1. 1
   Meselson and Stahl grew the bacterium Escherichia coli for many generations in a medium whose only nitrogen source was the heavy isotope ¹⁵N, so that all the bases in its DNA contained ¹⁵N. They then transferred the bacteria to a medium containing only the normal, lighter isotope ¹⁴N. DNA was extracted at intervals and centrifuged in a caesium chloride solution, which separates DNA molecules by density. Before the transfer, all the DNA formed a single heavy band. After one generation in ¹⁴N, all the DNA formed a single band of intermediate density. After two generations, there were two bands of equal size: one of intermediate density and one light.

   (a)

   Which result would have been obtained after one generation in ¹⁴N if DNA replication were conservative (the original double helix kept intact and a wholly new molecule made)?

   [1 mark]

   • AOne heavy band and one light band
   • BA single band of intermediate density
   • CA single heavy band
   • DA single light band

   (b)

   After two generations in ¹⁴N, what fraction of the DNA molecules contains any ¹⁵N?

   [1 mark]

   • AOne quarter
   • BOne half
   • CThree quarters
   • DAll of them

   (c)

   Explain how complementary base pairing ensures that the two DNA molecules produced are identical to the original molecule.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A new antibacterial compound binds to and inhibits helicase in a species of bacterium. In a culture growing at a constant rate, the mean mass of DNA per cell and the number of cells were measured every 20 minutes. Before the compound was added, the number of cells doubled every 20 minutes. Within 20 minutes of adding the compound, the mean mass of DNA per cell stopped increasing, and after a further 40 minutes the number of cells had stopped increasing. The cells remained alive for several hours.

   (a)

   Which of these is the role of helicase in DNA replication?

   [1 mark]

   • ALinking nucleotides together to form a new strand
   • BAdding complementary bases to the template strand
   • CJoining the sugar-phosphate backbones of the two strands
   • DUnwinding the double helix and breaking the hydrogen bonds between the two strands

   (b)

   Which statement describes the general role of DNA polymerase?

   [1 mark]

   • AIt breaks the hydrogen bonds between base pairs
   • BIt unwinds the DNA double helix
   • CIt links free nucleotides to form a new strand complementary to the template strand
   • DIt transcribes the template strand into RNA

   (c)

   Explain why the number of cells stopped increasing after the compound was added.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   In a paternity case, DNA profiles were produced for a child, the child's mother and two men, X and Y. At each of four short tandem repeat (STR) markers, each person has two fragments whose lengths are given in base pairs (bp). Marker 1: child 180 and 210; mother 180 and 240; man X 210 and 250; man Y 190 and 230. Marker 2: child 320 and 355; mother 355 and 370; man X 300 and 320; man Y 320 and 340. Marker 3: child 145 and 160; mother 145 and 150; man X 160 and 175; man Y 135 and 170. Marker 4: child 400 and 430; mother 400 and 415; man X 390 and 430; man Y 430 and 445.

   (a)

   Outline how the DNA fragments are separated by gel electrophoresis.

   [3 marks]

   (b)

   Deduce which man is more likely to be the child's father, explaining how the data support your answer and how reliable the conclusion is.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A cold case is reopened after 25 years. A stored swab from the crime scene contains about 20 picograms of degraded DNA, equivalent to the DNA in about three cells. The laboratory amplifies 20 STR markers by PCR and separates the fragments by gel electrophoresis. The profile matches a suspect at all 20 markers. The defence points out that when the same database of 5 million people was searched in an earlier case using only 6 markers, the profile matched two unrelated people. For the markers used, about 1 in 10 unrelated people share the pair of fragments of any one marker, and the markers are inherited independently.

   (a)

   Explain how the polymerase chain reaction (PCR) can produce enough DNA from this sample for a profile to be made.

   [6 marks]

   (b)

   Using the data, discuss how strongly the 20-marker profile supports the conclusion that the DNA at the crime scene came from the suspect.

   [6 marks]

   Total for question 4: 12 marks

End of questions