In simple terms
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Gene control
Cambridge 9700 Paper 4 — Gene control (16.3). A-Level Notes diagram-backed lesson with premium structure and live visuals.
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Regulatory gene (lacI): Located upstream of the operon, it encodes the lac repressor protein, which is constitutively expressed.
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Promoter (P): The binding site for RNA polymerase to initiate transcription of the structural genes.
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Operator (O): A segment of DNA within the promoter where the lac repressor binds.
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Structural genes (lacZ, lacY, lacA): These genes code for enzymes involved in lactose metabolism: β-galactosidase (lacZ) breaks down lactose into glucose and galactose; permease (lacY) is a membrane protein that transports lactose into the cell; and transacetylase (lacA) has a role in detoxifying by-products.
What this topic covers
The official Cambridge syllabus points this lesson works through.
- 16.3.1
Describe the differences between structural genes and regulatory genes and the differences between repressible enzymes and inducible enzymes
- 16.3.2
Explain genetic control of protein production in a prokaryote using the lac operon (knowledge of the role of cAMP is not expected)
- 16.3.3
State that transcription factors are proteins that bind to DNA and are involved in the control of gene expression in eukaryotes by decreasing or increasing the rate of transcription
- 16.3.4
Explain how gibberellin activates genes by causing the breakdown of DELLA protein repressors, which normally inhibit factors that promote transcription
Explore the concept
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Full topic notes
Formal explanation with the rigour you need for the exam.
Prokaryotic Gene Control: The Lac Operon
In prokaryotes like bacteria, genes are often organised into operons, which are groups of genes with related functions that are transcribed together. The lac operon in E. coli is the classic example, controlling the metabolism of lactose. It ensures that the bacteria only produce the enzymes needed to digest lactose when lactose is available and glucose (a preferred energy source) is scarce.
Components of the Lac Operon:
Regulatory gene (lacI): Located upstream of the operon, it encodes the lac repressor protein, which is constitutively expressed.
Promoter (P): The binding site for RNA polymerase to initiate transcription of the structural genes.
Operator (O): A segment of DNA within the promoter where the lac repressor binds.
Structural genes (lacZ, lacY, lacA): These genes code for enzymes involved in lactose metabolism: β-galactosidase (lacZ) breaks down lactose into glucose and galactose; permease (lacY) is a membrane protein that transports lactose into the cell; and transacetylase (lacA) has a role in detoxifying by-products.
Regulation of the Lac Operon (Negative Control):
Absence of lactose: The lac repressor protein, synthesised by the regulatory gene, is active and binds to the operator region. This physically blocks RNA polymerase from binding to the promoter and transcribing the structural genes. Result: No lactose-metabolising enzymes are produced.
Presence of lactose: Lactose acts as an inducer. It (or its isomer, allolactose) binds to the lac repressor protein, causing a conformational change. The repressor can no longer bind to the operator. RNA polymerase can then bind to the promoter and transcribe the structural genes. Result: Enzymes for lactose metabolism are produced.
Role of Cyclic AMP (cAMP) in Lac Operon Control (Positive Control):
Beyond just turning genes 'on' in the presence of lactose, bacteria also need to prioritise glucose when it's available. This is achieved through positive control involving cAMP and the Catabolite Activator Protein (CAP). This mechanism is also known as catabolite repression.
High glucose concentration: Inhibits the synthesis of cAMP. Low cAMP levels mean the cAMP-CAP complex does not form. RNA polymerase binds weakly to the promoter, resulting in a low level of transcription even if lactose is present.
Low glucose concentration: Leads to high cAMP levels. cAMP binds to CAP, forming the cAMP-CAP complex.
The cAMP-CAP complex binds to a specific site upstream of the lac promoter. This binding enhances the affinity of RNA polymerase for the promoter, significantly increasing the rate of transcription of the structural genes. This ensures that high levels of transcription only occur if glucose is scarce and lactose is present.
Eukaryotic Gene Control: A More Complex Picture
Eukaryotic gene control is far more complex, allowing for cell specialisation and developmental stages. Regulation can occur at multiple levels: transcriptional, post-transcriptional, translational, and post-translational.
Epigenetic Control: Histone Modification and DNA Methylation
Epigenetics refers to heritable changes in gene expression that occur without altering the underlying DNA sequence. Key mechanisms involve modifying chromatin structure, which can be loosely packed (euchromatin, associated with active genes) or tightly packed (heterochromatin, associated with inactive genes).
