deacetylases) that compact chromatin and reduce accessibility.
• May physically block the binding of activators or the assembly of the pre-initiation complex.

Combinatorial control

Because each promoter is regulated by multiple activators and repressors, gene expression is set by the integrated signal from many transcription factors. This combinatorial logic is what allows ~1,500 transcription factors to specify the distinct expression patterns of ~200 cell types.

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The Oestrogen Receptor: A Model of Hormone-Mediated Transcription

Key Definition: The oestrogen receptor (ER) is an intracellular protein that binds the steroid hormone oestrogen (oestradiol) and, once bound, functions as a sequence-specific transcription factor. ER illustrates how an extracellular signal can be translated into changes in gene expression without any membrane-bound receptor or second messenger.

The activation cascade

1. Hormone diffusion. Oestrogen is a small, lipid-soluble steroid; it diffuses freely across the plasma membrane of any cell.
2. Receptor binding. Inside the cytoplasm (or nucleus), oestrogen binds with high affinity to the ER. The unliganded receptor is held inactive by chaperone proteins (notably HSP90).
3. Conformational change. Hormone binding causes the receptor to change shape: chaperones are released, a hydrophobic cleft is reorganised, and the receptor becomes capable of dimerising.
4. Dimerisation. Two ligand-bound ER monomers pair to form an active homodimer.
5. Nuclear translocation (where dimerisation has not already occurred in the nucleus) and DNA binding. The ER dimer binds to specific DNA sequences called oestrogen response elements (EREs) in the regulatory regions of target genes.
6. Coactivator recruitment. The activated ER dimer recruits coactivators (e.g. members of the SRC/p160 family) and the mediator complex. Coactivators bring histone acetyltransferase activity, loosening chromatin near the target promoter.
7. Transcription initiation. Pol II is recruited and target genes are transcribed.

Target genes and physiological consequences

Oestrogen-responsive genes include those encoding proteins involved in:

• The uterine endometrial growth and menstrual cycle.
• Mammary gland development at puberty.
• Bone density maintenance (via osteoblast activity).
• Hepatic synthesis of lipoproteins (which contributes to the cardiovascular profile changes seen at menopause).

Mermaid: oestrogen activation cascade

flowchart TD
  A["Oestrogen (steroid hormone) in blood"] --> B["Diffuses through plasma membrane (lipid-soluble)"]
  B --> C["Binds intracellular oestrogen receptor (ER)"]
  C --> D["Conformational change: chaperones released, dimerisation"]
  D --> E["ER dimer binds Oestrogen Response Element (ERE) in target gene promoter/enhancer"]
  E --> F["Coactivators + mediator complex recruited"]
  F --> G["Histone acetylation: chromatin loosens"]
  G --> H["RNA polymerase II recruited"]
  H --> I["Transcription of target genes initiated"]
  I --> J["mRNA exported and translated → physiological response"]

Synoptic link

The oestrogen receptor mechanism is examined synoptically with AQA 7402 Section 3.6.3 — hormonal control (course 7: Homeostasis). Hormones with hydrophilic receptors (e.g. adrenaline, glucagon) act at the cell surface via G-protein-coupled receptors and second messengers (cAMP). Hormones with lipid-soluble cores (steroids, thyroid hormone) act through intracellular receptors that themselves become transcription factors. Both routes converge on changes in gene expression, but they differ in speed (seconds for second messengers; tens of minutes to hours for transcription factor cascades) and persistence (transient vs sustained).

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The Lac Operon: A Prokaryotic Teaching Analogue

The AQA 7402 specification focuses on eukaryotic gene regulation, but the historical and conceptual foundation of the field is the lac operon model proposed for E. coli. The framework was developed by François Jacob and Jacques Monod in the early 1960s and remains the cleanest illustration of how a small number of regulatory elements can produce a logically coherent on/off switch in response to environmental cues. (Their framework should be paraphrased — no verbatim quotation from the original papers is required.)

Components of the lac operon

• lacZ, lacY, lacA — three structural genes encoding β-galactosidase (hydrolyses lactose to glucose + galactose), lactose permease (transports lactose into the cell), and a transacetylase respectively. The three genes share a single promoter and are co-transcribed as a polycistronic mRNA.
• Promoter (P) — Pol binding site upstream of lacZ.
• Operator (O) — a short DNA sequence overlapping the promoter, bound by the lac repressor.
• lacI gene — encodes the lac repressor protein, transcribed independently from its own promoter.
• CAP binding site — upstream of the promoter; bound by the catabolite activator protein (CAP) in the presence of cAMP.

