Explain why a mutation in the promoter region of a structural gene may prevent the production of the encoded protein, even though the coding sequence of the gene remains unchanged.

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).

Transcription factors are proteins that bind to specific DNA sequences near a gene to regulate its expression. The promoter region is a non-coding sequence located upstream of a structural gene.

Model answer (4 marks)

1. The mutation alters the promoter sequence, so RNA polymerase cannot recognise and bind to it.
2. Without polymerase binding, transcription of the gene is prevented.
3. The mutation also disrupts binding sites for transcription factors, further blocking initiation.
4. Consequently no mRNA is produced, so the protein cannot be synthesised.

Examiner tips

Use the exact terms ‘RNA polymerase’, ‘promoter’, ‘transcription factors’, ‘mRNA’, ‘protein’
Show the logical chain: mutation → no binding → no transcription → no protein
Keep each point brief and separate

Common mistakes

Confusing the coding sequence with the promoter; stating the gene is unchanged but still not transcribed
Failing to mention transcription factors or the lack of mRNA
Using vague phrases like ‘gene is silenced’ without linking to promoter mutation

Mark scheme (4 marks)

RNA polymerase cannot bind to the promoter region
Transcription of the gene does not occur / is prevented
Transcription factors cannot bind to the mutated promoter
Without transcription, no mRNA is available for translation, so the polypeptide / protein is not synthesised

Key terms in this question

promoter region · structural gene