A-LevelBiologySelection and evolution9700/41 · May/June 2024 · Q415 Marks

White-clawed crayfish, Austropotamobius pallipes, live in rivers and lakes in Europe. In the 1850s, the North American signal crayfish, Pacifastacus leniusculus, was introduced to Europe. The introduced species carried a pathogen that causes a disease known as crayfish plague. This disease kills A. pallipes. Since 1850, the population size of A. pallipes has reduced in many areas of Europe due to the spread of crayfish plague.

(a)North American P. leniusculus can carry the crayfish plague pathogen without showing symptoms because they have evolved resistance to it. Explain how P. leniusculus could have evolved resistance to the crayfish plague pathogen.[4]
(b)(i)Outline and explain the steps that occur in PCR.[4]
(b)(ii)Suggest how the researchers were able to discover suitable sequences for the primers.[1]
(c)(i)State the letter of the curve with the highest starting concentration of DNA.[1]
(c)(ii)The quantity of A. pallipes DNA reaches a threshold (marked as a horizontal line on Fig. 4.1) at 28 cycles. The DNA in curve C reaches the threshold after 25 cycles. In one cycle of PCR the concentration of DNA doubles. Calculate the relative difference in the starting concentrations of A. pallipes DNA and the DNA in curve C.[1]
(d)(i)Identify the component that is taken up by normal bacteria to produce the genetically modified bacteria in Fig. 4.2 and state what this component contains to allow the bacteria to make green fluorescent protein.[3]
(d)(ii)Suggest why the gene for green fluorescent protein is sometimes transferred in addition to the desired gene in genetic engineering.[1]

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 2024). The mark or mark type is shown in brackets.

[1] (a) (random/spontaneous) mutation gave resistance ;
[1] (a) pathogen / disease / plague, was selection pressure ;
[1] (a) mutation, gave selective advantage / was selected for / allowed crayfish to survive (plague);
[1] (a) survivors / resistant crayfish, bred / reproduced / passed on mutation ;
[1] (a) frequency of, allele / mutation, (for resistance) increased ;
[1] (a) natural / directional, selection;
[1] (a) AVP;
[1] (b)(i) (heat to) 90-98 °C so DNA, denatures / strands separate ;
[1] (b)(i) (cool to) 50-65 °C so primers, bind / base pair / anneal (to DNA) ;
[1] (b)(i) (heat to) 68-75 °C so, Taq / DNA, polymerase, makes DNA / (new) strand ;
[1] (b)(i) repeat cycles to amplify DNA ;
[1] (b)(ii) used, a (named) database / bioinformatics;
[1] (c)(i) A;
[1] (c)(ii) (2^3) = 8 ;
[1] (d)(i) (named) plasmid / vector (used);
[1] (d)(i) (containing) gene / DNA, for (green) fluorescent protein ;
[1] (d)(i) (containing) a (prokaryotic) promoter ;
[1] (d)(ii) to act as a, marker / reporter, gene
[1] (d)(ii) or to identify, recombinant / GM / transgenic, bacteria / organisms
[1] (d)(ii) or to show expression of another transferred gene ;

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