A mutation occurs in the promoter region of a gene coding for an enzyme involved in melanin production. As a result, the gene is transcribed at a much higher rate than normal, leading to excess melanin deposition in skin cells. Explain how a mutation in the promoter region, rather than in the coding sequence of the gene, could lead to increased enzyme production.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Melanin is a pigment synthesised through a series of enzyme-catalysed reactions. The gene encoding one of these enzymes contains a promoter region upstream of the coding sequence. In a particular individual, a base substitution mutation has occurred within the promoter region of this gene.
Model answer (4 marks)
The promoter is the DNA sequence where RNA polymerase binds to start transcription. A base substitution in the promoter changes its sequence so that RNA polymerase binds more strongly or more often. This increased affinity leads to more frequent initiation of transcription, producing a larger amount of mRNA. The extra mRNA is translated by ribosomes, giving a greater quantity of the enzyme that synthesises melanin.
Examiner tips
- Mention the promoter as the RNA polymerase binding site
- Explain how the mutation increases binding affinity
- Show the chain: more binding → more transcription → more mRNA → more enzyme
Common mistakes
- Confusing the coding sequence with the promoter; writing that the mutation changes the enzyme’s structure
- Failing to link increased mRNA to increased enzyme production
Mark scheme (4 marks)
- The promoter region is the site where RNA polymerase binds to initiate transcription
- The mutation alters the base sequence of the promoter, increasing the affinity of RNA polymerase for that region
- Greater / more frequent binding of RNA polymerase leads to increased transcription of the gene, producing more mRNA
- More mRNA is translated at ribosomes, producing a greater quantity of the enzyme
Key terms in this question
promoter region · coding sequence · transcribed
Related
- All OCR A-Level Biology A (H420) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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