Explain why a mutation in a gene encoding a transcription factor protein could affect the expression of many other genes simultaneously.

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 regulatory DNA sequences and influence the transcription of target genes. A single transcription factor may have binding sites on the regulatory regions of multiple genes across the genome.

Model answer (4 marks)

A mutation in the gene for a transcription factor changes the amino‑acid sequence or tertiary structure of the protein. The altered protein can no longer bind correctly to its specific regulatory DNA sequences. Consequently transcription of all genes that normally use that transcription factor is affected – usually reduced or abolished. Because the same transcription factor regulates many genes across the genome, a single mutation therefore alters the expression of all those target genes at once.

Examiner tips

Show the chain: mutation → altered protein → loss of DNA binding → altered transcription of target genes.
Mention that the factor regulates multiple genes, so the effect is widespread.
Use the exact terms: transcription factor, regulatory/promoter region, binding, expression.
Keep the answer concise – 4 marks only.

Common mistakes

Failing to state that the mutation changes the protein’s structure or binding ability.
Confusing the mutation with a change in DNA sequence of the target genes rather than the factor itself.
Using vague terms like ‘gene expression’ without linking to transcription factor binding.

Mark scheme (4 marks)

Transcription factors bind to specific regulatory/promoter regions of DNA to influence transcription
A mutation in the transcription factor gene changes the amino acid sequence / tertiary structure of the transcription factor protein
The altered transcription factor can no longer bind correctly to its target regulatory sequences, so transcription of target genes is affected (reduced/prevented/increased)
Because the same transcription factor regulates multiple genes (has binding sites on many genes), a single mutation affects expression of all those target genes simultaneously

Key terms in this question

transcription factor · transcription · mutation