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A-Level Biology October/November 2024 Q3(c)(ii): Another method being investigated to treat LCA10 is to use a gene editing tool known as…
A-Level Biology · Paper 9700/42 · October/November 2024 · Question 3(c)(ii) · [3 marks]
Another method being investigated to treat LCA10 is to use a gene editing tool known as the CRISPR/Cas9 system. The CRISPR/Cas9 system uses a short length of RNA called guide RNA. Guide RNA is complementary to the target DNA and is linked to a nuclease enzyme called Cas9. Cas9 breaks phosphodiester bonds in DNA. The cell repair mechanisms repair the cut in DNA after the modification has taken place. • A vector delivers Cas9 and two specific guide RNAs to the photoreceptor cells. • They act on the section of DNA which contains the mutation. • Exon X is no longer added to the mRNA. Explain how this method used to treat LCA10 is an example of gene editing.
A full-marks model answer with a mark-by-mark examiner breakdown is below.
1 answer
- accepted ✓
Gene editing involves making a direct and precise change to an organism's own DNA sequence. In this treatment, the CRISPR/Cas9 system targets the mutated gene within the patient's photoreceptor cells.
The guide RNAs are complementary to specific DNA sequences, ensuring the Cas9 enzyme is directed to a precise location in the genome, on either side of the mutated exon. The Cas9 enzyme then cuts the DNA, causing the section containing the mutation to be deleted.
By removing the mutated exon, the resulting mRNA transcript no longer contains the faulty information. This allows the cell's ribosomes to synthesise a functional CEP290 protein, which restores the normal function of the photoreceptor cells and improves vision.
How the marks are awarded
- B1 — The model answer states that the section of DNA containing the mutation is 'deleted', which directly corresponds to the mark scheme point 'mutated DNA is, deleted / removed / cut out'.
- B1 — The answer explains that the Cas9 enzyme is directed to a 'precise location in the genome', fulfilling the requirement for 'specific (locations)'.
- B1 — The answer links the genetic change to the outcome: 'synthesise a functional CEP290 protein, which restores the normal function of the photoreceptor cells', earning the mark for 'functional protein / CEP290, now made OR correct functioning of photoreceptors'.
Common mistakes
- Confusing gene editing with gene therapy, for example by describing the addition of a new, correct copy of the gene rather than the modification of the existing one.
- Being too vague, for instance stating that 'the gene is changed' without specifying that a section of DNA is cut out or deleted.
- Failing to mention the specificity of the process, thereby omitting the key role of the guide RNA in targeting a precise location in the genome.
- Not completing the explanation, for example by describing the cutting of DNA but failing to link this to the production of a functional protein and the restoration of cell function.
Examiner tip: When explaining a genetic technology, always link the molecular action on the nucleic acid to its ultimate effect on the protein and the organism's phenotype.
AI-generated model answer, grounded in the official Cambridge mark scheme and reviewed by the MarkScheme team. Mark your own answer to this question →
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