What is CRISPR?

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.

pexels photo 3761509

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..

As found on YouTube

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