1. 1
   A student builds a physical model of a DNA molecule using coloured beads and rods to represent nucleotides, showing the double helix structure, the sugar-phosphate backbone and the bases that hold the two strands together. The teacher asks the student to explain the structure shown by the model in terms of real DNA chemistry.

   (a)

   One bead in the model represents a five-carbon sugar molecule. Which sugar does this bead represent in a real DNA nucleotide?

   [1 mark]

   • AGlucose
   • BRibose
   • CDeoxyribose
   • DSucrose

   (b)

   In the student's model, one strand of the double helix has the base sequence A-T-G-C-A reading along it. According to the rules of complementary base pairing, which base sequence must appear on the opposite strand, read in the same direction?

   [1 mark]

   • AT-A-C-G-T
   • BA-T-G-C-A
   • CG-C-A-T-G
   • DC-G-T-A-C

   (c)

   Describe the overall structure of a DNA molecule shown by the student's model, referring to the double helix, the sugar-phosphate backbone and the arrangement of bases.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A biotechnology company has inserted the human insulin gene into bacterial cells so that the bacteria produce human insulin protein for use as a medicine. Inside each bacterial cell, the inserted gene is used to direct the production of insulin molecules, which are then extracted and purified for patients with diabetes.

   (a)

   Inside the bacterial cell, an enzyme first copies the base sequence of the insulin gene into a molecule of messenger RNA. What is this process called?

   [1 mark]

   • ATranslation
   • BTranscription
   • CReplication
   • DRespiration

   (b)

   Once the mRNA copy of the insulin gene has been made, it moves to a cell structure where its base sequence is used to join amino acids together in the correct order to build the insulin protein. At which structure does this process occur?

   [1 mark]

   • ACell membrane
   • BMitochondrion
   • CNucleus
   • DRibosome

   (c)

   Explain how the base sequence of the inserted insulin gene ultimately determines the sequence of amino acids in the insulin protein produced by the bacteria.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A worker at a nuclear power plant is accidentally exposed to a high dose of ionising radiation during a maintenance procedure. Doctors are concerned that this exposure may have caused mutations in the DNA of some of the worker's cells, and they want to explain to the worker how such mutations arise and what effects they could have on the proteins the worker's cells produce.

   (a)

   Describe what is meant by a mutation and explain how ionising radiation can cause this type of change to occur in DNA.

   [3 marks]

   (b)

   One of the mutations caused by the radiation exposure occurs within a gene that codes for an enzyme involved in an important metabolic reaction in the worker's liver cells. Explain how a mutation in this gene could affect the structure and function of the enzyme it codes for.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Phenylketonuria (PKU) is an inherited metabolic disorder caused by a mutation in a gene that codes for an enzyme needed to break down the amino acid phenylalanine. In people with PKU, the mutated gene produces a faulty version of this enzyme, so phenylalanine builds up in the blood and can cause serious harm to the developing brain if the condition is not detected and managed early in life.

   (a)

   Explain, in terms of DNA, transcription, translation and protein structure, how a single mutation in this gene can lead to a non-functional enzyme and the metabolic problems seen in PKU.

   [6 marks]

   (b)

   Because PKU can cause serious, irreversible harm if untreated, many countries screen all newborn babies for the condition using a simple blood test shortly after birth, so that an affected baby can begin a controlled low-phenylalanine diet immediately. Discuss the biological reasoning behind newborn screening for a condition like PKU, and evaluate one benefit and one limitation of population-wide genetic or metabolic screening programmes of this kind.

   [6 marks]

   Total for question 4: 12 marks

End of questions