I keep hearing that CRISPR is gonna
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 simples 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
gonna be palindromes remember a
palindrome is a sequence of letters that
read the same left-to-right like never
odd or even so we're gonna 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 gonna 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 space or
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
cast genes now those cast genes will
make cast proteins the cast proteins in
general are going to be HeLa cases 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 bacteria
phage and that's exactly what's going on
so if we have a picture of the e coli
this would be the cell membrane cell
wall right here this would be the genome
of the bacteria I'm highlighting the
casts and the CRISPR system and so when
the bacteria phage injects its DNA what
normally would happen if you don't have
an immune system
this DNA would hijack the cell it could
become embedded in the genome but more
importantly it would make a bunch of
these bacteria phages and eventually
kill the cell but since it has this
CRISPR system what it's going to do is
it's going to transcribe and translate
proteins so this caste 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 does this is 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 caste system works there as
well it's going to create a different
class of protein a class one caste
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 infection 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 inactivate 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 caste 9 system this is
identified in the labs of Jennifer
Doudna and Emanuel Shep shop on taa and
what she was working on was
streptococcus pyogenes and their casts
CRISPR system and what's interesting
about it is that they only had one cast
protein we call that cast 9 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
casts 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 crisper 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 caste nine protein
but let's put our own sequence of DNA
right here and then if we could somehow
connect these two together we'd have a
really simple system and that's what
they did
they created the tracer RNA crispr RNA
Chimaera and so what's a camere it's
this ancient mythological beast it'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 cast
nine protein and then you've got this
Chimaera and since we're making this
simpler let's just call this the guide
RNA these are the two parts of a CRISPR
cast nine 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 this is what happens and so if
we 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 I'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 try
to fix it we'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 what 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
caste nine 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 CRISPR
caste 9 system is it does this in living
cells and it can cut the DNA and molt
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