The essence of CRISPR is simple:
it’s a way of finding a specific bit of DNA inside a cell. After that, the next step in CRISPR gene editing is usually to alter that piece of DNA.
However, CRISPR has also been adapted to do other things too, such as turning genes on or off without altering their sequence.

The key to CRISPR is the many flavours of “Cas” proteins found in bacteria, where they help defend against viruses. The Cas9 protein is the most widely used by scientists. This protein can easily be programmed to find and bind to almost any desired target sequence, simply by giving it a piece of RNA to guide it in its search.

When the CRISPR Cas9 protein is added to a cell along with a piece of guide RNA, the Cas9 protein hooks up with the guide RNA and then moves along the strands of DNA until it finds and binds to a 20-DNA-letter long sequence that matches part of the guide RNA sequence.
That’s impressive, given that the DNA packed into each of our cells has six billion letters and is two metres long. What happens next can vary. The standard Cas9 protein cuts the DNA at the target. When the cut is repaired, mutations are introduced that usually disable a gene. This is by far the most common use of CRISPR. It’s called genome editing – or gene editing – but usually the results are not as precise as that term implies.
CRISPR can also be used to make precise changes such as replacing faulty genes – true genome editing – but this is far more difficult.
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