A double-stranded DNA gene runs through the view. Cas9 must locate one 20-base-pair target sitting just beside a PAM. Press Play or step forward to begin editing.
Imagine you want to fix one typo buried in a book of $3$ billion letters — that is the scale of a human genome. CRISPR-Cas9 is the molecular version of find and replace. The "find" job is done by a short guide RNA; the "cut" is done by a protein called Cas9. Together they hunt down a single target sequence in a vast genome and snip the DNA there, so the cell can change that exact spot.
The guide RNA carries a 20-nucleotide "spacer" that is complementary to the target DNA, plus a folded scaffold that grips Cas9. Cas9 is an enzyme with two cutting tools, the HNH and RuvC domains. But Cas9 will only act next to a short DNA signal called the PAM (for the standard S. pyogenes enzyme, the sequence $5'\text{-}NGG\text{-}3'$, where $N$ is any base). Reprogramming CRISPR to a new gene needs nothing more than writing a new $20$-letter guide.
The order matters: Cas9 first checks for a PAM, then unwinds the local DNA so the guide can test for a match. A correct match lets the guide base-pair with the target strand over all $20$ bases, forming a stable R-loop that locks Cas9 down. Now the two nuclease domains fire — HNH cuts the target strand and RuvC cuts the non-target strand, both about $3$ base pairs from the PAM — leaving a blunt double-strand break. The cut alone is not the edit: the cell's repair, either error-prone NHEJ (random small insertions/deletions, an "indel") or template-guided HDR (a precise change), is what rewrites the gene. The repair pathway selector shows both outcomes of the same cut.
Press Play and watch the full sequence, then use Prev / Next to freeze on the R-loop step and rotate the helix to see the orange guide RNA zipped against the blue target strand. Stop on the cut step and look at how the break sits $3$ bp from the gold PAM. Finally, switch the repair pathway between NHEJ and HDR to compare a knockout against a precise edit.
| Stage | What happens | Key players |
|---|---|---|
| 1 · Target | A target gene with a 20-bp protospacer sitting next to an NGG PAM is selected | DNA, PAM |
| 2 · Complex | Cas9 binds the guide RNA, forming a search-ready ribonucleoprotein | Cas9, guide RNA (sgRNA) |
| 3 · PAM | Cas9 scans the DNA and docks only where it finds a 5'-NGG-3' PAM | Cas9 PAM-reading residues |
| 4 · R-loop | DNA unwinds; the guide base-pairs with the target strand across 20 bases | Guide RNA, target strand |
| 5 · Cut | HNH cuts the target strand, RuvC cuts the non-target strand, ~3 bp from the PAM | HNH & RuvC domains |
| 6 · Repair | NHEJ makes indels (knockout) or HDR copies a template (precise edit) | Cell repair machinery |
Before Cas9 will even test the guide, it must recognize a PAM in the DNA. This ordering speeds the search — Cas9 ignores the vast majority of the genome that lacks a nearby PAM — and, in the bacterium where CRISPR evolved, it protects the cell's own CRISPR memory bank from being cut, because those stored sequences are not flanked by PAMs.
A double-strand break is dangerous, so cells repair it fast. Non-homologous end joining (NHEJ) simply rejoins the ends and works in any cell, but it frequently gains or loses a few bases, producing indels that shift the reading frame and disable the gene — ideal for a knockout. Homology-directed repair (HDR) uses a supplied DNA template to copy in an exact, intended sequence, but it is much less efficient and mostly happens in dividing cells. Newer tools such as base editors and prime editors were developed to make precise changes without relying on a double-strand break at all.
CRISPR-Cas9 is a gene-editing tool made of two parts: the Cas9 protein, which acts like molecular scissors, and a short guide RNA, which tells the scissors where to cut. The guide is designed to match a chosen 20-base-pair stretch of DNA. Cas9 carries the guide along the genome until it base-pairs with its matching target next to a short signal called a PAM, then cuts both strands. The cell repairs the break, and that repair is what edits the gene.
Key takeaway: Cas9 is the scissors and the guide RNA is the address label that makes the cut land on one specific gene.The guide RNA carries a 20-nucleotide spacer complementary to one DNA strand at the target. As Cas9 unwinds short stretches of DNA, the guide tries to base-pair with the exposed strand. A wrong site will not form a stable match and Cas9 lets go; only at the correct site does the guide zip up with the target strand, forming an R-loop that locks Cas9 in place. Because matching depends on base pairing, scientists target a new gene just by changing the 20-letter guide.
Key takeaway: targeting is programmed by base pairing, so reprogramming CRISPR means writing a new 20-letter guide.A PAM (protospacer adjacent motif) is a very short DNA sequence sitting immediately next to the target. For the common S. pyogenes Cas9 it is 5'-NGG-3', where N is any base. Cas9 checks for the PAM before it will unwind the DNA or test the guide, so the PAM acts as a licence to cut. The PAM is part of the target DNA, not part of the guide RNA, and the guide does not pair with it.
Key takeaway: no correct PAM next to the target means no cut, even if the guide matches perfectly.Cas9 has two cutting (nuclease) domains. The HNH domain cuts the strand paired with the guide RNA (the target strand), and the RuvC domain cuts the other strand. Both cuts fall about 3 base pairs upstream of the PAM and line up to leave a clean, blunt double-strand break.
Key takeaway: two domains make two cuts 3 base pairs from the PAM, producing a blunt break across both strands.Cas9 only makes the cut; the cell's own repair machinery makes the edit. Non-homologous end joining (NHEJ) glues the broken ends back together but often adds or deletes a few bases, scrambling the gene and usually switching it off — useful for knockouts. Homology-directed repair (HDR) instead copies a DNA template the scientist supplies, writing in a precise intended change, but it is less efficient and works mainly in dividing cells.
Key takeaway: NHEJ gives quick but messy knockouts, while HDR with a template gives precise edits; the cut just opens the door.CRISPR is cheap, fast, and easy to reprogram, so it has spread across biology in just over a decade. Researchers use it to switch genes off to learn what they do, to build disease models, and to engineer crops and microbes. In medicine it underlies the first approved gene-editing therapy for sickle cell disease and beta thalassemia, with many more in trials. Jennifer Doudna and Emmanuelle Charpentier shared the 2020 Nobel Prize in Chemistry for developing it.
Key takeaway: CRISPR turned gene editing from slow and specialized into something almost any lab can do.CRISPR is a natural adaptive immune system in bacteria and archaea. When a virus attacks, the microbe stores a short piece of the viral DNA in a region of its own genome called a CRISPR array (Clustered Regularly Interspaced Short Palindromic Repeats). If the same virus returns, the cell transcribes that stored piece into a guide RNA that directs Cas proteins to chop up the matching viral DNA. In 2012 researchers showed the system could be simplified into a single guide RNA and reprogrammed to cut any chosen sequence.
Key takeaway: CRISPR is borrowed from a bacterial immune system that remembers viruses, then repurposed to edit DNA on demand.