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A-Level Biology October/November 2024 Q3(b)(i): Leber Congenital Amaurosis (LCA) is an inherited eye disease. In LCA, the photoreceptor…
A-Level Biology · Paper 9700/42 · October/November 2024 · Question 3(b)(i) · [3 marks]
Leber Congenital Amaurosis (LCA) is an inherited eye disease. In LCA, the photoreceptor cells in the retina die at an early age. This causes impaired vision (reduced eyesight) in children, which can progress to blindness. Mutations in different genes cause different forms of LCA. One form of this disease, LCA2, is caused by a mutation in the RPE65 gene. Gene therapy has been used to treat LCA2. Outline how an inherited eye disease, such as LCA2, is treated with gene therapy.
A full-marks model answer with a mark-by-mark examiner breakdown is below.
1 answer
- accepted ✓
Gene therapy for LCA2 involves using a vector, typically a harmless virus, to deliver a functional copy of the RPE65 gene into the patient's retinal cells.
- A normal, functional allele of the RPE65 gene is isolated and inserted into the genetic material of a modified virus (e.g., an adeno-associated virus).
- This genetically engineered virus is then injected directly into the retina of the patient's eye.
- The virus infects the retinal cells and delivers the functional RPE65 gene. The gene is then expressed through transcription and translation to produce the functioning RPE65 protein, which restores the function of the photoreceptor cells and improves vision.
How the marks are awarded
- B1 — The first mark is for stating that a normal/functional/healthy gene (or allele) is added to a virus. The model answer achieves this by saying 'A normal, functional allele of the RPE65 gene is...inserted into...a modified virus'.
- B1 — The second mark is for describing the delivery method. The model answer correctly states 'This genetically engineered virus is then injected directly into the retina of the patient's eye'.
- B1 — The third mark is for explaining the outcome at the molecular level. The model answer earns this by stating the 'gene is then expressed...to produce the functioning RPE65 protein'.
Common mistakes
- Stating that the faulty allele is 'removed' or 'replaced'. Somatic gene therapy typically involves adding a functional copy of the gene, not editing the existing faulty one.
- Describing an incorrect delivery method, such as injection into the bloodstream, which would be ineffective for a localised issue in the eye and could trigger a systemic immune response.
- Failing to mention the vector, or using an incorrect vector like a plasmid, which is not the standard method for this type of in vivo therapy.
- Omitting the final crucial step: that the new gene must be expressed (transcribed and translated) to produce a functional protein to have a therapeutic effect.
Examiner tip: For questions that ask you to 'outline' a biological process, structure your answer as a sequence of clear, logical steps to ensure you cover all key stages.
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