Section B Answer one question. (a) Describe the principles of the polymerase chain reaction (PCR).

Mark scheme

Marking points from the official mark scheme (Feb/Mar 2017). The mark or mark type is shown in brackets.

1. [1] (a) nine from: 1 production of a large number of copies of a length of DNA/amplification of DNA ;
2. [1] (a) 2 rapid;
3. [1] (a) 3 only small sample of DNA needed ;
4. [1] (a) 4 DNA, denatured/separated into two strands, by heat/at 95 °C;
5. [1] (a) 5 primer (DNA) added ;
6. [1] (a) 6 reference to annealing at, 60–65, °C;
7. [1] (a) 7 reference complementary base pairing;
8. [1] (a) 8 DNA/Taq, polymerase ;
9. [1] (a) 9 replicates (template) strand at, 70–75, °C;
10. [1] (a) 10 heated again to separate strands/process repeated ;
11. [1] (a) 11 Taq polymerase, is heat stable/has high optimum temperature ;
12. [1] (a) 12 does not need replacing each cycle ;
13. [1] (a) 13 efficient process ;
14. [1] (b) six from: 1 small/circular, piece of (double-stranded) DNA ;
15. [1] (b) 2 replicate independently/multiple origins of replication;
16. [1] (b) 3 high copy number ;
17. [1] (b) 4 easy to extract from bacteria;
18. [1] (b) 5 can be cut using restriction, enzyme/endonuclease ;
19. [1] (b) 6 gene/DNA, can be inserted;
20. [1] (b) 7 can be taken up by bacteria ;
21. [1] (b) 8 may contain genes for antibiotic resistance/can carry marker genes ;
22. [1] (b) 9 helps in identifying transformed bacteria ;
23. [1] (b) 10 acts as a vector;
24. [1] (b) 11 may carry promoter;