DNA profiling and PCREdexcel A-Level Biology A: Mind map

Exam questions on DNA profiling and PCR

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.
   Explain why primers are needed in PCR.2 marks
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.
   Deduce which man is more likely to be the child's father. Use the data to justify your answer.2 marks
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.
   Describe how DNA fragments are separated by gel electrophoresis and then made visible.3 marks