Logic of the switch

Environmental state	Lactose present?	Glucose present?	Repressor state	CAP state	Transcription
Glucose only	No	Yes	Bound to operator	Inactive (low cAMP)	OFF
Glucose + lactose	Yes	Yes	Allolactose displaces repressor	Inactive (low cAMP)	Low/basal
Lactose only	Yes	No	Allolactose displaces repressor	Active (high cAMP, binds DNA)	Strongly ON
Neither	No	No	Bound to operator	Active (high cAMP)	OFF

• Negative control: the lac repressor binds the operator and blocks Pol from reading through into the structural genes. When lactose enters the cell, a small fraction is isomerised to allolactose, which binds the repressor allosterically, dislodging it from the operator. This is induction.
• Positive control: cAMP-CAP is required for strong transcription. When glucose is plentiful, cAMP is low and CAP cannot bind; transcription remains weak even if the repressor is off. When glucose is exhausted, cAMP rises, CAP-cAMP binds upstream of the promoter, and Pol binding is strongly enhanced. This is catabolite repression — the cell preferentially uses glucose over lactose even when both are present.

Why this matters for the AQA spec

Although AQA 7402 emphasises eukaryotic control, the lac operon model establishes three deep principles that carry directly into eukaryotic biology:

1. Repressors and activators can act on the same gene. Eukaryotic promoters integrate activator and repressor inputs in much the same way.
2. Sequence-specific DNA binding is the central molecular event in transcriptional control.
3. Conformational change in a regulatory protein (allosterically induced by a small molecule, or by hormone binding in the oestrogen-receptor case) is the typical mechanism by which environmental or developmental cues are translated into changes in DNA binding.

Exam Tip: AQA questions on the lac operon are uncommon but appear occasionally as a synoptic anchor. If asked, frame the answer as a teaching analogue: introduce the operon structure briefly, then highlight the parallels with eukaryotic control. Do not over-extend the analogy — eukaryotes do not generally use operons.

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Epigenetic Crosstalk with Transcription Factors

Transcription factors do not act on naked DNA. They act on chromatin — DNA wrapped on histones — and chromatin accessibility is itself controlled by epigenetic modifications (the subject of Lesson 1).

The relationship is bidirectional:

• Epigenetics gates transcription factors. A transcription factor cannot bind its target sequence if that sequence lies in heterochromatin or if the relevant CpG island is heavily methylated. DNA methylation of an ERE, for example, will block ER binding even when ligand-bound ER is abundant.
• Transcription factors recruit epigenetic modifiers. Activators recruit histone acetyltransferases that loosen local chromatin; repressors recruit histone deacetylases and DNA methyltransferases that compact it. Once recruited, these enzymes can establish stable epigenetic marks that persist through cell division — so a transient transcription factor signal can produce a heritable change in expression state.

This crosstalk is the molecular basis of cellular memory in differentiation: a transcription factor cascade in an embryonic precursor cell establishes a chromatin state that, once locked in by methylation and histone modification, is propagated to all the descendants of that precursor.

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Common Errors and Mark-Loss Patterns

• Confusing the promoter with the operator. The promoter is where Pol binds. The operator is where the repressor binds. In eukaryotes, there is no operator in the prokaryotic sense — repressors bind silencers or compete with activators at the promoter.
• Saying that transcription factors "make" the protein. They do not. They regulate the transcription of the gene; ribosomes make the protein.
• Treating the lac operon as if it operates in human cells. It is a prokaryotic system. Most A* answers acknowledge this explicitly when asked about eukaryotic control.
• Stating that oestrogen binds a membrane receptor. Steroid hormones bind intracellular receptors. (Adrenaline binds membrane receptors. Do not confuse the two.)
• Forgetting that activators and repressors integrate — they are not binary switches. A typical eukaryotic gene is regulated by ~10 activators and repressors simultaneously.

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A-Level-Depth Misconceptions

• "Gene regulation = transcription factor binding." No. Transcription factor binding is the primary regulatory event, but the cell also regulates at the level of mRNA stability, mRNA export, translation initiation, post-translational modification, and protein degradation. AQA emphasises transcription factors, but A* answers note the wider hierarchy.
• "Methylation silences genes by changing the DNA sequence." False. Methylation adds a methyl group to cytosine; the sequence is unchanged. The mark is reversible. This is a foundational A* point — see Lesson 1 for the full development.
• "Operons exist in eukaryotes too." With rare exceptions (some nematodes), no. Eukaryotic protein-coding genes are transcribed individually, each with its own promoter.
• "Steroid hormones act faster than peptide hormones." Generally the reverse: peptide hormones acting through second messengers produce effects in seconds; steroid hormones acting through gene expression produce effects over tens of minutes to hours.

