- [Voiceover] All right, so
in this video, we're gonna be talking about something
known as restriction enzymes. Now, what are restriction enzymes? Well, let's go through an example, and hopefully that'll
help answer that question. So let's imagine that
we've got a bacteria. And it's just floating
around, it's doing its thing, and out of nowhere comes this virus. And the virus is just kinda upset today, so it decides that it wants
to infect this bacteria. So, what the virus does is
it goes over to the bacteria, and basically it attaches to it, and it injects viral
DNA into the bacteria. So this is the viral DNA, and
I'll just label that here, so viral DNA. And the bacteria, being a living creature, also has its own DNA. So this right here in the
blue, is the bacterial DNA, and I'll just label that here. So bacterial DNA. So basically, the virus
infected the bacteria. But the bacteria wants
to figure out some way to destroy the viral DNA. And one way that it can do that, is by labeling its own DNA. So let's imagine that the
bacteria labels its DNA with these purple dots. So what these purple dots actually are, is a methyl group. So we'll say that these
purple dots are methyl groups. And in order for the bacterial
DNA to be methylated, there's an enzyme known as methylase. So methylase is an enzyme
that basically floats around, and as bacterial DNA is synthesized, this enzyme methylase goes around and methylates bacterial DNA. Now, what this basically does, is it allows the bacteria
to recognize its own DNA, and recognize any DNA that's
not methylated as foreign DNA. So what we now have is another
enzyme that's floating around in the cytoplasm of the bacteria. And that enzyme is known
as a restriction enzyme. So this restriction enzyme. So this restriction enzyme
is kind of floating around, it's doing its thing, and
it recognizes the methylated bacterial DNA, but then
it sees this foreign, unmethylated DNA, and
it goes and destroys it. So this is basically a way for bacteria to protect itself from
being infected by viruses. And it basically does so
by methylating its own DNA, and destroying any other foreign
DNA that is unmethylated. So the reason that this is
called a restriction enzyme, is because researchers were noticing that certain bacteria were actually, they were resistant to
being infected by viruses. So they were basically
restricting viral growth. Thus, when they figured out
what enzyme is causing that, they basically called
it a restriction enzyme, 'cause it restricted
the growth of viruses. Now, in order for the
restriction enzyme to work, it needs to recognize a specific sequence, a specific bacterial sequence. So let's go ahead and write
out an examaple sequence. So imagine that, if we zoomed down to the nucleotide sequence of the bacterial DNA, let's imagine that maybe the
sequence was G-A-A-T-T-C. And of course, if this was one strand, then the sister strand would be G-A-A-T-T-C. Now, I don't know if you notice, but there's something unique
about this specific sequence. So if you read this top
line from left to right, G-A-A-T-T-C, and you read this bottom
strand from right to left, G-A-A-T-T-C, they're basically identical. And this is known as a
palindromic sequence. So palindromic sequence. Now, basically what a
palindromic sequence is, is exactly this and it's what
restriction enzymes recognize. So let's imagine that we
have a restriction enzyme, we'll just give it a
name, let's imagine that its name is EcoR1, so EcoR1. That's the name of one
type of restriction enzyme. EcoR1 is actually able to recognize this palindromic sequence. So as EcoR1 is floating
around the bacterial cell, it'll recognize this sequence, and if it's methylated, it won't touch it. But if it's unmethylated,
as in the viral DNA, it'll actually cleave it, so it'll come here, and it'll cleave it. Let me actually use a different color. So it'll cleave the DNA. And it'll cut it like so. And what that'll do is
it'll result in two strands. Because it just cut the DNA. So now what we're gonna
have is we're gonna have one strand that's gonna be G, and then this bottom strand, T-T-A-A, and then we're gonna have
this top part over here. So we're gonna have an A-A-T-T-C, and then a G down here. So basically, now these two strands are just floating around,
and these ends down here, so this end kinda hanging over the edge, they're known as sticky ends. So these are known as sticky ends. And the sticky ends
basically will float around, and if they contact one another, since they're complementary strands, they'll just reanneal. So normally if this gets cut, then you'll have two sticky ends, they'll float around and
then they'll reanneal. And we can actually take
advantage of the fact that we can have the strands reanneal, and we'll talk about different ways that we can actually use that
for medicinal purposes. So one way that we can
use this to our advantage, would be, let's say that we
wanna synthesize human insulin. So let's say that I
wanna make human insulin. So how can I make human insulin using the restriction enzyme technology? Well, let's imagine that we
have a bacteria over here. And this bacteria has
its bacterial DNA inside. And what we can basically do, is take this bacterial
DNA, so let's extract it out of the cell, so
we'll move it out here. So here's the bacterial DNA. What we'll do is we'll take EcoR1, and we'll basically cleave the
bacterial DNA at this site. So let's say that we cut it right here. So if we cut it right
there and then we kinda unroll the bacterial DNA,
now we have this space. And this space basically
has these sticky ends here. So I'll just fill those in. So let's say that there's a G C-T-T-A-A, and then over here on this side, I don't know if I have enough space, we'll have an A-A-T-T-C and a G. So now we've got these
sticky ends over here. And what we can go ahead and do is we can take the human insulin gene. So we'll take human insulin,
so this is the insulin gene. And we'll have exposed
it to EcoR1 already. So now it's got the sticky ends over here. So now on this side we've got an A-A-T-T-C and a G, and then over here on
this side we've got a G, and a C-T-T-A-A. So now what we can do is we
can take this whole thing and just plop it right in here. And as you can see, this part of the insulin gene will reanneal
with this right here. And then this part of the insulin gene will reanneal with this
part of the bacterial DNA. And basically now what we have is we have the insulin gene inserted
into the bacterial DNA. And what that'll basically
allow the bacteria to do is it'll allow it to
synthesize human insulin. And now what you do is
you just take that insulin and you purify it, and
now you can basically have a whole bunch of insulin that's made very cheaply and very quickly for diabetic patients that
might need the insulin. So that's one example of
how restriction enzymes can be used in the pharmaceutical and biotechnological world.