The big idea: CRISPR-Cas9 edits a gene at one chosen place: a guide RNA finds the target sequence and the enzyme Cas9 cuts the DNA there.
The points to remember
- A guide RNA is made with bases complementary to the target sequence in the gene.
- Cas9 is an enzyme (a nuclease) that carries the guide RNA.
- Where the guide RNA pairs with the DNA, Cas9 cuts both strands.
- As the cell repairs the cut, the gene can be knocked out, or a corrected sequence inserted from a supplied template.
- So bases can be replaced, added or removed at one precise site.
Remember it as: Guide finds, Cas9 cuts, repair edits.
Real example: Casgevy. In 2023 the UK and then the USA approved Casgevy, the first CRISPR-Cas9 treatment, for sickle-cell disease. A patient's blood stem cells are edited outside the body so that they make fetal haemoglobin, which does not sickle, then returned to the patient.
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Uses, ethics and rules: Editing can correct the three kinds of mutation, but some uses raise serious ethical questions.
| Mutation in the gene | What editing can do |
|---|---|
| Substitution (wrong codon) | Replace the wrong base with the correct one |
| Deletion | Put the missing bases back in |
| Insertion | Remove the extra bases |
| Harmful gene | Knock the gene out so it no longer works |
Ethics and regulation: Editing embryos changes the germ line, so the change is inherited. In 2018 a scientist in China edited embryos that became twin girls; he was condemned worldwide and jailed. Countries regulate gene editing differently, so there is an international effort to harmonise the rules.
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How it is asked: 1A items on the function of Cas9 or what CRISPR treatment consists of, and a longer Explain on correcting a mutation by gene editing.
A boy has a mutation that changes one codon in a gene, so his protein does not work. Explain how gene editing could correct the protein.
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