A-Level Biology May/June 2025 Q3(e): Suggest and explain how this gene editing procedure will prevent the accumulation of po…

1 answer

• ▲0
  accepted ✓

  The gene editing procedure uses a guide RNA (gRNA) that has a base sequence complementary to the target DNA sequence on the mutant CEP allele. This ensures the editing machinery is directed only to the faulty gene, preventing damage to other genes.

  At the target site, the editing process corrects the point mutation by carrying out a base substitution, replacing the incorrect thymine (T) base with a cytosine (C) base.

  This restores the correct DNA sequence, so transcription and translation will now produce a functional URO-synthase enzyme. This normal enzyme can metabolise porphyrins, preventing their accumulation and the symptoms of CEP.

  ---

  How the marks are awarded

  • M1 — The answer explains how off-target damage is avoided by stating that the guide RNA (gRNA) is specific to the mutant CEP allele, targeting only the faulty gene.
  • M1 — The answer correctly identifies the specific base substitution required, stating that thymine (T) is replaced by cytosine (C).
  • M1 — The answer explains that this correction leads to the production of a functional enzyme, which is the direct mechanism for preventing porphyrin accumulation.

  Common mistakes

  • Forgetting to explain how damage to other genes is prevented, by omitting any mention of the guide RNA's specificity.
  • Using vague language like 'the gene is fixed' or 'the mutation is corrected' without specifying the exact base substitution from T to C.
  • Failing to mention the production of a 'functional enzyme', instead just saying a 'new protein' is made, which is less precise.
  • Confusing the role of the enzyme with the porphyrins, for example by saying 'the enzyme accumulates'.

  Examiner tip: Always break down 'Suggest and explain' questions into their component parts and use precise biological terminology for each point in your explanation.

  ---

  AI-generated model answer, grounded in the official Cambridge mark scheme and reviewed by the MarkScheme team. Mark your own answer to this question →

  @markscheme_answers

Your answer

Sign in to answer this question.