Explain regulation of the lac operon
Trace how lactose controls repressor binding and transcription of prokaryotic structural genes.
Section 16.3 · Gene control
A Level; Cambridge International Biology 9700, 2028–2030. Written answers are self-checked; this is not practical-skills certification.
Before you begin
- Genes are DNA sequences that can be inherited; meiosis and fertilisation link parental genomes.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What does the lactose-derived inducer bind?
What you will learn
- Describe the roles of regulatory gene, repressor, operator, promoter and structural genes
- Explain lac-operon transcription with and without lactose, without relying on cAMP
Original teaching model. Counts and records are stipulated; drawings are schematic.
| Feature | Meaning |
|---|---|
| Shared DNA identities | E, C and R in both samples |
| Sample A messages | E 6; C 2; R 0 |
| Sample B messages | E 0; C 4; R 5 |
| C comparison | B has 2 more and twice A’s count |
Build the genetic explanation
The lac operon controls expression of structural genes needed to use lactose. Without lactose, an active repressor binds the operator and prevents RNA polymerase transcribing the structural genes. When lactose is present, a lactose-derived inducer binds the repressor and changes its shape; the repressor no longer binds the operator. RNA polymerase can then transcribe the structural genes, producing mRNA for proteins involved in lactose uptake and breakdown.
Check the evidence and limits
Keep the promoter, operator and structural genes distinct. The syllabus does not expect the role of cAMP, so do not use it to explain the core on/off sequence.
Pause and explain
A mutation prevents repressor binding to the operator. Predict transcription when lactose is absent.
Worked example
What happens to lac-operon transcription when lactose is absent?
Show the worked solution
- The repressor remains able to bind the operator.
- Operator binding blocks transcription of the structural genes.
Answer The repressor binds the operator and prevents transcription of the lac structural genes.
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Explain how lactose presence permits transcription of the lac structural genes.
Give me a hint
Set out the alleles or steps before deciding the outcome.
Compare my reasoning
- An inducer binds the repressor.
- The repressor leaves the operator, allowing RNA polymerase to transcribe.
A lactose-derived inducer binds the repressor and changes its conformation so it cannot bind the operator. RNA polymerase can then transcribe the structural genes.
Look for these in your work
- Genetic terms used correctly
- Steps linked
A mutation prevents the repressor from binding lactose but leaves operator binding intact. Predict expression when lactose is present.
Give me a hint
Use the given evidence and state your assumptions.
Compare my reasoning
- The repressor remains bound to the operator.
- Transcription stays blocked.
The structural genes remain repressed because lactose cannot inactivate the repressor; the repressor continues to bind the operator.
Look for these in your work
- Prediction or interpretation supported
- Limits stated
A mutation prevents repressor binding to the operator. Predict transcription when lactose is absent.
Give me a hint
Apply the inheritance or gene-control model to the new context.
Compare my reasoning
- The operator no longer binds the repressor.
- RNA polymerase can transcribe the genes.
Transcription may occur even without lactose because the repressor cannot bind the operator to block transcription.
Look for these in your work
- Model applied accurately
- Unsupported certainty avoided
Common mistakes
- Saying the inducer binds directly to the operator
- Adding cAMP as a required part of the syllabus explanation
What can you explain now?
What does the lactose-derived inducer bind?
Compare with the explanation
The repressor
Binding changes its shape so it no longer binds the operator.
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Make it stick
What changes at the repressor and operator when lactose is present?
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