The lac operon in E. coli is a well-studied example of gene regulation. Describe the role of the lac repressor protein in controlling the expression of the lac genes.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
The lac repressor binds to the operator sequence of the lac operon, blocking RNA polymerase from transcribing the lac genes.
When lactose is absent, the repressor is active and remains bound to the operator, preventing transcription.
Allolactose, a lactose isomer, binds to the repressor and induces a conformational change that reduces its affinity for the operator, causing the repressor to dissociate.
Thus, the repressor normally shuts off the lac genes, but in the presence of allolactose it releases the operator, allowing transcription of the genes needed for lactose utilisation.
When lactose is absent, the repressor is active and remains bound to the operator, preventing transcription.
Allolactose, a lactose isomer, binds to the repressor and induces a conformational change that reduces its affinity for the operator, causing the repressor to dissociate.
Thus, the repressor normally shuts off the lac genes, but in the presence of allolactose it releases the operator, allowing transcription of the genes needed for lactose utilisation.
Examiner tips
- Use the term ‘operator’ and ‘RNA polymerase’ to show understanding of the mechanism.
- Explain the conformational change caused by allolactose and its effect on DNA binding.
- Show the sequence of events: repressor binds → blocks transcription; allolactose binds → repressor releases → transcription resumes.
- Keep the answer concise and use correct terminology to hit all four marks.
Common mistakes
- Confusing the repressor with the inducer or the operator; students often say the repressor ‘activates’ transcription.
- Failing to mention that allolactose binds to the repressor, not to DNA.
- Using vague language such as ‘turns on’ or ‘turns off’ without explaining the binding mechanism.
Mark scheme (4 marks)
- Understanding the lac repressor's role in controlling gene expression
- Correctly describing the lac repressor's mechanism of action
- Describing the effect of allolactose on the lac repressor protein
- Correctly describing the overall effect of the lac repressor protein on lac gene expression
Related
- All OCR A-Level Biology A (H420) revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →
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