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how does crispr work step by step
Here's the twist: scientists didn't invent CRISPR. It's a bacterium's immune system, an old defense against viruses called phages, and we borrowed it.
A bacterium that survives an attack saves a snippet of the virus's DNA in its own genome, tucked between repeats, like a mugshot album. That album is CRISPR.
When that virus returns, the cell copies a snippet into a short RNA, the guide, and hands it to Cas9, a protein that cuts DNA. The guide tells Cas9 where.
Scientists borrowed the machine. Cas9 is the blade, and on its own it is blind. The guide RNA, twenty letters we can write to match a gene, is its address.
Cas9 slides along the cell's DNA checking one thing first: a short tag, N, G, G, called the PAM. No tag, no stopping, which also keeps it from cutting its own album.
Next, the guide unzips the DNA beside the tag and tries to pair with it, letter by letter, all twenty. A full match and Cas9 locks on. A bad mismatch, and it lets go.
Then Cas9's two blades each cut one strand, about three letters from the tag. The DNA snaps clean through. Notice what Cas9 did: it only cut.
So where does the edit come from? Not from Cas9. The cell sees a broken chromosome and rushes to repair it, and the repair is where editing happens.
Two routes. Fast, sloppy gluing usually adds or deletes a few letters and breaks the gene. Or, with a template we supply, the cell copies it in and rewrites the gene.
Here's the whole idea. Phage DNA snippets become a guide; the guide steers Cas9 to a matching spot beside a PAM; Cas9 cuts, and the cell's repair does the editing.
A bacterium's old defense became a tool that points at one spot in a genome and lets the cell do the rewriting.