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

1. 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.

   (a)

   Which enzyme is used to build the new DNA strands in a PCR, and why is it chosen?

   [1 mark]

   • ADNA ligase, because it joins DNA fragments together
   • BTaq polymerase, because it does not denature at the high temperature needed to separate the strands
   • CRestriction endonuclease, because it cuts DNA at specific sequences
   • DRNA polymerase, because it makes primers from nucleotides

   (b)

   At about 95 °C in the first stage of each PCR cycle, which change happens?

   [1 mark]

   • APrimers bind to the template strands
   • BTaq polymerase adds nucleotides to the primers
   • CNucleotides are joined by hydrogen bonds to form new strands
   • DHydrogen bonds between the two template strands break so the strands separate

   (c)

   Explain why primers are needed in a PCR.

   [2 marks]

   Total for question 1: 4 marks

2. 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.

   (a)

   After the current has run for 40 minutes, which fragment of this variety will be nearest the positive electrode?

   [1 mark]

   • A200 base pairs
   • B500 base pairs
   • C800 base pairs
   • D1200 base pairs

   (b)

   Why do the DNA fragments move towards the positive electrode?

   [1 mark]

   • AThe phosphate groups in the DNA backbone are negatively charged
   • BThe bases in DNA carry a positive charge
   • CThe agarose gel is negatively charged and repels DNA
   • DDNA is attracted to the buffer, which is positive

   (c)

   Explain how the gel separates fragments of different lengths.

   [2 marks]

   Total for question 2: 4 marks

3. 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.

   (a)

   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]

   (b)

   Describe the three stages of one PCR cycle and explain why Taq polymerase is used rather than human DNA polymerase.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A racehorse breeder suspects that a foal has been registered with the wrong sire. Blood samples were taken from the mare, the foal and two possible sires, stallion X and stallion Y. DNA from each animal was profiled using short tandem repeats (STRs) at two loci. The results give the number of repeats on each of the two chromosomes. Locus A: mare 9 and 12; foal 9 and 14; stallion X 11 and 14; stallion Y 12 and 13. Locus B: mare 6 and 6; foal 6 and 10; stallion X 7 and 10; stallion Y 6 and 8.

   (a)

   Describe how the DNA profiles of the four horses could have been produced from the blood samples.

   [6 marks]

   (b)

   Using the data, evaluate which stallion is more likely to be the sire of the foal. Discuss why using STRs from several loci makes the conclusion more reliable.

   [6 marks]

   Total for question 4: 12 marks

End of questions