A research team used CRISPR-Cas9 to introduce a loss-of-function mutation into a gene encoding a transcription factor in wheat. Explain why disrupting a gene that encodes a transcription factor is likely to have a more widespread effect on the wheat plant's phenotype than disrupting a gene that encodes a single structural protein.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Transcription factors are proteins that bind to specific regulatory sequences in the genome and control the rate of transcription of target genes. CRISPR-Cas9 is a genome-editing tool that uses a guide RNA to direct the Cas9 endonuclease to a precise location in the genome, where it introduces a double-strand break.
Model answer (4 marks)
A transcription factor regulates the expression of many target genes at the same time.
If the transcription factor is lost, all of those target genes are no longer activated, so many proteins are missing and several biochemical pathways are disrupted.
A structural protein, by contrast, is involved in only one specific structure or function; its loss therefore affects only that particular structure or pathway.
Thus the loss‑of‑function mutation in the transcription factor will produce a far more widespread, pleiotropic phenotypic change than a mutation in a single structural protein.
If the transcription factor is lost, all of those target genes are no longer activated, so many proteins are missing and several biochemical pathways are disrupted.
A structural protein, by contrast, is involved in only one specific structure or function; its loss therefore affects only that particular structure or pathway.
Thus the loss‑of‑function mutation in the transcription factor will produce a far more widespread, pleiotropic phenotypic change than a mutation in a single structural protein.
Examiner tips
- Use the word ‘regulates’ to show understanding of transcription factors. Show that loss of the factor stops many genes, not just one. Explain the difference between a widespread effect and a localised effect. Mention ‘pleiotropic’ to link to the mark scheme.
Common mistakes
- Saying the transcription factor only affects one gene. Confusing a structural protein with a regulatory protein. Failing to mention that many pathways are disrupted. Using vague terms like ‘big effect’ without explaining why.
Mark scheme (4 marks)
- A transcription factor regulates the expression of multiple target genes simultaneously
- Loss of the transcription factor therefore means multiple proteins are not produced / multiple biochemical pathways are disrupted
- A structural protein only contributes to one specific structure or function, so its loss has a limited / localised phenotypic effect
- Therefore the transcription factor mutation has a more widespread / pleiotropic effect on phenotype because many traits controlled by those target genes will be altered
Key terms in this question
CRISPR-Cas9 · transcription factor · loss-of-function mutation · structural protein
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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