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If you have studied restriction enzymes for Class 12 biology, you already know one way to cut DNA: an enzyme finds a fixed short sequence and cuts there. The CRISPR-Cas9 system goes a step further. You can choose the address. This post explains what Emmanuelle Charpentier and Jennifer Doudna found, how the "genetic scissors" work, a worked example on why a programmable address is so precise, and what a student can learn from how the discovery was made.
What did Charpentier and Doudna discover?
The Nobel Prize in Chemistry 2020 went to both of them "for the development of a method for genome editing". The story starts with bacteria. Many bacteria store short pieces of viral DNA between repeated sequences in their own genome, called CRISPR. These act as a memory of past infections.
Charpentier, studying *Streptococcus pyogenes*, found a previously unknown small RNA in large amounts in the bacterium. Her group showed that this molecule, named tracrRNA, is needed for the long RNA copied from the CRISPR region to mature into its working form. The work was published in Nature in March 2011.
That spring, Charpentier met Doudna, an RNA biochemist at Berkeley, at a conference in Puerto Rico and proposed working together. According to the Nobel Committee's popular account, their first test tube experiments failed: Cas9 and the CRISPR RNA alone left the DNA intact. Only when they added tracrRNA was the DNA cut in two.
How does CRISPR-Cas9 cut DNA at a chosen place?
Their 2012 paper in Science laid out the mechanism. The CRISPR RNA (crRNA) pairs with tracrRNA to form a two-RNA structure. This guides the Cas9 protein to DNA that matches the crRNA's guide sequence, and Cas9 makes a double-stranded break. Two parts of Cas9 do the cutting: the HNH domain cuts one strand and the RuvC-like domain cuts the other.
Then came the step that turned a bacterial curiosity into a tool. They fused the two RNAs into one molecule, a single guide RNA, and showed it still directed Cas9 to cut. Change the guide sequence and you change where the cut happens. The Nobel popular account describes how they picked five sites in a gene and cut each one where they intended.
Cas9 also needs a short signal next to the target, called the PAM. The Nobel Committee's scientific background notes that in *S. pyogenes* the PAM is 5'-NGG-3' (N is any base), and the cut falls 3 base pairs upstream of it.
Worked example: why is a programmable address so precise?
This is a back-of-envelope model, assuming DNA is a random string of four bases. Real DNA is not random, so treat the numbers as a feel for scale.
The NCERT Class 12 biotechnology chapter uses EcoRI, which recognises the six-base palindrome GAATTC.
The chance that any given six-base stretch is exactly GAATTC is (1/4)^6 = 1/4096.
So EcoRI's site turns up, on average, about once every 4,096 base pairs.
The NCERT molecular basis of inheritance chapter gives the human genome as 3164.7 million bp. Divide by 4,096 and you expect roughly 770,000 sites. A six-letter address is far too common to edit one gene.
Now suppose the address is 20 letters long. The chance of a match is (1/4)^20, which is about 1 in 1.1 × 10^12.
Expected chance matches in the genome: 3.16 × 10^9 ÷ 1.1 × 10^12 ≈ 0.003. A sequence that long is, in this simple model, very likely to be unique.
Common mistake: writing 4 × 20 instead of 4^20. Each position multiplies the possibilities by four, so the count grows as a power, not a product.
Where is CRISPR used today?
The Nobel Committee lists uses in basic research, crop breeding (including rice that takes up less cadmium and arsenic) and new medical treatments. On 8 December 2023 the US Food and Drug Administration approved Casgevy for sickle cell disease in patients 12 and older, calling it "the first FDA-approved therapy utilizing CRISPR/Cas9".
The Nobel account is also clear about risk: the tool can be misused, and laws in many places prohibit changing the human genome in ways that can be inherited. Ethics is part of the science here, not an afterthought.
What can a student take from how they worked?
Follow the odd result. Charpentier did not set out to build a gene editor. She noticed an abundant RNA she could not explain and kept asking what it did.
Combine different skills. A microbiologist and an RNA biochemist solved together what neither had solved alone.
Treat failure as information. The first experiments did nothing. Asking "what is missing?" led them to add tracrRNA.
Simplify once it works. Fusing two RNAs into one made the system easy for any lab to use.
To build the background for this story, revise how DNA is copied and read in our guide to DNA replication and transcription, and read how Rosalind Franklin's Photo 51 helped reveal the double helix that Cas9 cuts. If terms like PAM, palindrome or sticky end are still fuzzy, the Fermi Concept Library has a lesson for every concept in biology, each placed in a map so you can see what it builds on.
Next step: take the EcoRI calculation above, redo it for a four-base site and an eight-base site, and explain in one sentence why longer addresses are rarer.
