A-LevelBiologySelection and evolution9700/42 · Oct/Nov 2025 · Q46 Marks

The evolution of antibiotic resistance in bacteria has occurred as a result of natural selection.

(a)Name two ways in which a bacterium can become resistant to an antibiotic.[2]
(b)The World Health Organisation regularly analyses bacterial DNA sequence data. Suggest one way in which this contributes to solving the problem of antibiotic resistance in bacteria.[1]
(c)Some infectious bacterial diseases are treated with the antibiotic streptomycin. If a person does not finish the prescribed course of streptomycin, bacteria are more likely to become resistant to the antibiotic. Explain why a streptomycin-resistant strain of bacteria is more likely to develop as a result of natural selection when a person does not complete the prescribed course of antibiotics.[3]

Mark scheme

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

[2] (a) 1 mutation; 2 horizontal transmission / conjugation / transformation / transduction ;
[1] (b) idea of tracking, mutations / resistance genes / resistance alleles or identifies mechanisms of resistance or (informed) choice / production, of antibiotic ;
[3] (c) any three from: 1 streptomycin is selection pressure ; 2 bacteria with, mutation / resistance, survive / reproduce / have a selective advantage; ora 3 allele / gene, for resistance passed on ; 4 directional selection ;

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