The video you just watched does a really nice
job of laying out and animating the process of DNA replication. I want to spend a few slides focusing on the
chat the concepts that students find most challenging. And that has to do with this antiparallel
nature of DNA. And the fact that when you unzip a parental
DNA strand and make daughter strands, one of those strands is made continuously, that's
the leading strand. And then the other strand is made- synthesized
discontinuously. And that's the lagging strand. And of course, it's all about this five prime
to three prime structure of the two strands of DNA. So here's our parent strands right here. And one of the things you should know is that
this is unzipping this direction, okay, the DNA is unwinding this direction. And what's happening is that as you unwind
the DNA on this daughter strand, we're going to be synthesizing five prime to 3 prime And
we know that DNA likes the DNA polymerase, that's what it does, it synthesizes five prime
to three prime. So it's going to go in this direction. And as this continues to open up, it's just
going to keep going. Okay as this opens up now because this parent
strand was three prime to five prime that allowed this daughter strand to be synthesized,
five prime to three prime. This parent strand goes in this direction,
five prime, three prime, and that means this daughter strand is going to be three prime,
and you can't synthesize DNA, this direction, five, three prime to five prime, it can only
go this way. But the DNA is unwinding this direction. So what happens is it unwinds to here, and
a little bit of DNA gets synthesized five prime to three prime and then it unwinds a
little further and the DNA polymerase has to go back and synthesize another piece. And these pieces are called Okazaki fragments
because surprise, surprise, the person who discovered them was named Okazaki - Dr. Okazaki
discovered Okazaki fragments. And after these pieces are synthesized, then
the cell goes back and it hooks these pieces together. Alright, so that's the reason that we have
Okazaki fragments and a lagging strand and then a leading strand where synthesis is continuous. So we call this discontinuous synthesis because
it synthesizes the piece and then it has to go back and synthesize another piece this
just continues to synthesize, right, so it's continuous synthesis. Here's the bigger picture of the whole process. So here's our parent DNA and it's unwinding. So it's unwinding this direction this time. And this shows you our five prime and three
prime strands. So this is this parent strand here is the
this is oriented five prime to three prime, and this parent strand is oriented three prime
to five prime. Okay, so that's going to allow us to orient
ourselves in terms of the leading and lagging strand since the end of this strand is the
three prime end, that means that we can synthesize this daughter strand five prime to three prime,
so it's going to be continuous. And as this replication fork opens up, this
DNA polymerase can just keep going and synthesizing DNA. On this strand, though, remember that - this
is the five prime end of the parent strand, which means that this must be the three prime
and we can't synthesize three prime to five prime. So what has to happen? Well, the DNA has the DNA polymerase has to
synthesize this direction. And so it will make a piece and then as the
replication fork opens up, it'll make another piece and then it'll make another piece. And as this opens up, further, DNA polymerase
will come around and make another piece in here. There are a bunch of different enzymes involved
in this process. So and we're really simplifying the process
quite a bit. As you'll see in this next video, I'm going
to show you. Here's our, these are enzymes called helicase
and gyrase. They both unzip the DNA, they break those
hydrogen bonds and they also allow the unwinding to occur so that you don't cause a super twisting
of the DNA as a result of pulling it apart and, and causing tension in the helix. One of the things that happens when you pull
DNA apart is because the hydrogen bonds you know, releases energy when you form them,
these, these two strands want what they want to get back together, they want to reform
those hydrogen bonds. And so there are proteins called single-stranded
binding proteins that temporarily stabilize the DNA so it can stay single-stranded while
it's been copied. The enzyme DNA polymerase does the copying
of DNA and of course, it does it continuously on the leading strand, and discontinuously
on the lagging strand. It needs a little primer to be built with
RNA polymerase. So RNA polymerase comes in and builds a little
RNA primer and this basically gives the DNA polymerase a jumping-off point. So RNA polymerase doesn't need a primer but
DNA polymerase does. So a little primer synthesized - from that
primer, DNA polymerase makes a fragment. And then ultimately what happens is the RNA
is removed and the two pieces of DNA are hooked together using an enzyme called DNA ligase. So those are some of the major proteins, helicase,
and gyrase involved in an in unzipping and stabilizing the unzipped helix. The single-stranded binding proteins that
allow the DNA strands to remain single-stranded while they're being copied. And then we have RNA polymerase, and which
is also called primase. They refer to it here as primase. We have DNA polymerase and we have DNA ligase. All involved in this process of making DNA. I love this little GIF because it shows the
leading strand and the lagging strand. And this idea that you need these Okazaki
fragments, this is from our friends, the Amoeba sisters. It just really shows you that synthesis on
this end is continuous right? So that as it as the DNA unzips, you see,
it just keeps going. And we'll let it do its repeat. There are our Okazaki fragments on the other
end. So let's watch this, again and we'll focus
on the other strand. So here's our, our five prime to three prime
ends on the parent strand, but we have to go the other way on the daughter strand, right? So as this unzips, you see that we make our
little primer, that's our primase, then we make another piece of DNA. Right, so these are Okazaki fragments here,
it opens up some more. So we've made another Okazaki fragment. And now it's just going to show the process
of - I hope looking these together, maybe it doesn't show the hooking together, but
you get the idea that that's how the Okazaki fragments are formed. So this is really helpful to understand the
problems with leading lagging strand, and kind of fun to watch over and over again. Alright, so I've got one more picture to show
you before I show you the last video. And again, we're really saying the same thing
here over and over, but I think it bears repeating so that you understand it. Here's our DNA replication fork, right. So this is the DNA is unzipping this direction. And we can synthesize five prime to three
prime in this direction. But in this direction, we have to go five
prime to three prime the other way. So we're creating these Okazaki fragments
along the way and they will be connected by DNA ligase after they're synthesized Oh, this
is just a cute little cartoon showing ligase - the other enzyme says hey, ligase what you've
been up to and he says, just making ends meet. And this is just the job of DNA ligase so
if this cartoon helps you remember its role then enjoy. Okay, so I'm going to show you a video that
does a beautiful job showing the true much more accurately the true complexity of DNA
replication, and then we're going to talk a little bit about the specifics of replication
in bacteria. -