Ch 3 · 16.3 Gene control · AS & A Level Biology 9700
Topic 3 of 3 in Inheritance — AS & A Level Biology 9700 — 48 lessons.
Introduction
Not every gene in a cell is transcribed all of the time; cells switch genes on and off according to their needs and their environment. This lesson covers the lac operon as a bacterial model of gene control, transcription factors in eukaryotic cells, the difference between structural and regulatory genes and between inducible and repressible enzymes, and how gibberellin switches genes on by destroying DELLA proteins. (Syllabus section 16.3.)
Key ideas — The lac operon
The lac operon in Escherichia coli controls the transcription of three structural genes coding for enzymes needed to metabolise lactose. It consists of a regulatory gene, which is transcribed continuously and produces a repressor protein; a promoter, where RNA polymerase binds; an operator, adjacent to the promoter; and the structural genes. In the absence of lactose, the repressor protein binds to the operator, physically blocking RNA polymerase from transcribing the structural genes, so the operon is switched off. In the presence of lactose, lactose binds to the repressor protein, changing its shape so that it can no longer bind to the operator; RNA polymerase can then bind to the promoter and transcribe the structural genes, so the operon is switched on and the lactose-digesting enzymes are produced only when they are needed.
🧑🏫 Teacher's explanation
Think of gene control like a light switch in a smart home. You don't want your heater running at 100\% capacity when it is already summer; it would be a waste of energy. Instead, the system only turns on when a sensor detects a drop in temperature. In bacteria, the 'sensor' is the presence of a specific food source, like lactose.
📖 Definitions
🔍 Worked example — Easy
Identify whether the following scenario describes an inducible or repressible system: A bacterium produces an enzyme to break down a toxin only when that toxin is detected in the environment.
🔍 Worked example — Standard
Describe the role of the repressor protein in the lac operon when lactose is absent.
🔍 Worked example — Exam-style
Explain how gibberellin facilitates gene expression in plants.
⚠️ Common errors
🧠 Quick check 1
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🎯 Exam question
(a) Describe how the presence of lactose affects the transcription of structural genes in the lac operon. [3] (b) State the function of transcription factors in eukaryotes. [1] [4 marks]
Key ideas — Transcription factors in eukaryotic cells
In eukaryotic cells, transcription factors are proteins that bind to DNA and control gene expression by decreasing or increasing the rate of transcription. They move from the cytoplasm into the nucleus and bind to specific DNA sequences near a gene, either promoting or preventing the binding of RNA polymerase to the promoter. A transcription factor that promotes transcription is called an activator; one that prevents it is a repressor. Because different cell types produce different sets of transcription factors, the same genome can be expressed differently in different tissues, switching particular genes on or off to produce cells with specialised structures and functions during differentiation.
🧑🏫 Teacher's explanation
Think of your DNA as a massive library of cookbooks. Every cell in your body has the exact same library, but a skin cell doesn't need to cook pasta, and a muscle cell doesn't need to bake bread. To manage this, the cell uses 'Transcription Factors'—these are like librarians who stand by specific recipes.
📖 Definitions
🔍 Worked example — Easy
A student is asked to distinguish between a structural gene and a regulatory gene. Describe the fundamental difference in their function.
🔍 Worked example — Standard
Explain the role of DELLA proteins in plant gene expression in the absence of gibberellin.
🔍 Worked example — Exam-style
Compare the effect of an activator versus a repressor on the rate of transcription in a eukaryotic cell.
⚠️ Common errors
🧠 Quick check 1
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🎯 Exam question
(a) Explain how this occurs despite both cell types having the same genome. [2] (b) Describe how the presence of a DELLA protein would affect the production of this protein. [2] [4 marks]
Key ideas — Structural and regulatory genes; inducible and repressible enzymes
Enzymes made under gene control are of two kinds:
🧑🏫 Teacher's explanation
Think of your cell like a massive factory. A structural gene is like a blueprint for a specific machine (like a conveyor belt or a drill) that does the actual work. A regulatory gene is like the factory manager's instruction manual; it doesn't build the machines itself, but it tells the workers when to start or stop building them. This ensures the factory doesn't waste money and materials building things it doesn't need right now.
📖 Definitions
🔍 Worked example — Easy
A student is asked to classify the following genes: Gene A codes for the enzyme β-galactosidase. Gene B codes for a repressor protein that binds to the operator region of the lac operon. Identify which is the structural gene and which is the regulatory gene.
🔍 Worked example — Standard
Explain why the synthesis of β-galactosidase in E. coli is described as 'inducible'.
🔍 Worked example — Exam-style
Describe the role of DELLA proteins in plant gene expression and explain how gibberellin influences this process.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Key ideas — Gibberellin and DELLA proteins
Gibberellin (15.2, 16.2) works by switching genes on in an unusual way, by removing a brake:
🧑🏫 Teacher's explanation
Think of gene expression like a car engine. Most of the time, the car is parked with the handbrake pulled tight. The 'handbrake' in this biological system is a protein called DELLA. Even if you have the key (transcription factors) ready to start the engine, the handbrake (DELLA) prevents the car from moving.
📖 Definitions
🔍 Worked example — Easy
Identify the role of DELLA proteins in a plant cell when gibberellin concentration is low.
🔍 Worked example — Standard
Describe the sequence of events that occurs when gibberellin binds to its receptor.
🔍 Worked example — Exam-style
A mutant barley plant has a mutation that prevents the breakdown of DELLA proteins. Predict the effect on amylase production during germination.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) Describe how gibberellin leads to the production of amylase in barley seeds. [3]
Worked example
Question. A mutation in the regulatory gene of the lac operon in a strain of E. coli results in a repressor protein that cannot bind to lactose, but can still bind to the operator. (a) Predict the effect of this mutation on the transcription of the structural genes, in the presence and in the absence of lactose. [3] (b) Explain your prediction. [3]
🔍 Worked example — Easy
Identify the type of enzyme produced by the lac operon when lactose is present in the medium.
🔍 Worked example — Standard
A researcher observes that a specific protein is only synthesized when its precursor molecule is present in the cytoplasm. Classify this enzyme and the role of the gene involved.
🔍 Worked example — Exam-style
A mutation occurs in the regulatory gene of the lac operon. The resulting repressor protein is unable to bind to the operator but is still produced. Predict the effect on lactose metabolism in a medium containing lactose. [3]
⚠️ Common errors
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Practice
Answers