A team of researchers used CRISPR-Cas9 to edit the genome of a crop plant, disabling a gene that normally suppresses disease resistance. Explain how the CRISPR-Cas9 system locates and cuts the target gene at a specific position in the plant's genome.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
CRISPR-Cas9 is a genome-editing tool adapted from a bacterial immune defence mechanism. It has been used in agriculture to modify crop plants by making precise cuts at specific locations in the genome. The system relies on two key components: a guide RNA and the Cas9 endonuclease protein.
Model answer (4 marks)
The guide RNA (gRNA) is designed to be complementary in base sequence to the target DNA sequence.
The gRNA binds to the target DNA strand by hydrogen bonding through complementary base pairing.
The gRNA is bound to the Cas9 endonuclease, forming a complex that directs Cas9 to the correct location in the genome.
Cas9 acts as an endonuclease, cutting both strands of the DNA double helix at the target site, producing a double‑strand break.
The gRNA binds to the target DNA strand by hydrogen bonding through complementary base pairing.
The gRNA is bound to the Cas9 endonuclease, forming a complex that directs Cas9 to the correct location in the genome.
Cas9 acts as an endonuclease, cutting both strands of the DNA double helix at the target site, producing a double‑strand break.
Examiner tips
- Use the exact terms ‘guide RNA’, ‘complementary base pairing’, ‘complex with Cas9’, ‘endonuclease’
- Show the sequence of events: gRNA design → hybridisation → complex formation → DNA cleavage
- Keep each point brief and separate to match the four marks
Common mistakes
- Confusing the gRNA with the target DNA sequence; they are complementary, not identical
- Forgetting to mention that Cas9 cuts both strands, creating a double‑strand break
- Using vague language such as ‘cuts DNA’ without specifying the role of Cas9 as an endonuclease
Mark scheme (4 marks)
- The guide RNA (gRNA) is complementary in base sequence to the target DNA sequence
- The guide RNA binds / hybridises to the target DNA strand by hydrogen bonding (complementary base pairing)
- The guide RNA is bound to / forms a complex with the Cas9 endonuclease, directing it to the correct location in the genome
- Cas9 acts as an endonuclease, cutting both strands of the DNA double helix at the target site, producing a double-strand break
Key terms in this question
Related
- All OCR A-Level Biology A (H420) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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