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This lesson covers how gene expression is controlled in both prokaryotes and eukaryotes, as required by the Edexcel A-Level Biology specification (9BI0, Topic 7). Understanding gene regulation explains how cells with identical DNA can have different structures and functions.
Every cell in a multicellular organism contains the same DNA (the same genome). However, a liver cell looks and functions very differently from a neuron or a muscle cell. This is because different genes are expressed (switched on) in different cell types — a process called differential gene expression.
Gene regulation ensures that:
Exam Tip: The key principle is that all cells in an organism have the same genome but express different genes. Differentiation is the result of differential gene expression, not differences in DNA content.
Gene expression can be regulated at multiple levels:
| Level | Mechanism |
|---|---|
| Chromatin remodelling | Modification of histones or DNA to alter accessibility |
| Transcriptional | Control of whether a gene is transcribed into mRNA |
| Post-transcriptional | Alternative splicing, mRNA editing, mRNA stability |
| Translational | Control of whether mRNA is translated into protein |
| Post-translational | Protein modification, activation, degradation |
The most common and most important level of regulation is transcriptional control — determining whether or not RNA polymerase transcribes a gene.
The lac operon in Escherichia coli is a classic example of gene regulation in prokaryotes. It controls the expression of genes needed to metabolise lactose.
| Component | Function |
|---|---|
| lacZ | Codes for beta-galactosidase (breaks lactose into glucose and galactose) |
| lacY | Codes for lactose permease (transports lactose into the cell) |
| lacA | Codes for transacetylase |
| Promoter | Binding site for RNA polymerase |
| Operator | Binding site for the lac repressor protein |
| lacI (regulatory gene) | Codes for the lac repressor protein (constitutively expressed) |
This is an example of inducible gene expression — the genes are normally off and are switched on by an inducer.
Exam Tip: The lac operon is a favourite exam topic. Make sure you can explain both states (lactose absent and lactose present) clearly. Use terms like inducer, repressor, operator and allosteric change.
Eukaryotic gene regulation is far more complex than prokaryotic regulation. Key mechanisms include:
Transcription factors are proteins that bind to specific DNA sequences near the promoter of a gene and either activate or repress transcription.
Steroid hormones (such as oestrogen, testosterone and cortisol) can directly regulate gene expression:
Exam Tip: Steroid hormones regulate gene expression by acting as transcription factors (indirectly, via their receptor). Peptide hormones cannot enter the cell and instead use cell-surface receptors and second messenger systems.
As discussed in the transcription lesson, alternative splicing allows one gene to produce multiple different mRNA molecules, and therefore multiple different proteins. Which exons are included depends on splicing factors present in the cell.
The lifespan of an mRNA molecule in the cytoplasm affects how much protein is produced. mRNA stability is influenced by:
Small RNA molecules called microRNAs (miRNAs) and small interfering RNAs (siRNAs) can silence gene expression post-transcriptionally:
Even if mRNA is present in the cytoplasm, translation can be regulated:
Once a protein has been synthesised, its activity can be regulated by:
| Mechanism | Example |
|---|---|
| Phosphorylation | Adding phosphate groups activates or deactivates enzymes (e.g. kinase cascades) |
| Ubiquitination | Tagging proteins with ubiquitin marks them for degradation by the proteasome |
| Proteolytic cleavage | Inactive precursor proteins (zymogens) are activated by cutting (e.g. trypsinogen → trypsin) |
| Allosteric regulation | Binding of molecules changes protein conformation and activity |
Cells communicate using signalling molecules that can alter gene expression:
This explains how external signals — such as hormones, growth factors and cytokines — can change which genes are expressed in a cell.
| Level of regulation | Example mechanism |
|---|---|
| Chromatin | Histone modification, DNA methylation |
| Transcription | Transcription factors, enhancers/silencers, lac operon |
| Post-transcription | Alternative splicing, RNA interference, mRNA stability |
| Translation | Initiation factor regulation, ribosome blocking |
| Post-translation | Phosphorylation, ubiquitination, proteolytic cleavage |
Exam Tip: When asked how a particular cell type acquires its specialised function, explain that all cells have the same DNA but different transcription factors are active in different cells, leading to different genes being transcribed and different proteins being produced.
