Replication Fork

by Kevin Ahern, PhD

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    00:01 Now, this schematic shows a replication fork.

    00:04 In this diagram, we can see several proteins.

    00:09 The replication fork is that fork that's occurring on the right so we see the splitting of the strands that are occurring.

    00:15 We see the single-strand binding proteins that we've seen before, attaching to the single-strands.

    00:21 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 circular fashion that I used in the last diagram to show you an easy way of looking at it.

    00:41 In fact, DNA replication occurs on both strands in almost the same time. That's pretty cool.

    00:48 This overall process has to be orchestrated by the enzymes that you see.

    00:54 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'll see why that's the case in a minute.

    01:10 The DNA-polymerase which is at the replication fork has to be handling both of those in e-coli.

    01:16 In eukaryotic cells, it's a little simpler and that there are two polymerases that are handling, one handling the top and one handling the bottom.

    01:24 That's not the case in prokaryotes- one is handling both.

    01:27 We see the helicase which is peeling apart the strands.

    01:30 We see the DNA polymerases that are involved and the topoisomerase which is involved in helping to relieve that tension ahead of the replication fork.

    01:42 The DNA ligase is coming in and filling in the pieces and what we're going to see is at the top strand is replicating in a different way than the bottom strand.

    01:53 The top strand is part of what we call the lagging strand and the bottom strand is we call the leading strand.

    01:59 It turns out that the leading strand is made in one continuous piece.

    02:03 Whereas the lagging strength is made up of many, many little individual pieces known as Okazaki fragments.

    02:10 Each of these fragments contains an RNA primer and its five prime end The DNA polymerase, in this case, on the bottom strand is heading to the right.

    02:18 The DNA polymerase on the top strand is heading to the left.

    02:22 So there's the leading strand and the lagging strand and we'll 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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