In this video, we continue our discussion of DNA replication in terms of the lagging strand and the leading strand. Now, here, after separating the double helix, replication creates 2 new strands. We're going to say the strand types are dependent on the direction of the replication fork. Here we have our leading strand and our lagging strand. We're going to say the leading strand is the continuous replication in the same direction as the replication fork movement, and only 1 RNA primer is required for replication. If we come down here and take a look at this image, remember, we have our old DNA in blue, our new DNA being laid down in green, and then our RNA primer that's being put down by primases to help initiate replication. Now, imagine that we have our origin of replication, and it's over here. Everything that's going on is to the left of that right now. We're just paying attention to half of our replication bubble.
If we take a look at this, we're going to look at our leading strand first. We say that we put down our primer and remember, when putting down our primer and starting to create new DNA, it always goes in the 5' to 3' direction. So here, this is the 3' end of our old DNA. If we're going from 5' to 3', that means we have to lay down our initial primer and then the new DNA on the 5' end of itself. And it's going to go this way towards 3' for itself. 3' for itself would correlate to 5' of the old DNA strand. Now, we have our helicase here which is cutting through the hydrogen bonds of our nitrogenous bases. So this is our replication fork here. Our helicase is moving in this general direction, and so our replication fork is moving in that same direction. The leading strand puts down new DNA in that same direction. It's moving this way, in the same direction as the helicase and the replication fork. So as this is becoming more and more open, we're putting more and more DNA down.
The lagging strand though is the opposite. Here, it is a discontinuous replication in the opposite direction as the replication fork movement. Here we're going to create what are called Okazaki fragments, which are multiple, small replicated segments that require an RNA primer. So if we take a look here, we're going to say that an Okazaki fragment is just a small segment of new DNA coupled with our primer that's been put down. So we can see that we have 1, 2, 3, 4, 5 Okazaki fragments shown here. Collectively altogether, this is what is the lagging strand. So our lagging strand is just a collection of Okazaki fragments.
Now, how does this work? Well, our replication fork and our helicase are going in this direction. We see that new DNA is being put down in this direction. It's going the opposite way. This indicates a lagging strand. Now, the way it works is as we're opening up this DNA helix with our replication bubble being formed we're putting down a primer. So, a primer gets laid down here as we open up and then new DNA starts getting created. But then what happens? This mechanism keeps sliding down this way, opening up the DNA strand even more. And as it opens up even more, then another primer has to be put down here and new DNA moves forward. So, we're basically waiting for it to open up so we could put a primer there and then grow this way. It opens up a little bit further down here, we put another primer, it moves down that way. We're waiting for it to open up even more the DNA double helix, so we can put a primer and then move in the opposite direction. This is going to cause small gaps in between our Okazaki fragments. Later on, we'll see how we can connect these gaps together at the end of DNA replication. But for now, our lagging strand, it puts down new DNA in the opposite direction of the replication fork movement. This creates these Okazaki fragments and they have small gaps between them. This is different from the leading strand which just continuously puts more and more DNA down as long as it needs to to make a whole strand of new DNA. It's moving in the same direction as the helicase and the replication fork movement. We don't have these small gaps that we would see in the lagging strand when it comes to the leading strand.
Alright. So just keep in mind, the difference between our leading strand and our lagging strand is really in reference to the direction of our replication fork movement and the helicase enzyme.