Describe the function of a repressor protein in the regulation of gene expression.

OCR A-Level Biology A (H420) — 6.1 Cellular control · Describe · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

A repressor protein binds to the operator region of DNA, blocking RNA polymerase from initiating transcription.

1. It recognises a specific DNA sequence (operator) adjacent to a promoter.
2. By occupying the operator, it prevents RNA polymerase from binding or progressing.
3. This stops the synthesis of mRNA for the associated genes.
4. Thus the repressor controls gene expression, allowing the cell to turn genes off when they are not needed.

Examiner tips

  • Use the term ‘operator’ and ‘RNA polymerase’ – examiners look for specific vocabulary.
  • Show the sequence of events: binding → blockage of polymerase → no transcription.
  • Mention that the repressor can be released by an inducer if the cell needs the gene product.
  • Keep the answer concise – 4 marks, one sentence per point.

Common mistakes

  • Confusing a repressor with an activator – they have opposite effects.
  • Failing to mention the operator or RNA polymerase – missing key mechanism.
  • Writing a vague statement like ‘it stops gene expression’ without explaining how it blocks transcription.

Mark scheme (4 marks)

  1. Understanding the role of a repressor protein
  2. Link to gene regulation
  3. Mechanism of action
  4. Function in gene regulation

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