A-LevelBiologyNucleic acids and protein synthesis9700/43 · May/June 2025 · Q39 Marks

An operon is a section of DNA found in prokaryotes.

(a)Explain why the enzymes coded for by the lac operon are described as inducible enzymes.[2]
(b)An investigation into the induction and action of the lac operon was carried out using the bacterium, Escherichia coli, grown in a growth medium containing glucose. When the bacteria had used all the glucose, an excess of lactose was added to the growth medium. The activity of β-galactosidase was measured from this time (0min), as shown in Fig. 3.1. [Figure 3.1] With reference to the lac operon, explain the shape of the curve in Fig. 3.1.[7]

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Mark scheme

Marking points from the official mark scheme (May/June 2025). The mark or mark type is shown in brackets.

[2] 3(a) 1 enzymes produced when, allolactose / lactose / substrate, is present or enzymes only produced when required ; 2 genes not, expressed / switched on / transcribed, continuously or genes, expressed / switched on / transcribed, when, allolactose / lactose / substrate, binds to repressor ; I inducer
[7] 3(b) any seven from: 1 lactose enters the bacterium ; 2 lactose binds to the repressor ; 3 causing a shape change; 4 the repressor detaches from the operator ; 5 promoter unblocked / AW ; 6 RNA polymerase binds to the promoter ; 7 gene(s) transcribed ; 8 ß-galactosidase is made ; 9 ß-galactosidase breaks down lactose ; galactosidase activity levels off as: 10 ß-galactosidase activity at maximum / galactosidase active sites full ;

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