How does CRISPR-cas9 work as “molecular scissors”?

Question

I’ve recently learned that CRISPR is like an immine system for some bacteria. How does combining it with the enzyme cas9 make it able to rearrange genes? Those two don’t seem to connect to me

In: Biology

Answer 1

Simplified explanation:

In nature, bacteria are under constant attack of viruses (bacteriophages). These viruses replicate by inserting their DNA into the bacterial DNA and hijacking the internals of the cells to produce more viruses.

Usually bacteria die when they are infected by a bacteriophage. But occasionally a single bacterium can survive the attack and parts of the viral DNA can be left behind. These DNA parts left in the cell (and also passed on to further generations) are CRISPR sequences. They can basically serve as a memory and fingerprint how the viral DNA from the attack in the past looked like.

Cas9 is an enzyme that some bacteria evolved to recognize new viral DNA by their similarity with the CRISPR sequences / guides they already “know” from the past. Cas9 removes that viral DNA from the cell by cutting it out, so it is like an immune system fighting back against viruses.

Researchers have used Cas9 as scissors by basically supplying their own “custom CRISPR sequence” (not quite, but you can think of it like this. It is a guide RNA). Cas9 will then cut similar sequences out like originally in the bacteria, but the researchers can precisely control with the guide what is cut instead of only viral DNA.