CRISPR-Cas9 and genome editing !

CRISPR-Cas9 is RNA-guided 'molecular scissors' that cut DNA at an exact spot to switch off, correct or insert a gene.

Updated 2026-11-01

Key facts

Origin: the immune system bacteria use against viruses (bacteriophages).
The guide RNA (about 20 nt) leads the Cas9 nuclease to the complementary sequence, which must sit next to a PAM motif (NGG).
Cas9 makes a double-strand break; the cell repairs it by NHEJ (small insertions or deletions that knock out the gene) or by homologous recombination with a template (precise correction).
Variants without a double-strand break: base editors and prime editors.
There is already an approved therapy for sickle cell disease and β-thalassemia that reactivates fetal haemoglobin.

The single guide RNA (sgRNA) joins the search sequence and the scaffold that binds Cas9.
NHEJ: useful to knock out a gene. Homology-directed repair: to correct a mutation, but it is less efficient and happens mainly in dividing cells.
Base editors (a nicking Cas9 fused to a deaminase) change C→T or A→G without cutting both strands.
Dead Cas9 (dCas9) fused to activators or repressors switches genes on or off without changing the sequence.
Risks: off-target cuts, mosaicism, immune response to Cas9 and the ethics of germline editing.

The approved therapy edits the BCL11A erythroid enhancer ex vivo in the patient's stem cells, which then make fetal haemoglobin.
In research it is used to build cell and animal disease models and for large-scale gene screens.

Think of the answer, then tap the question to see it.

What sets the exact spot where Cas9 cuts?

The guide RNA sequence, complementary to the target, which must also sit next to a PAM motif.

Which repair route is used to knock out a gene after a Cas9 cut?

NHEJ: it introduces small insertions or deletions that usually shift the reading frame.

Where does the CRISPR-Cas9 system come from?

From the adaptive immune system of bacteria, which stores pieces of viruses to recognise and cut them.

Educational information, not medical advice. For any symptom, see a health professional.

Sources: What are genome editing and CRISPR-Cas9?: MedlinePlus Genetics (MedlinePlus, US National Library of Medicine, 2026); CRISPR (National Human Genome Research Institute (NHGRI), 2026); What is gene therapy?: MedlinePlus Genetics (MedlinePlus, US National Library of Medicine, 2026); Textbook of Biochemistry for Medical Students, 6th ed. (Jaypee Brothers Medical Publishers (Vasudevan DM, Sreekumari S, Vaidyanathan K), 2011).

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Representative model, for educational purposes only.