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Going Further

• Undergraduate reading: Alberts et al., Molecular Biology of the Cell (Chapter 7 — control of gene expression). Lewin's GENES (chapters on eukaryotic transcriptional regulation). For the historical foundation, paraphrased accounts of the Jacob–Monod operon model are accessible in any standard genetics textbook.
• Interview prompt: "A drug is being developed that mimics oestrogen but binds the receptor without recruiting coactivators. What would such a drug do, and why might it be useful clinically?" (Discussion: selective oestrogen receptor modulators — SERMs — such as tamoxifen, used in oestrogen-receptor-positive breast cancer. The drug binds ER, blocks oestrogen access, but recruits corepressors instead of coactivators in breast tissue, suppressing target genes.)

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Specimen Question 1 — 6 marks (modelled on the AQA paper format)

Question: Explain how oestrogen, a steroid hormone, can lead to the transcription of specific target genes in a cell.

Mark scheme decomposition by AO:

Mark	AO	Awarded for
1	AO1	Oestrogen is lipid-soluble / diffuses through the plasma membrane
2	AO1	Binds an intracellular (cytoplasmic / nuclear) oestrogen receptor
3	AO2	Hormone binding causes conformational change / dimerisation of receptor
4	AO2	Active complex binds DNA at oestrogen response element (ERE)
5	AO1	Recruits coactivators / mediator complex / RNA polymerase II
6	AO2	Transcription of target gene increased / mRNA produced

Grade C model answer (~150 words)

Oestrogen is a steroid hormone, so it can pass through the cell membrane because it is lipid-soluble. Inside the cell it binds to its receptor. The receptor changes shape when oestrogen binds to it. The receptor and hormone together act as a transcription factor. The complex goes to the nucleus and binds to the DNA at a specific sequence. This helps RNA polymerase bind to the gene. Then transcription of the gene happens and mRNA is produced. The mRNA is then translated into a protein. This is how oestrogen controls gene expression. The protein made will then do its job in the cell, for example by causing cell growth in the uterus during the menstrual cycle. So the hormone signal becomes a change in which genes are expressed in the target cell.

Examiner commentary: Awarded approximately 4/6 (M1, M1, M1, M1). The candidate identifies the lipid-soluble diffusion, intracellular binding, change in receptor shape, and transcription of the target gene. Mark losses: no mention of the oestrogen response element by name; coactivators / mediator complex absent; dimerisation not specified. Many candidates lose marks here by treating the receptor as a generic protein rather than naming it as a sequence-specific transcription factor with a defined DNA-binding site.

Grade A* model answer (~230 words)

Oestrogen is a steroid hormone and is lipid-soluble. It therefore diffuses freely across the phospholipid bilayer of the plasma membrane and enters the cell without need for a membrane receptor. In the cytoplasm (or nucleus) it binds with high affinity to the intracellular oestrogen receptor (ER), which in its unliganded state is held in an inactive conformation by chaperone proteins (notably HSP90). Hormone binding triggers a conformational change: chaperones are released, a hydrophobic cleft is reorganised, and two ligand-bound monomers dimerise to form the active ER homodimer. This dimer translocates to (or remains in) the nucleus and binds, via its zinc-finger DNA-binding domain, to a specific sequence called the oestrogen response element (ERE) in the regulatory region of target genes.

DNA binding alone is insufficient for high-level transcription. The activated dimer recruits coactivators (notably the SRC/p160 family) and the mediator complex, which together bridge the receptor to RNA polymerase II at the promoter. Coactivators deliver histone acetyltransferase activity, loosening local chromatin and increasing promoter accessibility. The pre-initiation complex assembles and Pol II initiates transcription. The resulting mRNAs are processed, exported and translated, producing the protein products responsible for the physiological response (uterine endometrial growth, mammary gland development, bone density maintenance).

Examiner commentary: Full marks (6/6). The answer identifies the lipid-soluble diffusion, the intracellular receptor, the hormone-induced conformational change and dimerisation, the named ERE, coactivator and mediator-complex recruitment, and the transcription step. Precise vocabulary (HSP90, zinc finger, mediator complex, histone acetyltransferase) and the molecular-to-physiological closure place this firmly in the A* band.

