DNA profiling and PCREdexcel A-Level Biology A: Subtopic test

1. 1
   A forensic laboratory has recovered a very small amount of DNA from a single hair left at a crime scene. The DNA must be amplified by the polymerase chain reaction (PCR) before it can be profiled. The thermal cycler repeats a cycle of three temperature stages, using a DNA polymerase taken from the bacterium Thermus aquaticus.

   (a)

   Why is DNA polymerase from Thermus aquaticus used in PCR rather than a human DNA polymerase?

   [1 mark]

   • AIt copies DNA more slowly, which reduces errors
   • BIt does not need primers to start copying
   • CIt is not denatured at the temperature used to separate the DNA strands
   • DIt joins nucleotides containing ribose instead of deoxyribose

   (b)

   Starting from one double-stranded DNA molecule, how many double-stranded copies are present after 6 complete PCR cycles?

   [1 mark]

   • A64
   • B32
   • C12
   • D128

   (c)

   Explain why primers are needed in PCR.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Short tandem repeat (STR) regions of DNA were amplified by PCR and the fragments separated by gel electrophoresis to compare a child with her mother and with two men, A and B, who might be her father. The fragment sizes in base pairs (bp) were: child 420, 310, 250 and 180; mother 420, 250 and 150; man A 310, 220 and 180; man B 390, 260 and 140.

   (a)

   Which fragment in the child's profile must have been inherited from her father?

   [1 mark]

   • A420 bp
   • B250 bp
   • C150 bp
   • D310 bp

   (b)

   Why do the DNA fragments move towards the positive electrode during electrophoresis?

   [1 mark]

   • AThe bases of DNA carry a positive charge
   • BThe phosphate groups of DNA carry a negative charge
   • CAgarose gel is positively charged and attracts DNA
   • DThe buffer carries the fragments by capillary action

   (c)

   Deduce which man is more likely to be the child's father. Use the data to justify your answer.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A student separated DNA fragments by electrophoresis in an agarose gel. A ladder of fragments of known size was loaded alongside the samples. The ladder fragments of 1000, 800, 600, 400 and 200 bp travelled 12, 17, 23, 31 and 45 mm from the wells respectively. Fragment X from a sample travelled 27 mm and fragment Y travelled 10 mm.

   (a)

   Describe how DNA fragments are separated by gel electrophoresis and then made visible.

   [3 marks]

   (b)

   Estimate the size of fragment X and explain why the size of fragment Y cannot be estimated reliably from this ladder.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A conservation laboratory studies a rare species of parrot. It uses DNA profiling to plan breeding pairs, and its researchers also study a gene whose primary transcript contains five exons. In brain cells the mature mRNA contains exons 1, 2, 3 and 5, whereas in gut cells it contains exons 1, 2, 4 and 5.

   (a)

   Explain how DNA profiling using PCR and gel electrophoresis can be used to decide which pairs of parrots would be the most suitable to breed together.

   [6 marks]

   (b)

   Explain how one gene can give rise to more than one protein, using the information about the brain and gut mRNA.

   [6 marks]

   Total for question 4: 12 marks

End of questions