Discovery
| Parameter | Value |
|---|---|
| Publication date | June 28, 2012 (Science, vol. 337, pp. 816–821) |
| Lead researchers | Jennifer Doudna (UC Berkeley) and Emmanuelle Charpentier (Umeå, Sweden) |
| Model organism | Streptococcus pyogenes (Group A streptococcus bacterium) |
| Effector protein | Cas9 (1,368 amino acids, 158 kDa) |
| Guide RNA length | 20 nucleotides (crRNA) + tracrRNA (~80 nt), fused into sgRNA of ~100 nt |
| Required PAM sequence | 5'-NGG-3' (protospacer adjacent motif) |
| Cut precision | Double-stranded, 3 bp upstream of PAM |
| Nobel Prize | Chemistry 2020 (Doudna and Charpentier) |
Technical Explanation
1. Target recognition — The single chimeric guide RNA (sgRNA) forms a complex with Cas9. The complex scans double-stranded DNA searching for a PAM sequence (5'-NGG-3'). When the PAM is found, Cas9 locally separates the two strands and the guide RNA hybridizes by Watson-Crick complementarity with the 20 nucleotides adjacent to the PAM. If complementarity is sufficient (≤3 mismatches tolerated in the distal region), Cas9 activates its catalytic conformation.
2. Double-strand break — Cas9 has two nuclease domains: RuvC cuts the non-complementary strand, HNH cuts the complementary strand. The cut produces blunt ends 3 base pairs upstream of the PAM. The result: a targeted double-strand break (DSB) in the genome.
3. Cellular repair and editing — The cell repairs the break through two pathways: NHEJ (Non-Homologous End Joining), which introduces random insertions/deletions (indels) and inactivates the targeted gene (knock-out), or HDR (Homology-Directed Repair), which uses a DNA template provided by the researcher to insert a precise sequence (knock-in). The NHEJ:HDR ratio is typically 9:1 in mammalian cells.
4. Delivery — The sgRNA + Cas9 complex is delivered as a plasmid, mRNA + synthetic sgRNA, or pre-assembled ribonucleoprotein (RNP). The RNP method is preferred for therapy: Cas9 is active for 24–48 hours then degraded, limiting off-target effects.
Why It Worked
Zinc finger nucleases (ZFN, 1996) and TALENs (2011) already enabled genome editing, but each new target required engineering a new protein — a 6 to 12-month effort costing $5,000–25,000.CRISPR−Cas9replacesproteinengineeringwithasimplechangeof20nucleotidesinanRNAsynthesizablein48hoursfor75. This shift from protein complexity to the simplicity of DNA-RNA base pairing makes genome editing accessible to any molecular biology lab in the world.
Causal Chain
Yoshizumi Ishino identifies CRISPR sequences in E. coli (1987) → Mojica finds them in archaea (1993) → Bolotin identifies Cas9 (2005) → Barrangou proves adaptive immune function (2007, Science) → Doudna and Charpentier reprogram Cas9 in vitro (2012) → Zhang applies it to human cells (2013) → first approved CRISPR therapy: Casgevy for sickle cell disease (MHRA, November 2023)
Anecdote
The word "CRISPR" was coined in 2002 by Francisco Mojica and Ruud Jansen in an email exchange. Mojica had first proposed "SRSR" (Short Regularly Spaced Repeats), but Jansen found the acronym unpronounceable. CRISPR (Clustered Regularly Interspaced Short Palindromic Repeats) stuck, even though the repeats are not always palindromic.
Sources
References verified during the August 2026 fact-checking audit: these are the pages
against which this bulletin's claims were checked.