Replication Fork

by Kevin Ahern, PhD

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    00:01 Now this schematic shows a replication fork.

    00:04 It actually shows a replication fork for a eukaryotic cell. But it's essentially the same as a prokaryotic cell with one minor difference.

    00:13 In this diagram, we can see several proteins.

    00:17 The replication fork is that fork that's occurring on the right. So we see the splitting of the strands that are occurring.

    00:23 We see the single strand binding proteins, that we have seen before, attaching to the single strands.

    00:29 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.

    00:48 In fact, DNA replication occurs on both strands at almost the same time. Now that's pretty cool.

    00:56 This overall process has to be orchestrated by the enzymes that you see.

    01:02 So the RNA primer is made and on the top strand the replication is proceeding from right to left.

    01:10 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.

    01:17 With DNA polymerase which is at the replication fork has to be handling both of those in E-coli.

    01:24 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.

    01:36 We see the helicase which is peeling apart the strands.

    01:39 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.

    01:50 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.

    02:01 The top strand is part of what we call the lagging strand and the bottom strand is what we call the leading strand.

    02:08 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.

    02:21 The DNA polymerase, in this case, on the bottom strand is heading to the right.

    02:25 The DNA polymerase on the top strand is heading to the left.

    02:29 So there is the leading strand and the lagging strand and we will see more about those in the figure coming up.

    About the Lecture

    The lecture Replication Fork by Kevin Ahern, PhD is from the course DNA Replication and Repair.

    Included Quiz Questions

    1. Lagging strand replication is continuous
    2. Primase is responsible for making the primer necessary to start the process
    3. DNA Polymerase I removes the RNA primer
    4. DNA ligase joins together Okazaki fragments

    Author of lecture Replication Fork

     Kevin Ahern, PhD

    Kevin Ahern, PhD

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