Leading and Lagging Strand

by Kevin Ahern, PhD

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    00:01 So it's important to understand what leading strand replication and lagging strand replication are and why they occur. Why does the cell make it hard? Well the answers to the "Why?" is rooted in two things.

    00:13 One we know that the strands of DNA are anti-parallel.

    00:17 The top strand is going 5 prime to 3 prime and the bottom strand is going 3 prime to 5 prime, okay? The other reason why this is important is that DNA polymerases can only work in the 5 prime to 3 prime direction.

    00:32 Now that's important; because, that means that we can't, for example, start replicating the top strand and remember that the top strand here has the 3 prime end on the left side and the 5 prime end on the right side.

    00:45 Which means that when replication of it starts in the anti-parallel fashion it's going to start 5 prime with a new strand.

    00:52 Here is the replication of the top strand and here is the replication of the bottom strand if it would occur in the same direction.

    01:00 Well it can't occur in the same direction; because, this would mean the polymerase would have to be working 3 prime to 5 prime.

    01:07 And I have already given you the rule that polymerase only works 5 prime to 3 prime.

    01:13 So this scheme that we see on this up here cannot happen.

    01:18 Otherwise we will make either a DNA that's not anti-parallel or we would be violating the rule of the DNA polymerase that it can't go 3 prime to 5 prime. There is a problem.

    01:28 So instead of doing what I showed on this screen, the replication of the bottom strand has to start internally and then move leftwards and we will see how that happens here.

    01:40 So we have seen now that the top strand which is actually the leading strand on this replication system, the top strand has advanced far along and it has opened up some regions on the bottom strand in green.

    01:54 As the top strand has been peeled further and further away more of the bottom strand has been open and the primase, which made the primer, and the DNA polymerase have started pointing things that way. And the further in the replication fork gets in, the more of the bottom strand is exposed.

    02:13 So the more exposure allows for more fragments.

    02:16 So we can see here is the leading strand in one piece and here is the lagging strand.

    02:21 And by the way, as the figure says, the lagging strand has fragments have a name, they are called Okazaki Fragments, named for the person who discovered them.

    02:31 As this replication proceeds we can now see that the leading strand is still staying in one piece.

    02:37 Where the lagging strand is having the catch up in the individual pieces moving leftwards here.

    02:42 So this ultimately results in the bottom strand, even though it starts out in multiple pieces being joined together into one bigger strand and that happens because of what we saw earlier.

    02:57 The removal of the RNA primers and the joining of the pieces by DNA ligase.

    03:02 So that ligase plays a really important role in making that bottom Okazaki Fragment strand contiguous.

    03:11 And what happens with that then is we will ultimately completely replicate both strands and have the situation that you see on the screen.

    03:19 Now I showed you in the circuital figure earlier the removal of primers and I did that for a purpose even though I cheated a little bit in making that replication.

    03:29 I did it to show you an important concept.

    03:33 The important concept is that as we go around a circular DNA molecule it will always come to back where you started even if it took couple of steps to get there.

    03:43 That means it's easy to remove the primer and when you remove the primer it's easy to fill in the gap and you have two complete strands.

    03:50 Eukaryotic cells don't have that luxury.

    03:53 The DNA in eukaryotic chromosomes is in linear, as you can see on the screen.

    03:59 Now if you replicate a linear system and you start with a RNA primer you can remove them but then you can't replace them; because, you have nothing else coming back around to help you with that, okay? Those sequences that were primers at the end of a linear eukaryotic DNA are lost with each round of replication.

    04:22 That means, therefore, that linear eukaryotic chromosomes get shorter every time they replicate.

    04:28 And that means that the DNAs in a person like me who is probably older than a person like you, has shorter DNAs; because, of this phenomena. This has enormous human health implications.

    04:42 An important question about the linear chromosomes in eukaryotic cells is "Why do they not disappear?" Well to answer that question we have to learn a little bit about the structure of eukaryotic chromosomes.

    04:53 So we can see on this diagram a eukaryotic linear chromosome that has gone through different levels of replication. The one on the top has had the least number of replications and the one on the bottom has had the most.

    05:04 As you can see that blue portion of the chromosome is getting shorter and shorter with each round of replication.

    05:11 Now that portion of the chromosome is a very critical part of it.

    05:15 That critical part of the chromosome is known as a telomere.

    05:19 And the telomere is actually built to be short, as we shall see.

    About the Lecture

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

    Included Quiz Questions

    1. They occur on the lagging strand
    2. They refer to the short RNA primers used to start DNA replication
    3. They are made 3’ to 5’
    4. They are not found in eukaryotic cells
    1. DNA ligase
    2. DNA polymerase III
    3. Primase
    4. Helicase
    5. DNA gyrase
    1. The length of the chromosomes keeps on reducing
    2. The length of the chromosomes keeps on increasing
    3. The sex chromosomes get joined at the free ends by covalent bonding
    4. The number of chromosomes decreases
    5. The length of each gene gets decreased to half

    Author of lecture Leading and Lagging Strand

     Kevin Ahern, PhD

    Kevin Ahern, PhD

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