QUESTION 81 (6 marks)
PCR begins with 5 double-stranded copies of a target region. Assume ideal doubling each cycle until the stated comparison.
a) Calculate the expected number of copies after 6 cycles and after 9 cycles. [2 marks]
b) Describe the purpose of each of the three repeated stages: denaturation, annealing and extension. [3 marks]
c) A real reaction yields fewer copies than the ideal model. State one plausible technical explanation. [1 marks]
Practice marking scheme
Answer
a) 320 and 2560 copies. b) Separate strands; bind primers; extend new DNA using polymerase. c) For example, incomplete primer binding or reduced polymerase activity.
Working
The model gives and . Heating separates the two DNA strands; cooling allows primers to bind complementary target sequences; DNA polymerase extends from the primers to synthesise new strands. In practice, efficiency can be reduced by inhibitors, insufficient reagents, unsuitable temperatures or incomplete extension.
Marking criteria
- Calculates 320. [1 mark]
- Calculates 2560. [1 mark]
- Describes denaturation as strand separation. [1 mark]
- Describes annealing as primer binding. [1 mark]
- Describes extension as polymerase synthesis of new DNA. [1 mark]
- Identifies an appropriate cause of less-than-ideal amplification. [1 mark]
Practice question aligned to the current QCAA syllabus; review the worked solution and marking criteria.
View the QCAA syllabusCompare your working with the guide above.