This material sits in Edexcel 9BI0 Topic 8 (Grey Matter — Coordination, Response and Gene Technology), which expects candidates to compare prokaryotic operon-level regulation (the lac operon as the worked example: promoter, operator, structural genes lacZ/lacY/lacA, the lacI regulatory gene encoding the repressor, the inducer allolactose, and the catabolite activator protein (CAP) — cAMP system as the second, glucose-sensing layer of control) with eukaryotic regulation by transcription factors, enhancers, chromatin remodelling and alternative splicing at multiple stages of expression. Synoptic links run backwards to lesson 2 on transcription (which produces the mRNA whose synthesis is regulated here) and lesson 3 on translation (regulation can also act on mRNA stability and translation initiation); forwards to lesson 5 on epigenetics (heritable changes in expression without DNA-sequence change — DNA methylation, histone modification) and lesson 6 on gene technology (engineered promoters drive recombinant-protein expression in bacterial and mammalian systems); laterally to Topic 2 (cells) for stem cells and differentiation as stable, heritable patterns of differential gene expression, and to Topic 5 (Energy, Exercise and Coordination) for insulin/glucagon coordinating fed/fasted metabolism partly at the gene-expression level (e.g. insulin upregulates glucokinase, glucagon upregulates PEPCK for gluconeogenesis). Refer to the official Pearson Edexcel 9BI0 specification document for exact wording.
Question (8 marks):
(a) Describe how the lac operon is regulated when Escherichia coli is grown in a medium containing lactose but no glucose. (4)
(b) The same bacteria are then transferred to a medium containing both lactose and glucose. Predict and explain the change in transcription rate of the lac operon, referring to CAP and cAMP. (4)
Solution with mark scheme:
(a) M1 (AO1) — repressor inactivation. When lactose enters the cell, a small amount is converted by basal levels of β-galactosidase to allolactose, which acts as the inducer. Allolactose binds the lac repressor protein (encoded by the constitutively expressed lacI gene), causing an allosteric conformational change that lowers the repressor's affinity for the operator. The repressor releases from the operator.
A1 (AO1) — RNA polymerase access. With the operator unblocked, RNA polymerase can bind the promoter and transcribe the structural genes lacZ, lacY and lacA as a single polycistronic mRNA. The mRNA is translated to produce β-galactosidase (lacZ — hydrolyses lactose to glucose + galactose), lactose permease (lacY — imports lactose into the cell) and transacetylase (lacA).
A1 (AO1) — positive feedback / induction. More lactose enters via permease and more allolactose is generated, reinforcing repressor inactivation. This is inducible expression — genes normally off, switched on by the substrate they metabolise.
A1 (AO2) — economy. The cell does not waste resources synthesising lactose-metabolising enzymes when no lactose is present, but produces them rapidly when lactose becomes available — a textbook example of substrate induction as metabolic economy.
(b) M1 (AO1) — CAP role. The lac promoter is a weak promoter — RNA polymerase binds it poorly without help. Maximum transcription requires the catabolite activator protein (CAP) to bind a site upstream of the promoter and recruit RNA polymerase. CAP only binds DNA when complexed with cyclic AMP (cAMP).
A1 (AO2) — glucose effect on cAMP. Intracellular glucose lowers cAMP levels (via inhibition of adenylyl cyclase). When glucose is present, cAMP is low, CAP-cAMP cannot form, CAP does not bind the promoter, and RNA polymerase recruitment is poor.
A1 (AO2) — predicted result. Even though lactose is present and the repressor is inactivated, transcription of the lac operon is substantially reduced in the presence of glucose because the CAP-cAMP positive control is missing. This is catabolite repression — preferential use of glucose over lactose.
A1 (AO3.1) — synthesis. The lac operon integrates two independent signals: lactose (presence sensed via repressor) AND glucose (absence sensed via cAMP/CAP). Maximum transcription requires lactose high AND glucose low — a logical AND gate, not a simple on/off switch. This dual control is more sophisticated than the textbook "lactose switches it on" caricature.
Total: 8 marks (M2 A6).
Question (6 marks): A scientist studies a eukaryotic gene that is highly expressed in liver but silent in muscle. She finds that the DNA sequence of the gene and its promoter are identical in both tissues, but the chromatin structure and transcription factor profile differ. In liver, the promoter region is in a euchromatin state with histone H3K4 methylation; in muscle, it is in heterochromatin with H3K9 methylation. A liver-specific transcription factor (HNF4α) is present in liver but absent in muscle.
