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
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.
Viruses inject DNA into bacteria in hopes that the bacteria will use that DNA to make more copies of the virus. Its hard for the bacteria to actually have a mechanism of figuring out what is its own DNA and what isn’t so the bacteria will usually just mindlessly make the stuff the DNA encodes, in this case that virus.
CRISPR is a system where some virus DNA is actually stored as part of the bacteria’s DNA in a special section. Then you have the CAS9 protein which kind of acts as police, it goes around and if it finds a floating piece of DNA out there, it will try to compare it to the DNA in the bacteria’s genes in the “enemy virus” section and if its a match, it snips the DNA.
Usually double stranded snips like this mean the end for the virus’s schemes, that DNA can no longer really be used for anything nefarious as its like building a house but with only half the blueprint, wont work.
However other organisms do have other proteins that can be used for double strand repair here. And if we inject some other DNA along side these proteins for double strand repair, there’s a good change the this additional DNA will be incorporated in the middle of the snip when the cut is fixed.
So with this system, we can generally design something that cuts at a specific place in DNA with CAS9 then add some more DNA in the middle and repair it. Up until this point we have been able to mess around with and make DNA in a lab setting with specific sequences but we could never actually get it into the genome of something alive. This solves that problem.
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