A mutation in a structural gene results in a non-functional repressor protein. Explain how this mutation affects the transcription of the genes controlled by this repressor.

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

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

In prokaryotes, the lac operon provides a well-studied example of gene regulation. A repressor protein, encoded by a regulatory gene, can bind to the operator region and prevent RNA polymerase from transcribing structural genes. When a repressor protein is non-functional, the normal regulatory mechanism is disrupted.

Model answer (4 marks)

The mutation makes the repressor protein non‑functional, so it cannot bind to the operator region.
RNA polymerase can then bind to the promoter and transcribe the structural genes without obstruction.
The structural genes are transcribed continuously, i.e. constitutively, even when the substrate is absent.
Thus the normally inducible, regulated expression becomes permanent, unregulated expression, which is energetically wasteful for the cell.

Examiner tips

Use the exact terms ‘non‑functional repressor’, ‘operator’, ‘constitutive transcription’, and ‘inducible expression’ – these are the key words the mark scheme looks for.
Show the logical sequence: mutation → loss of repressor binding → free promoter → continuous transcription – this demonstrates understanding of the mechanism.

Common mistakes

Confusing the mutation as being in the repressor gene instead of the structural gene; the question specifies the structural gene mutation leads to a non‑functional repressor. Using vague phrases like ‘more genes are made’ instead of ‘constitutive transcription’. Forgetting to mention that the change is from inducible to permanent expression.

Mark scheme (4 marks)

The non-functional repressor cannot bind to the operator region
RNA polymerase can bind to the promoter and move along the DNA unimpeded
The structural genes are transcribed continuously / constitutively, regardless of whether the substrate is present
This represents a change from inducible / regulated expression to permanent / unregulated expression, which is energetically wasteful for the cell

Key terms in this question

repressor protein · transcription · structural gene