It's easy for us to plot all these enzymes on a two-dimensional piece of paper or slide, but it's important for us as biologists to understand that none of these enzymes works independently. They aggregate to form what is called a replisome, which is what we're seeing here. So in here are all the enzymes and they're all working together.
So we have our DNA polymerase 3 subunits here. Remember, this is actually a whole subunit here, but we can see the ones that are involved in the replication, the ones that are clamping on. So these are the beta clamps.
We can see these proteins that are holding together the complex. We can see DNA helicase here. We've got DNA gyrase here that are moving in this direction.
We've got these single-stranded... binding proteins all the way along here. Okay, we've got our lagging strand, and we've got our leading strand right here as well.
Now, the important thing is here, and I will post some videos, and we'll potentially look at them during class too, is that the replisome is pulling that DNA template through. But as it does so, the lagging strand is going to loop out and around to allow that to occur. simultaneously. So this is continuous, so replication is going continuous in this direction, but this allows us to put a primer down, replicate this, release that molecule.
Put another primer down, replicate this, release that molecule. Many prokaryotes that have a circular chromosome have replication that is bidirectional. What that means is we have replication forks that are going in both. direction. So let's talk about that in a little bit more detail.
Replication, remember, starts at the origin of replication or the REC, where all our enzymes start to bind, and then that generates the replication fork. So here we have a replication fork going in this direction. We've also got a replication fork going in this direction. And what that does, it means that DNA replication can occur as rapidly as possible.
And we can also start a round of replication before the last round actually finished. And I'll talk about that here in a moment. As these replication forks move around the structure, around the chromosome and replicate the DNA, they are going to form what is called a theta structure.
So you can see that here. And if you know anything about Greek symbols, this is the theta symbol. So they create a theta structure.
and they will continue to move those replication forks in both directions and eventually we'll get to the ter site which is right here at the conclusion of replication i'll talk about how we finish that process off to end up with two new chromosomes here in a moment because that complex has to involve a number of proteins so as soon as we've cleared this origin right here there's no reason why another set of enzymes involved in DNA replication can bind, they can, and then they can start DNA replication right behind the first set of replication forks. So it can continue doing this process, which means that bacterial replication and archaea replication can be very, very fast.