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Specimen Question 2 — 9 marks (modelled on the AQA paper format)

Question: Discuss how transcription factors can lead to the activation of specific genes in eukaryotic cells. In your answer, refer to the role of hormonal signalling and explain how this is integrated with epigenetic regulation.

Mark scheme decomposition by AO:

Mark	AO	Awarded for
1	AO1	Transcription factor = protein that binds specific DNA sequence and regulates transcription
2	AO1	Activator binds enhancer; recruits RNA polymerase II to promoter
3	AO2	Hormone (e.g. oestrogen) is signal that activates a transcription factor
4	AO1	Steroid hormone diffuses into cell, binds intracellular receptor
5	AO2	Hormone-receptor complex acts as transcription factor at ERE
6	AO1	Coactivators / mediator complex bridge to Pol II
7	AO2	Chromatin must be accessible — histone acetylation / DNA demethylation required
8	AO2	Methylated promoters silence the gene even if TF is present
9	AO3	Synthesis: combinatorial integration of TFs + epigenetic state → cell-type-specific expression

Grade C model answer (~180 words)

Transcription factors are proteins that bind to specific bits of DNA and switch genes on or off. Activators turn genes on by helping RNA polymerase bind. Repressors stop the gene being expressed. Hormones can act through transcription factors. Oestrogen is an example. Oestrogen is a steroid so it can pass through the cell membrane. It binds to its receptor inside the cell. The receptor then changes shape and binds to a special part of the DNA. This means the gene gets transcribed and mRNA is made. So the hormone signal becomes a change in which genes are switched on. Epigenetics is also important. DNA can be methylated which switches the gene off. If a gene's promoter is methylated then transcription factors cannot bind so the gene cannot be switched on. Histones can also be modified. Acetylation makes the chromatin looser so genes can be transcribed. So transcription factors and epigenetics work together to control which genes are expressed in each cell type.

Examiner commentary: Awarded approximately 5/9 (M1, M1, M1, M1, M1). The candidate identifies the basic transcription-factor concept, the oestrogen example, the methylation effect on TF access, and acetylation of histones. Mark losses: no mention of EREs or coactivators by name; mediator complex absent; no AO3 synthesis of combinatorial integration. Many candidates lose marks here by treating transcription factors and epigenetics as independent topics rather than as integrated controls.

Grade B model answer (~240 words)

Transcription factors (TFs) are sequence-specific DNA-binding proteins that regulate the rate of transcription of target genes. Activators bind to enhancer sequences and recruit coactivators and the mediator complex, which in turn promote assembly of the RNA polymerase II pre-initiation complex at the promoter. Repressors bind silencers and recruit corepressors that compact chromatin.

Hormones act through this machinery. Oestrogen, a lipid-soluble steroid, diffuses through the plasma membrane and binds an intracellular receptor (ER). The hormone-bound ER dimerises and binds an oestrogen response element in the regulatory region of target genes, where it functions as a transcription factor by recruiting coactivators and the mediator complex.

Epigenetic state regulates whether transcription factors can access their target sequences. DNA methylation at CpG islands in a promoter blocks transcription-factor binding directly and recruits methyl-CpG-binding proteins that compact chromatin. Histone acetylation by HATs loosens chromatin and is associated with active transcription; deacetylation by HDACs is associated with silencing. So a transcription factor cannot activate a gene whose promoter is heavily methylated or whose histones are deacetylated, regardless of the abundance of the TF itself.

The combinatorial integration of multiple transcription factors, hormonal inputs, and the chromatin state of the locus produces the cell-type-specific gene-expression profile that distinguishes one differentiated cell from another.

Examiner commentary: Awarded approximately 7/9 (M1 × 7). Solid mark-scheme coverage. Mark losses: the dimerisation step of ER is not stated; the AO3 synthesis is present but compressed; no quantitative anchor for the number of TFs at a typical promoter.

Grade A* model answer (~310 words)

A transcription factor is a sequence-specific DNA-binding protein that regulates the rate of transcription of target genes. Eukaryotic transcription factors are functionally classified as activators (which increase transcription) or repressors (which decrease it). Activators bind to enhancer elements that may lie tens or hundreds of kilobases from the gene they regulate, and recruit coactivators and the mediator complex — a large multi-subunit bridge that stabilises pre-initiation complex assembly and recruits RNA polymerase II to the promoter. The activator–enhancer–mediator interaction is brought into physical contact with the promoter by DNA looping.

