Leading and lagging strands in DNA replication | MCAT | Khan Academy

- [Voiceover] Let's talk
a little bit in more depth

about how DNA actually copies itself,

how it actually replicates,

and we're gonna talk about

the actual actors in the process.

Now, as I talk about
it, I'm gonna talk a lot

about the 3' and 5' ends
of the DNA molecule,

and if that is completely
unfamiliar to you,

I encourage you to watch the video

on the antiparallel structure of DNA.

And I'll give a little bit
of a quick review here,

just in case you saw it but
it was a little while ago.

This is a zoom-in of DNA,

it's actually the zoom-in from that video,

and when we talk about the 5' and 3' ends,

we're referring to what's
happening on the riboses

that formed part of this
phosphate sugar backbone.

So we have ribose right over here,

five-carbon sugar,

and we can number the carbons;

this is the 1' carbon,
that's the 2' carbon,

that's the 3' carbon,
that's the 4' carbon,

and that's the 5' carbon.

So this side of the ladder,

you could say,

it is going in the ...

it is going, let me
draw a little line here,

this is going in the 3' to 5' direction.

So this end is 3' and then this end is 5'.

It's going 3' to 5'.

Notice three, this phosphate
connects to the 3',

then we go to the 5'
connects to a phosphate,

this connects to a 3',

then it connects--

then we go to the 5'
connects to a phosphate.

Now on this end, as we
said it's antiparallel.

It's parallel, but it's
oriented the other way.

So this is the 3', this is the 5',

this is the 3', this is the 5'.

And so this is just
what we're talking about

when we talk about the
antiparallel structure.

These two backbones,

these two strands are
parallel to each other,

but they're oriented
in opposite directions.

So this is the 3' end
and this is the 5' end.

And this is gonna be really important

for understanding replication,

because the DNA polymerase,

the things that's adding
more and more nucleotides

to grow a DNA strand;

it can only add nucleotides on the 3' end.

So if we were talking
about this right over here,

we would only be able to add …

We would only be able
to add going that way.

We wouldn't be able to add going …

We wouldn't be able to add going that way.

So one way to think about it
is you can only add nucleotides

on the 3' end or you can only extend …

You can only extend DNA
going from 5' to 3'.

If you're only adding on the 3' end,

then you're going from the
5' to the 3' direction.

You can't go from the
3' to the 5' direction.

You can't continue to add on
the 5' side using polymerase.

So what am I talking
about with polymerase.

Well let's look at this
diagram right over here

that really gives us an overview

of all of the different actors.

So here is just our of our DNA strand,

and it's, you can imagine
it's somewhat natural,

in it's natural unreplicated form,

and you could see we've labeled
here the 3' and the 5' ends,

and you could follow
one of these backbones.

This 3', if you follow
it all the way over here,

it goes, this is the corresponding 5' end.

So this and this are the same strand,

and this one, if you follow it along,

if you go all the way over
here, it's the same strand.

So this is the 3' end,

and 3' end of it and then
this is the 5' end of it.

Now the first thing,

and we've talked about
this in previous videos

where we give an overview of replication,

is the general idea is that
the two sides of our helix,

the two DNA, the double-helix
needs to get split,

and then we can build another,

we can build another side of the ladder

on each of those two split ends.

You could really view this
as if this is a zipper,

you unzip it and then you put
new zippers on either end.

But there's a lot of--

in reality, it is far more
complex than just saying

"Oh, let's open the zipper
and put new zippers on it."

It involves a whole bunch of
enzymes and all sorts of things

and even in this diagram,

we're not showing all
of the different actors,

but we're showing you the primary actors,

at least the ones that
you'll hear discussed

when people talk about DNA replication.

So the first thing that needs to happen,

right over here, it's all
tightly, tightly wound.

So let me write that,

it is tightly, tightly wound.

And it actually turns out,

the more that we unwind it on one side,

the more tightly wound
it gets on this side.

So in order for us to unzip the zipper,

we need to have an enzyme

that helps us unwind
this tightly wound helix.

And that enzyme is the topoisomerase.

