A-LevelBiologyNucleic acids and protein synthesis9700/43 · May/June 2023 · Q310 Marks

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. (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. lacI Fig. 3.1

(a)(i)Outline the main features of the lac operon.[3]
(a)(ii)Explain the role of the lacl gene in the regulation of the lac operon.[4]
(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]

This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.

Mark scheme

Marking points from the official mark scheme (May/June 2023). The mark or mark type is shown in brackets.

the operon has:
[1] promoter ;
[1] operator ;
[1] three structural genes / named three structural genes ;
lacz or ẞ galactosidase gene
lacA or lactose / ẞ galactoside, permease gene
lacy or transacetylase gene
I regulatory genes/order of named parts
any four from:
laci gene
[1] is always expressed ;
[1] controls (structural) gene expression ;
[1] codes for the repressor (protein) ;
[1] repressor, binds to the operator / blocks the promoter ;
[1] prevents, (structural) gene expression / RNA polymerase binding to promoter ;
[1] lactose /allolactose, binds to repressor ;
[1] (so) repressor cannot bind to operator / promoter unblocked / gene expression can occur ;
[1] enzymes / proteins, made continuously / all the time ;
[1] (because) enzymes / proteins, needed / necessary (for cell) ;
[1] end product inhibition / made until product concentrations too high ;

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