Gene regulation and the lac operon - TCE Biology (Tasmania)
“Gene expression and regulation: prokaryotic cells only, using the lac operon example; the phenotypic expression of genes depends on factors controlling transcription and translation, the products of other genes and the environment; differential gene expression controls cell differentiation”
Cells only express the genes they need. In the bacterium E. coli, the lac operon controls the genes for using lactose. A regulator gene makes a repressor protein that binds to the operator and blocks RNA polymerase, so when lactose is absent the structural genes lacZ (beta-galactosidase, which breaks lactose into glucose and galactose) and lacY (permease, which takes lactose into the cell) are not transcribed. When lactose is present, a form of lactose binds to the repressor and changes its shape, the repressor leaves the operator, and the genes are transcribed. When glucose is available, E. coli uses it first and the lac genes stay largely switched off.
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What this dot point is asking
Gene regulation is examined under criterion 5 (Section B of the TASC exam). The course uses only prokaryotic regulation, specifically the lac operon. The external assessment specifications require the functions of lacZ and lacY and their products, and exclude lacA, CAP and cAMP, eukaryotic gene regulation, methylation, HOX genes and epigenetics.
Why regulate genes?
Making enzymes costs energy and materials. By transcribing genes only when their products are needed, a bacterium saves resources and can respond quickly to changes in its environment, such as which sugars are available.
Parts of the lac operon
An operon is a group of genes controlled together by one promoter and one operator.
| Part | Role |
|---|---|
| Regulator gene (lacI) | Separate gene that codes for the repressor protein; always transcribed |
| Promoter | Where RNA polymerase binds to begin transcription |
| Operator | Where the repressor binds; lies between the promoter and the structural genes |
| lacZ | Codes for beta-galactosidase, which hydrolyses lactose into glucose and galactose |
| lacY | Codes for permease, a membrane protein that transports lactose into the cell |
Lactose absent: operon off
- The regulator gene produces the repressor protein.
- The repressor binds to the operator.
- RNA polymerase is blocked, so lacZ and lacY are not transcribed.
- No beta-galactosidase or permease is made.
Lactose present: operon on
- Lactose enters the cell, and some is converted to allolactose.
- Allolactose binds to the repressor and changes its shape.
- The repressor can no longer bind to the operator.
- RNA polymerase transcribes lacZ and lacY, and the mRNA is translated into beta-galactosidase and permease.
- Lactose is taken up and broken down. When it is used up, the repressor is free to bind the operator again, and the operon switches off.
Lactose does not switch the genes on directly. It inactivates the repressor. The operon is "off" by default and "on" only when the repressor is removed from the operator.
When glucose is available
Glucose is E. coli's preferred energy source. When glucose is present, transcription of the lac operon stays low even if lactose is also present, so the bacterium uses glucose first and switches to lactose only when glucose runs out. You need this outcome, but not the mechanism (which involves CAP and cAMP).
| Glucose | Lactose | Lac operon |
|---|---|---|
| Present | Absent | Off (repressor bound) |
| Absent | Absent | Off (repressor bound) |
| Present | Present | Low (glucose used first) |
| Absent | Present | On (high transcription) |
Differential gene expression
The same principle, switching genes on and off, explains why cells in a multicellular organism have identical DNA but different structures and functions. Differential gene expression controls cell differentiation as tissues form: a root hair cell, a guard cell, a neuron and a white blood cell each express different sets of genes. The phenotype depends on factors that control transcription and translation, on the products of other genes, and on the environment.
Two mutant strains
- Strain 1: the repressor gene has a mutation so the repressor cannot bind the operator. Result: lacZ and lacY are transcribed all the time, with or without lactose.
- Strain 2: the repressor has a mutation so allolactose cannot bind to it. Result: the repressor stays on the operator even when lactose is present, so the bacterium cannot use lactose.
Marker's note: trace the effect through repressor, operator, RNA polymerase and transcription.
- Saying lactose binds to the operator
- Allolactose binds to the repressor, not the operator.
- Mixing up promoter and operator
- RNA polymerase binds the promoter; the repressor binds the operator.
- Mixing up the products
- lacZ codes for beta-galactosidase (breaks down lactose); lacY codes for permease (transports lactose in).
- Writing about eukaryotic transcription factors
- They are excluded; use the lac operon.
Exam-style questions
Questions in the style of TASC exam questions on this dot point, each with a worked answer. They are written by ExamExplained unless tagged "Past paper"; the year shows the paper a question is modelled on.
Original6 marksExplain how the lac operon in E. coli is switched off when lactose is absent and switched on when lactose is present.Show worked answer →
Three marks for each condition.
Lactose absent. The regulator gene produces a repressor protein. The repressor binds to the operator, which lies between the promoter and the structural genes. This blocks RNA polymerase from moving along the DNA, so lacZ and lacY are not transcribed, and the cell does not waste energy making enzymes it cannot use.
Lactose present. A form of lactose (allolactose) binds to the repressor and changes its shape, so the repressor can no longer bind to the operator. RNA polymerase binds to the promoter and transcribes lacZ and lacY. The cell makes beta-galactosidase, which breaks lactose into glucose and galactose, and permease, which transports lactose into the cell.
Original3 marksA mutation in the operator stops the repressor from binding to it. Predict and explain the effect on the production of beta-galactosidase when lactose is absent.Show worked answer →
Prediction (1 mark). Beta-galactosidase would be produced even when lactose is absent.
Explanation (2 marks). The repressor normally switches the operon off by binding to the operator. If it cannot bind, RNA polymerase is never blocked, so lacZ is transcribed and translated all the time, wasting the cell's energy and materials.