QUESTION 4 (6 marks)
a) Describe the roles of helicase and DNA polymerase in DNA replication.
[2 marks]
b) Explain two ways the structure of DNA allows helicase and DNA polymerase to carry out their roles.
[2 marks]
c) Explain how errors during DNA replication can lead to point and frameshift mutations.
[2 marks]
QCAA guide · typeset solution
QCAA sample response and mark allocation
4a) | Helicase unwinds the double helix and separates the strands. Polymerase adds nucleotides to the new strand. | • describes the role of helicase [1 mark] • describes the role of DNA polymerase [1 mark] |
4b) | The weak nature of hydrogen bonds makes it easy for helicase to separate strands. Complementary base pairing allows DNA polymerase to be guided by the template strand. | • explains one way the structure of DNA aids replication [1 mark] • explains a second way the structure of DNA aids replication [1 mark] |
4c) | Mutations occur when DNA polymerase makes an error by inserting, removing or adding an incorrect nucleotide. Substitution reactions involve A, T, C or G being swapped for a different nucleotide, and result in point mutations. Because of the way that codons work, these mutations will only affect one amino acid. Additions or deletions cause frameshift mutations, where all codons after the mutation will be affected. | • explains how errors in DNA replication cause point mutations [1 mark] • explains how errors in DNA replication cause frameshift mutations [1 mark] |
QCAA sample response and marking criteria reproduced from the official guide.
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