Hormonal signalling exploits this architecture. Oestrogen is a lipid-soluble steroid; it diffuses through the plasma membrane and binds an intracellular receptor (ER) held inactive in the cytoplasm by HSP90 chaperones. Hormone binding triggers chaperone release, conformational change, and dimerisation. The activated ER dimer binds the oestrogen response element in target gene promoters via its zinc-finger DNA-binding domain, and then recruits coactivators (SRC/p160 family) and the mediator complex — functioning, in effect, as a hormone-activated transcription factor.

Transcription factors do not act on naked DNA; they act on chromatin. Epigenetic modifications determine whether the target sequence is accessible. CpG-island methylation in the promoter blocks TF binding directly and recruits methyl-CpG-binding proteins that compact chromatin; histone deacetylation by HDACs has a similar compacting effect. Conversely, histone acetylation by HATs (often recruited by activators themselves) loosens chromatin and licenses transcription. The relationship is reciprocal: epigenetic state gates TF binding, but TFs recruit the enzymes that establish or remove epigenetic marks. This crosstalk is the molecular basis of cellular memory in differentiation.

The cell-type-specific gene-expression profile that distinguishes a hepatocyte from a neurone is therefore the integrated output of a combinatorial set of transcription factors (often dozens per gene), hormonal inputs, and the chromatin state of each locus — a regulatory logic of remarkable depth and biological flexibility.

Examiner commentary: Full marks (9/9). The candidate develops the activator/repressor framework, the oestrogen receptor case in molecular detail, and the bidirectional crosstalk between epigenetic state and transcription factor activity. The closing AO3 synthesis on combinatorial control and cellular memory is the move that distinguishes A* from B.

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Required Practical Note

AQA 7402 has no required practical specifically anchored in section 3.8. Techniques relevant to this topic (gel shift / electrophoretic mobility shift assays, chromatin immunoprecipitation, reporter gene assays) are encountered at undergraduate level. Section 3.8 connects synoptically to RP6 (chromatography) when separating protein fractions, and to gel electrophoresis (covered in Lesson 5 of this course) when sizing DNA fragments produced by restriction digest of regulatory regions.

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Synoptic Links

This lesson connects to three other sections of the AQA 7402 specification:

1. Section 3.4.2 — Transcription and translation (course 4: DNA, Genes and Inheritance). The basic mechanism of transcription (Pol II, mRNA synthesis from a DNA template) is the substrate on which transcription factors act.
2. Section 3.6.3 — Hormonal control (course 7: Homeostasis). Steroid hormones (oestrogen, testosterone, cortisol, aldosterone) act through intracellular receptors that function as transcription factors. Peptide and amine hormones act through membrane receptors and second messengers; their downstream effects often converge on phosphorylation of transcription factors (e.g. CREB phosphorylation by PKA in response to glucagon).
3. Section 3.8.2 — Epigenetics (this course, Lesson 1). Epigenetic modifications regulate the accessibility of transcription factor binding sites and recruit or exclude transcription factor complexes.

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Summary

• Eukaryotic gene regulation is dominated by sequence-specific transcription factors that bind to promoters, enhancers and silencers.
• Activators recruit coactivators and the mediator complex; repressors recruit corepressors and histone-modifying enzymes.
• The oestrogen receptor is the canonical example of a hormone-activated transcription factor: steroid diffuses in, binds intracellular receptor, the complex binds an oestrogen response element, and target genes are transcribed.
• Prokaryotic operons (lac operon) provide the conceptual foundation: a single regulatory event (repressor binding, allosteric induction by allolactose) switches a cluster of genes on or off. The eukaryotic equivalent is more elaborate but rests on the same principle of sequence-specific DNA binding.
• Transcription factors act on chromatin, not naked DNA: epigenetic state gates their access, and transcription factors recruit the enzymes that establish epigenetic marks.
• Combinatorial integration of dozens of transcription factors, hormonal inputs and chromatin state produces the cell-type-specific gene expression that distinguishes one differentiated cell type from another.

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Spec alignment: AQA 7402 Section 3.8.2; linked synoptically to 3.4.2 (transcription mechanism), 3.6.3 (hormonal control — oestrogen and other steroid hormones), 3.8.2 (epigenetics, Lesson 1 of this course). Refer to the official AQA specification document for exact wording.