Histone Acetylation: Adding acetyl groups to the positively charged lysine residues on histone tails. This neutralises their positive charge, weakening the interaction between histones and the negatively charged DNA. The chromatin becomes less compact (euchromatin), making DNA more accessible to transcription factors and RNA polymerase, thus increasing gene expression.
Histone Methylation: Adding methyl groups to histones. The effect can vary; it can either activate or repress gene expression depending on the specific histone and site of methylation. Often, methylation is associated with tighter chromatin packing and reduced gene expression (heterochromatin).
DNA Methylation: Adding methyl groups (–CH₃) to cytosine bases, typically in CpG islands (regions rich in cytosine and guanine) located in promoter regions. This usually represses gene expression by either physically blocking the binding of transcription factors or by recruiting proteins that condense chromatin.
Transcriptional Control: Transcription Factors
Transcription factors are proteins that bind to specific DNA sequences, regulating the rate of gene transcription. They are crucial for orchestrating gene expression patterns.
General Transcription Factors: Essential for the binding of RNA polymerase II to the promoter region of protein-coding genes. They form part of the initiation complex.
Specific Transcription Factors (Regulatory Transcription Factors): Bind to enhancer or silencer sequences, which can be far from the promoter. They can be activators (increase transcription) or repressors (decrease transcription).
Activators: Bind to enhancers, often bending the DNA to bring the enhancer closer to the promoter. They help recruit RNA polymerase and other general transcription factors, thereby stimulating transcription.
Repressors: Bind to silencer sequences or directly interfere with activators or RNA polymerase, preventing or reducing transcription.
Post-Transcriptional Control: MicroRNA (miRNA)
MicroRNAs (miRNAs) are small, non-coding RNA molecules (typically 20-22 nucleotides long) that play a significant role in regulating gene expression after transcription has occurred.
miRNAs are transcribed from specific genes and processed into mature, single-stranded molecules.
They bind to complementary sequences on target messenger RNA (mRNA) molecules.
This binding can lead to either degradation of the mRNA (if the complementarity is perfect) or inhibition of its translation (if the complementarity is partial).
Ultimately, miRNAs reduce the amount of protein produced from a specific gene, providing a fine-tuning mechanism for gene expression.
Worked examples
See the formulas applied — reveal one step at a time, like the exam.
Compare and contrast the mechanisms of gene control in prokaryotes (using the lac operon) and eukaryotes, focusing on how different cellular conditions lead to changes in gene expression.
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Organisation: In prokaryotes (e.g., E. coli), genes with related functions are often grouped into operons, allowing coordinated regulation. Eukaryotic genes are typically regulated individually, though genes in a pathway can be co-regulated by common transcription factors.
An experiment measured the activity of β-galactosidase (encoded by lacZ) in E. coli under different growth conditions. The results are shown below:
- Growth medium with 2% glucose only: 10 units of β-galactosidase activity.
- Growth medium with 2% lactose only: 4500 units of β-galactosidase activity.
- Growth medium with 2% glucose and 2% lactose: 450 units of β-galactosidase activity.
(a) Calculate the fold-induction of the lac operon when cells are switched from a glucose-only medium to a lactose-only medium. (b) Explain the molecular basis for the difference in β-galactosidase activity between the lactose-only medium and the medium containing both glucose and lactose.
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(a) Calculation of Fold-Induction:
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Glossary
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Quick check
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Revision flashcards
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What is an operon?
A cluster of genes with related functions that are transcribed together as a single mRNA molecule. They are common in prokaryotes, with the lac operon being a classic example.
Key takeaways
Review these before you close the topic — retrieval beats re-reading.
- ✓
Regulatory gene (lacI): Located upstream of the operon, it encodes the lac repressor protein, which is constitutively expressed.
- ✓
Promoter (P): The binding site for RNA polymerase to initiate transcription of the structural genes.
- ✓
Operator (O): A segment of DNA within the promoter where the lac repressor binds.
- ✓
Structural genes (lacZ, lacY, lacA): These genes code for enzymes involved in lactose metabolism: β-galactosidase (lacZ) breaks down lactose into glucose and galactose; permease (lacY) is a membrane protein that transports lactose into the cell; and transacetylase (lacA) has a role in detoxifying by-products.
Practice — then mark it
The whole point: a real Cambridge question, marked mark-by-mark.
9700/42 · Q3(c)(ii)
The normal allele for the gene An-1 codes for a transcription factor that has a role in awn development and in the number of grains of rice produced. When the transcription factor is present there is: • • an increase in the expression of genes involved in awn development (positive regulation) a decrease in the expression of genes involved in the number of grains produced (negative regulation). Suggest and explain how changes at the An-1 locus can cause rice plants to have grains with no awns and an increased grain yield.
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