1. 1
   A student is building a physical model of a short RNA molecule and a short DNA molecule for a class display, using different coloured beads for phosphates, sugars and each type of base.

   (a)

   Which pair of components should appear in the RNA model but not in the DNA model?

   [1 mark]

   • ADeoxyribose and thymine
   • BRibose and thymine
   • CDeoxyribose and uracil
   • DRibose and uracil

   (b)

   In the models, which bonds should link adjacent nucleotides within one strand?

   [1 mark]

   • ACovalent bonds between the phosphate of one nucleotide and the sugar of the next
   • BHydrogen bonds between bases
   • CCovalent bonds between the bases of adjacent nucleotides
   • DPeptide bonds between sugars

   (c)

   Apart from the type of sugar, outline two structural differences between the DNA and RNA models.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   In 1952 Alfred Hershey and Martha Chase grew T2 bacteriophages (viruses that infect bacteria) in two batches: one with radioactive sulfur (³⁵S) and one with radioactive phosphorus (³²P). Each batch was allowed to infect Escherichia coli. The mixtures were then shaken in a blender to knock the phage coats off the bacteria and centrifuged, so that the bacteria formed a pellet and the empty phage coats stayed in the liquid (supernatant). In the ³⁵S batch about 80% of the radioactivity was in the supernatant. In the ³²P batch most of the radioactivity was in the pellet. New phages released from the infected bacteria contained some ³²P but almost no ³⁵S.

   (a)

   Why was ³⁵S used to label the protein and ³²P used to label the DNA?

   [1 mark]

   • ASulfur is found only in DNA and phosphorus only in proteins
   • BProteins contain sulfur but DNA does not, while DNA contains phosphorus in its backbone but proteins contain little or none
   • CBoth elements are found equally in DNA and proteins, so either could be used
   • D³²P is less radioactive than ³⁵S, so it is safer for labelling DNA

   (b)

   Which development made the Hershey–Chase experiment possible?

   [1 mark]

   • ADNA sequencing
   • BThe polymerase chain reaction
   • CThe availability of radioisotopes as research tools
   • DGenetic modification of bacteria

   (c)

   Explain how the results support the conclusion that DNA, not protein, is the genetic material.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Erwin Chargaff measured the base composition of DNA from many species. His results, as percentages of all bases, included: human — adenine 30.9, thymine 29.4, guanine 19.9, cytosine 19.8; Escherichia coli — adenine 24.7, thymine 23.6, guanine 26.0, cytosine 25.7; wheat — adenine 27.3, thymine 27.1, guanine 22.7, cytosine 22.8. Before his work, the tetranucleotide hypothesis proposed that DNA consists of a simple repeating sequence of the four bases.

   (a)

   Calculate the ratio of purines to pyrimidines in human DNA and explain the significance of this ratio for the structure of the DNA helix.

   [3 marks]

   (b)

   Discuss how Chargaff's data relate to the tetranucleotide hypothesis, with reference to the problem of induction and falsification.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A human cell contains about 2 m of DNA packed into a nucleus only about 6 μm across. Much of this DNA is wound into nucleosomes. When a gene is to be transcribed, the DNA in that region is unpacked and RNA polymerase moves along one strand, building an mRNA molecule that a ribosome will later translate.

   (a)

   Explain the directionality of DNA and RNA strands and its significance for replication, transcription and translation.

   [6 marks]

   (b)

   Describe the structure of a nucleosome and suggest how nucleosomes affect the storage and use of DNA in the nucleus.

   [6 marks]

   Total for question 4: 12 marks

End of questions