# A mutation in a structural gene results in a non-functional repressor protein. Explain how this mutation affects the transcription of the genes controlled by this repressor.

> OCR A-Level Biology A (H420) — 6.1 Cellular control · Explain · 4 marks

> In prokaryotes, the lac operon provides a well-studied example of gene regulation. A repressor protein, encoded by a regulatory gene, can bind to the operator region and prevent RNA polymerase from transcribing structural genes. When a repressor protein is non-functional, the normal regulatory mechanism is disrupted.

## Mark scheme (4 marks)

1. The non-functional repressor cannot bind to the operator region
2. RNA polymerase can bind to the promoter and move along the DNA unimpeded
3. The structural genes are transcribed continuously / constitutively, regardless of whether the substrate is present
4. This represents a change from inducible / regulated expression to permanent / unregulated expression, which is energetically wasteful for the cell

## Key terms

- [repressor protein](https://www.gradenine.co.uk/glossary/repressor-protein)
- [transcription](https://www.gradenine.co.uk/glossary/transcription)
- [structural gene](https://www.gradenine.co.uk/glossary/structural-gene)

## Related

- [Revision notes for OCR A-Level Biology A (H420)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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Source: [GradeNine](https://www.gradenine.co.uk/q/a-mutation-in-a-structural-gene-fd597e4b) · Published by Druglandscape Ltd.