Using the data, explain how the same gene can be expressed in liver but silent in muscle, and discuss what this reveals about the multiple levels of eukaryotic gene regulation.
Mark scheme decomposition by AO:
| Mark | AO | Earned by |
|---|---|---|
| 1 | AO1.1 | Stating that all cells have identical DNA but differential expression produces tissue-specific phenotypes |
| 2 | AO1.2 | Naming chromatin remodelling, transcription factor binding, and histone modification as distinct regulatory mechanisms |
| 3 | AO2.1 | Linking euchromatin (H3K4me) to accessible chromatin allowing transcription factor and RNA polymerase access in liver |
| 4 | AO2.7 | Linking heterochromatin (H3K9me) to compacted chromatin blocking transcription factor and RNA polymerase access in muscle |
| 5 | AO3.1 | Concluding that gene expression requires BOTH permissive chromatin AND the correct tissue-specific transcription factor (HNF4α) — two layers operate together |
| 6 | AO3.2 | Synoptic — connecting tissue-specific expression to differentiation (Topic 2 stem cells) and to the heritability of expression patterns through mitosis (Topic 5 epigenetics) |
Total: 6 marks (AO1 = 2, AO2 = 2, AO3 = 2). Edexcel reliably tests eukaryotic regulation through "same DNA, different expression" prompts; candidates who name only transcription factors, ignoring chromatin state, lose AO3 marks. The mark scheme rewards layered explanations.
Lesson 2 (transcription) — what regulation acts on. Transcriptional regulation modulates whether RNA polymerase initiates transcription. In prokaryotes, repressors and activators bind operators/CAP sites near the promoter; in eukaryotes, general transcription factors (TFIID etc.) plus specific transcription factors plus enhancer-bound activators (looped to the promoter via Mediator) determine initiation rate. Without the substrate of transcription itself, regulation has nothing to regulate.
Lesson 3 (translation) — regulation also acts post-transcriptionally. mRNA stability (poly-A length, 3' UTR sequences, miRNA binding), translation initiation rate (eIF2α phosphorylation, mTOR), and protein turnover (ubiquitin–proteasome) are all regulatory layers. Transcription is the most common but not the only point of control.
Lesson 5 (epigenetics) — heritable expression states. DNA methylation (5-methylcytosine at CpG islands → silencing) and histone modifications (H3K4me/H3K27ac → active; H3K9me/H3K27me3 → silent) are stable through mitosis, transmitting expression states to daughter cells. Epigenetics is the molecular memory of differentiation.
Lesson 6 (gene technology) — engineered regulation. Recombinant-protein expression systems exploit regulated promoters: lac promoter (IPTG-inducible — IPTG is a non-hydrolysable allolactose mimic), T7 promoter (orthogonal phage RNA polymerase), CMV promoter (constitutive in mammalian cells), tetracycline-inducible (Tet-On/Off) systems. Understanding native regulation enables synthetic regulation.
Topic 2 (cells — stem cells and differentiation). Differentiation is the stable establishment of tissue-specific transcription factor networks and chromatin states. Liver, muscle and neurons share identical genomes but express different gene sets because different transcription factors (HNF4α in liver, MyoD in muscle, NeuroD in neurons) drive different chromatin landscapes. Stem cells are pluripotent because they have permissive chromatin and broad transcription factor accessibility.
Topic 5 (energy, exercise and coordination — hormonal control of metabolism). Insulin (fed state) signals through PI3K/Akt to activate transcription factors (e.g. SREBP-1c) that upregulate glycolytic and lipogenic genes (glucokinase, fatty acid synthase). Glucagon (fasted state) raises cAMP, activating CREB which upregulates gluconeogenic genes (PEPCK, glucose-6-phosphatase). The lac operon's CAP-cAMP system is the prokaryotic ancestor of eukaryotic cAMP-CREB control — a strong synoptic connection.
| AO | Typical share on regulation questions | Earned by |
|---|---|---|
| AO1 (knowledge) | 35–45% | Naming operon, promoter, operator, repressor, inducer, transcription factor, enhancer, chromatin |
| AO2 (application) | 35–50% | Predicting expression in given conditions (lactose ± glucose); explaining tissue specificity from chromatin/TF data |
| AO3 (analysis / evaluation) | 15–20% | Integrating multiple regulatory layers; arguing why same DNA gives different cell phenotypes; evaluating evidence for chromatin vs TF primacy |
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