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CAIE A-Level Biology A2 16.3 Gene Control Questions

Practise gene control by interpreting lac operon data, comparing inducible and repressible systems and tracing transcription-factor or gibberellin signals to gene expression.

Syllabus
2028–2030
Course
Biology 9700
Level
A2

Exam points

  • Distinguish structural genes that code functional products from regulatory genes that control transcription.
  • Compare inducible and repressible enzymes using substrate and end-product effects on transcription.
  • Use lac operon diagrams or lactose/glucose data to predict repressor binding and gene expression.
  • Explain how eukaryotic transcription factors bind promoters or enhancers to alter transcription rates.
  • Trace gibberellin signalling through DELLA breakdown and PIF release to stem-elongation gene expression.

Covered syllabus points

16.3.1—Structural genes vs regulatory genes16.3.2—Repressible enzymes vs inducible enzymes16.3.3—Genetic control in prokaryotes16.3.4—Transcription factors in eukaryotes16.3.5—Gibberellin and gene activation

Question 1

[Maximum number: 13]

Question (a)

(a)

The lac operon of prokaryotes contains a group of structural genes that are under the same control and are transcribed together.

Another operon found in prokaryotes is the trp operon.
Fig. 6.1 summarises the structure and control of the trp operon.

Fig. 6.1

Fig. 6.1

[ 8 ]

Question (i)

(i)

Describe the differences in structure and control between the lac operon and the trp operon.

[ 3 ]

Question (ii)

(ii)

Suggest why structural genes in operons are transcribed together.

[ 1 ]

Question (iii)

(iii)

trp⁡A\operatorname{trp} A is an example of a structural gene and trp⁡R\operatorname{trp} R is an example of a regulatory gene. Describe the differences between the functions of structural genes and regulatory genes.

[ 2 ]

Question (iv)

(iv)

trp⁡A\operatorname{trp} A codes for the enzyme tryptophan synthase.

Tryptophan synthase catalyses the formation of the amino acid tryptophan.
Explain why tryptophan synthase is an example of a repressible enzyme.

[ 2 ]

Question (b)

(b)

Control of gene expression in eukaryotes is more complex than in prokaryotes.

In plants, the control of gene expression can involve plant hormones, such as gibberellin, and proteins known as JAZ and MYC.

[ 5 ]

Question (i)

(i)

Describe how gibberellin activates genes in plant cells.

[ 4 ]

Question (ii)

(ii)

Transcription of some plant genes is prevented when JAZ proteins bind to other proteins known as MYC.
When JAZ proteins are broken down, MYC proteins are free to bind to DNA. This allows transcription to begin.
State the term that is used to describe proteins such as MYC proteins.

[ 1 ]

Question 2

[Maximum number: 10]

Gene expression in a cell is controlled. When a gene is expressed (switched on), the gene is transcribed. When a gene is not expressed (switched off), the gene is not transcribed.

Environmental changes can cause some genes to be switched on or switched off.

Question (a)

(a)

An example of control of gene expression in prokaryotes is regulation in the lac operon.

The lac operon is a length of DNA that is made up of different parts.
Fig. 3.1 shows a simple diagram representing the lacI (regulatory) gene and the lac operon.

Fig. 3.1

Fig. 3.1

[ 7 ]

Question (i)

(i)

Outline the main features of the lac operon.

[ 3 ]

Question (ii)

(ii)

Explain the role of the l a c I gene in the regulation of the lac operon.

[ 4 ]

Question (b)

(b)

The lac operon codes for inducible enzymes. Repressible operons code for repressible enzymes.

Suggest and explain why it is an advantage to a prokaryote to have a repressible operon.

[ 3 ]
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