Now this schematic shows a replication fork.
It actually shows a replication fork for a
eukaryotic cell. But it's essentially the same
as a prokaryotic cell with one minor difference.
In this diagram, we can see several proteins.
The replication fork is that fork that's
occurring on the right. So we see the splitting
of the strands that are occurring.
We see the single strand binding proteins, that
we have seen before, attaching to the single strands.
We have the DNA primase which
is laying down the primer
that's used to start replication. But interestingly
this primer is having to deal with both strands;
because, it turns out that in replication
it doesn't occur in that circuital fashion that I used in
the last diagram to show you an easy way of looking at it.
In fact, DNA replication occurs on both strands at
almost the same time. Now that's pretty cool.
This overall process has to be orchestrated
by the enzymes that you see.
So the RNA primer is made and on the top strand
the replication is proceeding from right to left.
And on the bottom strand the replication is
proceeding from left to right and we will see
why that's a case in a minute.
With DNA polymerase which is at the replication
fork has to be handling both of those in E-coli.
In eukaryotic cells that's a little simpler
in that it's two polymerases that are handling,
one handling the top and one handling
the bottom. That's not the case in the
prokaryotic. One is handling both.
We see the helicase which
is peeling apart the strands.
We see the DNA polymerases
that are involved, okay?
And the topoisomerase which is
involved in helping to relieve
that tension ahead of the replication fork.
The DNA ligase is coming in and filling in the pieces
and what we are gonna see is that the top strand
is replicating in a different
way than the bottom strand.
The top strand is part of what we call the lagging strand
and the bottom strand is what we call the leading strand.
It turns out that the leading strand is
being made in one continuous piece
and the lagging strand is being made in many many
many little pieces, each of which, has an RNA primer.
The DNA polymerase, in this case, on the
bottom strand is heading to the right.
The DNA polymerase on the top
strand is heading to the left.
So there is the leading strand and the lagging strand
and we will see more about those in the figure coming up.