A2 · Practice questions · Gene control

Repressors, operators and DELLA.

Six original Cambridge-style questions on how genes are switched on and off in bacteria and plants.

Original questions All questions on this page are original work, written in the Cambridge AS & A Level style. They are not from past papers. They test the same concepts and skills the syllabus rewards.

Keep these straight

Who binds what.

Repressor binds the operator; RNA polymerase binds the promoter.
Lactose binds the repressor: an inducible system.
Transcription factors are proteins that raise or lower transcription rate.
Gibberellin causes DELLA breakdown, freeing transcription factors.

Recall

[2 marks]

Distinguish between a structural gene and a regulatory gene, giving an example of each from the lac operon.

Structural gene: codes for a protein used in the cell's metabolism or structure, for example lacZ (β-galactosidase) or lacY (lactose permease). ✓
Regulatory gene: codes for a protein that controls the expression of other genes, for example lacI (the repressor). ✓

Explain

[5 marks]

Explain how the lac operon allows E. coli to make β-galactosidase only when lactose is present.

The regulatory gene lacI is transcribed and translated to make a repressor protein. ✓
Without lactose the repressor binds the operator, so RNA polymerase cannot bind the promoter / cannot transcribe the structural genes. ✓
When lactose is present it binds to the repressor, changing its shape so it cannot bind the operator. ✓
RNA polymerase binds the promoter and transcribes lacZ, lacY and lacA as one mRNA, which is translated into β-galactosidase, permease and transacetylase. ✓
When lactose is used up the repressor binds the operator again; this saves energy and materials. ✓

Explain

[3 marks]

Compare inducible and repressible enzymes.

Inducible enzymes (e.g. β-galactosidase) are made only when their substrate is present; the substrate inactivates the repressor. ✓
Repressible enzymes (e.g. tryptophan synthesis enzymes) are normally made, and their synthesis stops when their product accumulates. ✓
In a repressible system the product binds and activates the repressor, which then binds the operator. ✓

Analysis

[3 marks]

Predict, with reasons, whether β-galactosidase is made in each case: (a) a mutant operator to which the repressor cannot bind, no lactose; (b) a mutant repressor that cannot bind lactose, lactose present; (c) a promoter mutation that prevents RNA polymerase binding, lactose present.

(a) Made: the repressor cannot block the operator, so the genes are transcribed all the time. ✓
(b) Not made: lactose cannot change the repressor's shape, so it stays on the operator. ✓
(c) Not made: RNA polymerase cannot bind the promoter, so the structural genes are not transcribed even though the repressor has left. ✓

Explain

[4 marks]

Explain how gibberellin causes stem elongation by activating genes.

Gibberellin (GA₁) binds to a receptor protein in the cell. ✓
This causes DELLA proteins to be broken down. ✓
DELLA proteins normally bind to and inhibit transcription factors needed to transcribe growth genes. ✓
Once DELLA is removed, the transcription factors bind to DNA (the promoter) and increase transcription; the proteins made cause cell elongation in the stem. ✓

Recall

[2 marks]

State what is meant by a transcription factor and where transcription factors act.

A protein that binds to DNA (a specific sequence, often near the promoter) and controls gene expression. ✓
It increases or decreases the rate of transcription of a gene in eukaryotes (in the nucleus). ✓

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