Forensic biology

Sample questions

Original practice questions written in the style of the exam.

1. Question 14 marks

   Describe how the polymerase chain reaction (PCR) is used to produce many copies of a DNA sample.

   Mark scheme

   • The DNA is heated to about $95\,^\circ\text{C}$ to break the hydrogen bonds and separate the strands (1)
   • The mixture is cooled to about $55\,^\circ\text{C}$ so primers bind (anneal) to complementary sequences at the ends of the target DNA (1)
   • At about $72\,^\circ\text{C}$, Taq polymerase adds free nucleotides to build new complementary strands (1)
   • The cycle is repeated many times, doubling the amount of DNA each cycle (1)
2. Question 23 marks

   A body is found in a room. Its core temperature is $31\,^\circ\text{C}$ and rigor mortis is present in the face and jaw but not in the legs.

   Explain how these observations can be used to estimate the time of death, and suggest one factor that would make the estimate less reliable.

   Mark scheme

   • After death, the body cools towards the temperature of its surroundings at a roughly predictable rate, so the temperature drop from about $37\,^\circ\text{C}$ indicates time since death (1)