DNA and anticancer drugsAQA A-Level Chemistry: Subtopic test

1. 1
   DNA is the molecule that carries genetic information in the nucleus of a cell. A single strand of DNA is a polymer, built up from repeating units called nucleotides.

   (a)

   Which row correctly gives the three components of one DNA nucleotide?

   [1 mark]

   • APhosphate ion, glucose and a base
   • BPhosphate ion, ribose and a base
   • CPhosphate ion, 2-deoxyribose and a base
   • DTwo phosphate ions and 2-deoxyribose

   (b)

   Which statement correctly describes how two nucleotides are joined in the same strand?

   [1 mark]

   • AA covalent bond forms between the phosphate group of one nucleotide and the 2-deoxyribose of the next
   • BA hydrogen bond forms between the phosphate group of one nucleotide and the base of the next
   • CAn ionic bond forms between the phosphate groups of two nucleotides
   • DA covalent bond forms between the two bases

   (c)

   Describe the structure of a single strand of DNA, including the position of the bases.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   In cells, DNA exists as two strands wound together in a double helix. The two strands are complementary, and each strand can be used as a template when the DNA is copied.

   (a)

   Which pair of bases is held together by two hydrogen bonds in the double helix?

   [1 mark]

   • ACytosine and guanine
   • BAdenine and guanine
   • CCytosine and thymine
   • DAdenine and thymine

   (b)

   A sample of double-stranded DNA contains 28% guanine by number of bases. What is the percentage of adenine in the sample?

   [1 mark]

   • A28%
   • B22%
   • C44%
   • D56%

   (c)

   Explain how hydrogen bonding between bases leads to the two strands of DNA being complementary.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Cisplatin, cis-Pt(NH₃)₂Cl₂, is a Pt(II) complex given by injection to treat some cancers. A dose is calculated from body surface area: 75 mg of cisplatin for every square metre. A patient has a surface area of 1.80 m². Relative atomic masses: Pt = 195.1, N = 14.0, H = 1.0, Cl = 35.5.

   (a)

   Cisplatin binds to DNA by a ligand replacement reaction. Explain what this means and how the bond between platinum and DNA forms.

   [3 marks]

   (b)

   Calculate the number of cisplatin molecules in one dose for this patient. Use the Avogadro constant, L = 6.02 × 10²³ mol⁻¹. Give your answer to three significant figures.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A hospital cancer team is reviewing how DNA is copied before a cell divides, and how the drug cisplatin interferes with this process. The team must also explain to a patient why the drug can be both life-saving and unpleasant to receive.

   (a)

   Describe the structure of DNA and explain how hydrogen bonding between bases allows the two strands to be separated and then copied accurately.

   [6 marks]

   (b)

   Evaluate the use of cisplatin as an anticancer drug. Include an explanation of how it works and why it can have adverse effects.

   [6 marks]

   Total for question 4: 12 marks

End of questions