Hey everyone. So in our continued discussion of DNA replication, we can say that replication begins with helicase unwinding DNA at a specific site called the origin of replication. Now, this origin of replication or ori, this is basically where helicase decides where it wants to cut into the hydrogen bonds between our nitrogenous bases. This typically happens when it's scanning for a specific sequence of nucleotides, and that's where it starts to cut.
So, if we take a look here, we'd say that this orange box that is on top of this Phosphate Sugar Backbone of our DNA Double Helix represents our origin of replication. You can see here we have a helicase and it looks like it's moving this way, and another helicase that's moving this way. They are bending and then breaking these hydrogen bonds that are connecting the different nitrogenous bases together. Now, here we're going to say that our two strands of DNA are separated, forming what we call 2 replication forks. These replication forks are Y-shaped regions at each end of the bubble where DNA is unwound.
Now, if I take a look here, we still have our origin of replication here, we have our helicase here and our helicase here. They're just continually cutting through the hydrogen bonds, opening up our DNA Double Helix where we have this gap now. This empty space in here represents our replication bubble. And we're going to say that the regions that are highlighted in yellow, well, those are our replication forks. Remember, we say they are Y-shaped regions. If you look, it kind of looks like a Y. Looks like a Y here too. So they are Y-shaped regions.
Now, DNA replication proceeds bidirectionally in both directions. So, as you can see here, we have our old DNA in blue and then we have our new DNA being laid down once we've opened up the double helix. We can see that DNA is being laid down in this direction and DNA is being laid down in this direction. We also see that we have a magenta tail, that's our RNA primer, which remember, RNA primers are put down by our enzyme, primase, that first has to be put down before our new DNA can start being put down, complementary to the DNA template strands.
Alright. So just remember, when we're talking about replication forks, we're talking about this region highlighted in yellow, looks like a Y. It's being created by helicase just continuously moving down the double helix and cutting through the hydrogen bonds that connect the nitrogenous bases together. Opening it up creates a replication bubble which then allows new DNA to come in and start being laid down on the DNA template strands. With this new DNA, we have DNA replication.