PCR, gel electrophoresis and DNA profilingEdexcel International A Level Biology: Flashcards

Exam questions on PCR, gel electrophoresis and DNA profiling

1. A forensic laboratory receives a swab from a crime scene that contains only a few skin cells, so the DNA recovered is far too little to analyse directly. The laboratory copies the target region of the DNA using the polymerase chain reaction (PCR) in a thermal cycler.
   Explain why primers are needed in a PCR.2 marks
2. A seed company wants to check that a batch of rice is the premium variety it is sold as. DNA is extracted from the grains and cut with restriction enzymes. The mixture of fragments from each sample is loaded into separate wells in an agarose gel, covered in buffer, and a direct current is applied. One variety gives fragments of 200, 500, 800 and 1200 base pairs.
   Explain how the gel separates fragments of different lengths.2 marks
3. A researcher starts a PCR with a single double-stranded DNA molecule containing the target sequence. The thermal cycler runs 30 cycles. The reaction mixture contains template DNA, primers, free nucleotides, buffer and Taq polymerase.
   Calculate the number of double-stranded copies of the target sequence after 30 cycles, assuming that the number doubles in every cycle. Give your answer in standard form to 3 significant figures.3 marks