Explain why a mutation in the promoter region of a gene may result in a non-functional protein being produced at a lower than normal rate, even though the coding sequence of the gene is unchanged.
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 genes and influence the rate at which RNA polymerase initiates transcription. Mutations can affect any region of a gene, including regulatory sequences upstream of the coding region.
Model answer (4 marks)
A mutation in the promoter changes the DNA sequence that RNA polymerase and transcription factors recognise. The altered sequence reduces binding affinity, so RNA polymerase initiates transcription less efficiently. Consequently fewer mRNA molecules are synthesised. With less mRNA available, ribosomes translate fewer copies, giving a lower amount of protein. The protein produced is still non‑functional because the coding sequence is unchanged, but its quantity is reduced.
Examiner tips
- Mention the promoter as the RNA polymerase binding site
- Explain how a sequence change reduces binding and transcription
- Link reduced mRNA to reduced translation
- State that the protein is still non‑functional because the coding region is intact
Common mistakes
- Confusing the coding sequence with the promoter; claiming the protein sequence changes
- Forgetting to mention the role of transcription factors
- Over‑emphasising the protein being non‑functional when the question is about production rate
Mark scheme (4 marks)
- The promoter is the region where RNA polymerase binds to initiate transcription
- A mutation in the promoter alters the base sequence, reducing the ability of RNA polymerase (or transcription factors) to bind
- Less / reduced transcription occurs, so less mRNA is produced
- Less mRNA means less translation occurs, so less protein is produced; the protein itself is non-functional because the coding sequence is unaltered but production rate is reduced
Key terms in this question
promoter region · coding sequence
Related
- All OCR A-Level Biology A (H420) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Cellular control questions
- A mutation in a structural gene results in a non-functional repressor protein. E…
- A mutation occurs in the promoter region of a gene coding for an enzyme involved…
- Explain why a mutation in a gene encoding a transcription factor protein could a…
- Explain why a mutation in the promoter region of a structural gene may prevent t…
- Describe the function of a repressor protein in the regulation of gene expressio…
- The lac operon in E. coli is a well-studied example of gene regulation. Suggest …