Distinguish structural and regulatory genes
Compare genes coding for functional products with genes that regulate expression and contrast inducible with repressible enzymes.
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 is the product of a regulatory gene used for?
What you will learn
- Describe structural and regulatory genes
- Compare inducible and repressible enzyme production
Original teaching model. Counts and records are stipulated; drawings are schematic.
| Feature | Meaning |
|---|---|
| Available information | Shared supplied gene set |
| Selective use | Different genes expressed |
| Product | Proteins needed for cell function |
| Qualifier | Most body cells, not every cell without exception |
Build the genetic explanation
A structural gene codes for a polypeptide or functional RNA used by the cell. A regulatory gene codes for a product, often a protein, that controls transcription of other genes. An inducible enzyme is produced when a signal or substrate switches on expression; a repressible enzyme is produced unless a signal, often the end product, switches expression off. These patterns allow cells to adjust gene expression to conditions.
Check the evidence and limits
The gene’s role in regulation is not the same as the enzyme’s metabolic role. “Inducible” and “repressible” describe control of expression, not whether the enzyme itself can be activated after it is made.
Pause and explain
A substrate binds a repressor, causing it to leave the operator so enzymes can be made. Explain why this is inducible control.
Worked example
A gene codes for a repressor protein that binds another gene’s operator. Classify the first gene.
Show the worked solution
- It codes for a product controlling another gene.
- That is a regulatory gene.
Answer It is a regulatory gene.
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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.
Compare structural and regulatory genes by their products or roles.
Give me a hint
Set out the alleles or steps before deciding the outcome.
Compare my reasoning
- Structural genes code for products used in cell function.
- Regulatory genes influence expression of other genes.
A structural gene codes for a functional product such as a polypeptide. A regulatory gene codes for a product that controls transcription of other genes.
Look for these in your work
- Genetic terms used correctly
- Steps linked
An end product binds a repressor and stops transcription of a biosynthetic pathway. Is the enzyme system inducible or repressible?
Give me a hint
Use the given evidence and state your assumptions.
Compare my reasoning
- The pathway is normally expressed until the product accumulates.
- The product switches expression off.
It is repressible: the end product acts as a corepressor that enables repression of genes for the biosynthetic enzymes.
Look for these in your work
- Prediction or interpretation supported
- Limits stated
A substrate binds a repressor, causing it to leave the operator so enzymes can be made. Explain why this is inducible control.
Give me a hint
Apply the inheritance or gene-control model to the new context.
Compare my reasoning
- The substrate signal switches transcription on.
- The enzymes are produced in response to the substrate.
It is inducible because the substrate-related signal removes repression and induces expression of the enzyme genes.
Look for these in your work
- Model applied accurately
- Unsupported certainty avoided
Common mistakes
- Calling every gene a structural gene
- Confusing enzyme activation with induction of its gene expression
What can you explain now?
What is the product of a regulatory gene used for?
Compare with the explanation
Controlling expression of other genes
Its product can affect transcription of structural genes.
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Make it stick
How do inducible and repressible systems differ in their default expression pattern?
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