Community Q&A
A-Level Biology May/June 2025 Q3(c)(ii): The normal allele for the gene An-1 codes for a transcription factor that has a role in…
A-Level Biology · Paper 9700/42 · May/June 2025 · Question 3(c)(ii) · [3 marks]
The normal allele for the gene An-1 codes for a transcription factor that has a role in awn development and in the number of grains of rice produced. When the transcription factor is present there is: • • an increase in the expression of genes involved in awn development (positive regulation) a decrease in the expression of genes involved in the number of grains produced (negative regulation). Suggest and explain how changes at the An-1 locus can cause rice plants to have grains with no awns and an increased grain yield.
A full-marks model answer with a mark-by-mark examiner breakdown is below.
1 answer
- accepted ✓
A mutation in the An-1 gene, such as a change in the base sequence, would alter the primary structure of the transcription factor it codes for. This leads to a change in the folding of the polypeptide and therefore a different, non-functional tertiary 3D shape.
As a result, the faulty transcription factor can no longer bind to the promoter regions of its target genes. This has two effects:
- The genes for awn development are no longer activated by the transcription factor. Their expression is therefore decreased, leading to rice grains with no awns.
- The genes for the number of grains produced are no longer repressed by the transcription factor. This lack of negative regulation means their expression increases, resulting in a higher grain yield.
How the marks are awarded
- MP1 — For identifying the initial cause as a mutation or change in the base sequence of the An-1 gene, as stated in 'A mutation in the An-1 gene, such as a change in the base sequence...'
- MP2 — For explaining that the mutation results in a non-functional transcription factor with an altered tertiary/3D shape, as seen in '...a different, non-functional tertiary 3D shape.'
- MP3 — For stating that the consequence of the changed shape is that the transcription factor can no longer bind to the promoter region of the DNA, as mentioned in '...the faulty transcription factor can no longer bind to the promoter regions...'
Common mistakes
- Stating that the gene is 'damaged' or 'broken' without specifying a mutation or change in base sequence.
- Failing to link the mutation to the change in the protein's tertiary/3D structure, simply saying 'the protein doesn't work'.
- Confusing the regulation, for example stating that the lack of the transcription factor causes a decrease in grain number gene expression.
- Incorrectly stating that the transcription factor binds to mRNA or ribosomes instead of the promoter region of the DNA.
Examiner tip: For questions on gene expression, always trace the full sequence of events: from the change in DNA (mutation), to the altered protein (structure), to its changed molecular function (binding), and finally to the resulting phenotype.
AI-generated model answer, grounded in the official Cambridge mark scheme and reviewed by the MarkScheme team. Mark your own answer to this question →
Your answer
Sign in to answer this question.