And the way that it actually works is

it breaks up parts of the
back bones temporarily,

so that it can unwind and
then they get back together,

but the general high-level
idea is it unwinds it,

so then the helicase enzyme,

and the helicase really doesn't look like

this little triangle
that's cutting things.

These things are actually
far more fascinating

if you were to actually see a--

the molecular structure of helicase.

But what helicase is doing

is it's breaking those
hydrogen bonds between our …

Between our nitrogenous bases,

in this case it's an adenine
here, this is a thymine

and it would break that
hydrogen bond between these two.

So, first you unwind it,

then the helicase,

the topoisomerase unwinds it,

then the helicase breaks them up,

and then we actually think about

these two strands differently,

because as I mentioned,

you can only add nucleotides

going from the 5' to 3' direction.

So this strand on the
bottom right over here

which we will call our leading strand,

this one actually has a
pretty straightforward,

remember this is the
5' end right over here,

so it can add,

it can add going in that direction,

it can add going in that
direction right over here.

This is the 5' to 3',

so what needs to happen here
is to start the process,

you need an RNA primer

and the character that puts an RNA primer,

that is DNA primase.

We'll talk a little bit more about

these characters up here
in the lagging strand,

but they'll add an RNA,

let me do this in a color you can see,

an RNA primer will be added here,

and then once there's a primer,

then DNA polymerase can just
start adding nucleotides,

it can start adding
nucleotides at the 3' end.

And the reason why the leading
strand has it pretty easy

is this DNA polymerase right over here,

this polymerase, and once again,

they aren't these perfect
rectangles as on this diagram.

They're actually much more
fascinating than that.

You see the polymerase up there,

you also see you one
over here, polymerase.

This polymerase can just,

you can kind of think of it
as following the opened zipper

and then just keep adding,

keep adding nucleotides at the 3' end.

And so this one seems
pretty straightforward.

Now, you might say wouldn't it be easy

if we could just add
nucleotides at a 5' end,

because then we could say

well this is going from 3' to 5',

well maybe that polymerase
or different polymerase

could just keep adding
nucleotides like that,

and then everything would be easy.

Well, it turns out that
that is not the case.

you cannot add nucleotides at the 5' end,

and let me be clear,
this 3' right over here,

this, I'm talking about this strand.

This strand right over here,

this, let me do this in another color,

this strand right over here,

this is the 3' end, this is the 5' end,

and so you can't,

you can't just keep adding
nucleotides just like that,

and so how does biology handle this?

Well it handles this by adding primers

right as this opening
happens, it'll add primers,

and this diagram shows the
primer is just one nucleotide

but a primer is typically
several nucleotides,

roughly 10 nucleotides.

So it'll add roughly 10 RNA
nucleotides right over here,

and that's done by the DNA primase.

So the DNA primase is
going along the lagging,

is going along this side,
I can say the top strand,

and it's adding,

it's adding the RNA primer,

which won't be just one nucleotide,

it tends to be several of them,

and then once you have that RNA primer,

then the polymerase can add
in the 5' to 3' direction,

it can add on the 3' end.

So then it can just start adding,

it can just start adding DNA like that.

And so you can imagine this process,

it's kind of, you add the
primase, put some primer here,

and then you start building
from the 5' to 3' direction.

You start building just like that,

and then you skip a little bit
and then that happens again.

So you end up with all
these fragments of DNA

and those fragments are
called Okazaki fragments.

So, it's a Okazaki fragments,

and so what you have happening
here on the lagging strand,

you can think of it as,

why is it called the lagging strand?

Well you have to do it in this kind of …

it feels like a sub-optimal way

where you have to keep creating
these Okazaki fragments

as you follow this opening,

and so it lags, it's going
to be a slower process,

but then all of these
strands can be put together

using the DNA ligase.

The DNA ligase; not only will
the strands be put together,

but then you also have the
RNA being actually replaced

with DNA and then when all is said done,

you are going to have a strand
of DNA being replicated,

or being created right up here.

So when it's all done, you're
gonna have two double strands,

one up here for on the lagging strand,

and one down here on the leading strand.