The lac operon in E. coli is a well-studied example of gene regulation. Suggest how the lac repressor protein ensures that the lac genes are only transcribed when lactose is present in the cell.

Model answer (4 marks)

Examiner tips

• Use the term ‘operator’ and ‘RNA polymerase’ to show understanding of the mechanism.
• Explain the conformational change caused by lactose binding – this shows depth of knowledge.
• Show that transcription is blocked when the repressor is bound and released when lactose is present – this links cause and effect.
• Keep the answer concise, 1–2 sentences per point – full marks are awarded for a clear, accurate explanation.

Common mistakes

• Confusing the inducer (lactose) with the repressor – students sometimes say the repressor binds lactose instead of lactose binding the repressor.
• Failing to mention the operator sequence or RNA polymerase – omitting key components loses marks.
• Writing a vague statement such as ‘the repressor stops transcription’ without explaining how lactose reverses this.”