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  • I keep hearing that CRISPR is going to revolutionize medicine.

  • The way we fight disease, cure cancer, and maybe even create new humans and I agree with that.

  • But I haven't been able to find great videos out there that explain what CRISPR is.

  • They tend to be too complex or too simple.

  • So I thought I'd throw another video into the mix.

  • I proudly present:

  • Like most things in molecular biology, CRISPR was first identified in E.coli. And if we break apart the acronym it stands for

  • Clustered Regularly Interspaced Short Palindromic Repeats.

  • Now that's a mouthful.

  • But it does tell you the two main parts found in CRISPR.

  • First of all, we have the repeats.

  • These are going to be short segments of DNA, so 20 to 40 letters in length.

  • And they're going to be palindromes.

  • Remember a palindrome is a sequence of letters that read the same left-to right.

  • Like in "never odd or even".

  • So we're going to have these letters that are palindrome.

  • The reason why is that when you transcribe that DNA you make RNA that forms these little hairpin turns.

  • So we've got the repeats. Those are all identical one after another after another after another.

  • But they're interspaced. And so what's in the middle?

  • We're going to have what's called spacer DNA. Now what's interesting about the spacer DNA is that it's not identical.

  • Each segment of spacer DNA is going to be unique. And this puzzled scientists when they identified this back in the 80s and 90s.

  • But in the 2000s, what they found is that that spacer DNA, that's the important DNA, matches up perfectly with viral, especially bacteriophage, DNA.

  • They also identified a number of genes associated with CRISPR.

  • So these are the CRISPR-associated or cas genes.

  • Now those cas genes will make cas proteins.

  • The cas proteins in general are going to be helicases, those are proteins that unwind DNA. And then nucleases, those that cut the DNA.

  • And so the idea was perhaps, this is an immune system for bacteria,

  • a way they could fight their old nemesis, the bacteriophage. And that's exactly what's going on.

  • So if we have a picture of E. coli this would be the cell membrane, cell wall right here.

  • This would be the genome of the bacteria.

  • I'm highlighting the cas and the CRISPR system.

  • And so when the bacteriophage injects its DNA,

  • what normally would happen, if you don't have an immune system, is this DNA would hijack the cell.

  • It could become embedded in the genome. But more importantly it would make a bunch of these bacteriophages and eventually kill the cell.

  • But since it has this CRISPR system, what it is going to do is it's going to transcribe and translate proteins, so this cas complex.

  • And it's also going to transcribe that DNA to make what's called CRISPR RNA and it'll fit right into this protein like this.

  • What is this? It's a way to fight that viral DNA.

  • It essentially breaks it apart and so before the infection starts, the infection essentially has ended.

  • Now you might say "that's interesting, but what happens if it's injecting DNA where we don't have a spacer that matches?"

  • Well the CRISPR-Cas system works there as well.

  • It's going to create a different class of protein, a class 1 cas protein.

  • And what that'll do is it takes the DNA in, it breaks it apart, but more importantly it takes that DNA and copies it into the CRISPR system.

  • So what is CRISPR? It is spacer, repeat, spacer, repeat.

  • But the spacers are essentially history of old infections, so we won't be infected again.

  • This is exactly the way your immune system works on a much larger level.

  • You're making antibodies, and then you have white blood cells that will envelop that invader.

  • But what scientists thought is if we could hijack this CRISPR system, we could perhaps use it.

  • Because this is a living cell here, To either in activate genes or maybe even embed new genes.

  • And so the search was on. And the one that you'll hear most about is the CRISPR Cas9 system.

  • This was identified in the labs of Jennifer Doudna and Emmanuelle Charpentier. and what she was working on was Streptococcus pyogene,

  • and their Cas-CRISPR system. And what's interesting about it is that they only had one cas protein.

  • We call that Cas9

  • Now It doesn't look like this.

  • It looks like this.

  • But if we look at its major structure, it has a nuclease.

  • So it's got this section right here where it can cut DNA here and it can cut dna here as well.

  • In S. pyogenes, they also are creating two long strips of RNA.

  • We have the CRISPR RNA.

  • Crispr RNA is going to fit into the cas.

  • But they also have what's called tracer RNA.

  • So if we look at what that looks like in this bacteria.

  • You've got the spacer segment. That's going to be the part that matches up with the corresponding viral DNA.

  • You have this tracer RNA that essentially holds the CRISPR RNA in place.

  • And then this whole thing together forms this complex where we can break DNA.

  • But what the lab thought is wouldn't it be cool if we could modify this whole system.

  • Use the one Cas9 protein, but let's put our own sequence of DNA right here.

  • And then if we could somehow connect these 2 together, we'd have a really simple system.

  • And that's what they did.

  • They created the tracer RNA-CRISPR RNA chimera.

  • And so what's a chimera? It's this ancient mythological beast, that's a combination of all these different species.

  • And so what they've done is created a new type of RNA.

  • And they've got a system that's really simple.

  • It's got two parts in it.

  • You've got the Cas9 protein and then you've got this chimera.

  • And since we're making the simpler, let's just call this the guide RNA.

  • These are the two parts of a CRISPR-Cas9 system.

  • This is going to be the CRISPR part.

  • It's going to be the RNA that's got the information of where we want to cut.

  • And then we've got the protein that's actually going to do the cutting.

  • And this is what happens.

  • And so if we've got a little bit of DNA,

  • so this is the DNA that we want to cut,

  • we create a guide RNA that's going to have a corresponding bit of RNA.

  • What happens is the DNA'll feed into it like that.

  • Once it's in place, we're going to cut it right here, and we're going to cut it right here.

  • And so we do this little snip, and now we have an inactivated gene, we've broken the gene.

  • Now the cell will try to fix it.

  • It'll do some insertions and deletions, creates mutations.

  • But what we can do a lot of the time is we can inactivate that gene.

  • That's what the bacteria are going to do.

  • But since we've created it, we can cut the DNA wherever we want to cut the DNA.

  • We essentially just have to know what is the sequence of DNA that we want to cut.

  • Put that into the guide RNA.

  • And then we can cut it.

  • Now let's say we want to make this more complex.

  • Not only do we want to break a gene,

  • but let's say we want to insert a new gene.

  • Well now the system is going to just have three parts.

  • We've got the Cas9. We've got the guide RNA.

  • And then we've got the host RNA that we want to put in.

  • So as we break the DNA, the host DNA is going to be added and then the DNA is going to fix it.

  • So essentially we've added the gene to the cell.

  • Now what's cool about the CRISP-Cas9 system, is it does this in living cells

  • and it can cut the DNA in multiple different places.

  • So how could we use this?

  • Well, let's say somebody has cystic fibrosis.

  • What we could do is use a system like this to fix the genes in that person.

  • Or in the future we could engineer a new embryo.

  • You can kind of see where this is going.

  • But more importantly, I hope you know what a CRISPR system is.

  • In review, a CRISPR system is an immune system that was identified in bacteria, and then modified in humans.

  • And I hope that was helpful.

I keep hearing that CRISPR is going to revolutionize medicine.

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CRISPRとは? (What is CRISPR?)

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    zywang に公開 2021 年 01 月 14 日
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