In this video, we're going to talk about the steps of DNA replication. DNA replication in prokaryotes can actually be simplified into just 7 steps that we have numbered down below 1 through 7. These numbers 1 through 7 correspond with the numbers 1 through 7 that you see in our image for the steps of DNA replication. Notice that below our image right here, we have this key, which is going to be helpful for labeling each of the enzymes and proteins that we see throughout and also the DNA molecules and the RNA primers as well.
Starting with the very first step of DNA replication, the enzyme topoisomerase is going to bind to the origin of replication or the ori, and recall that the topoisomerase's function is to relieve or remove the strain that's due to DNA supercoiling. Topoisomerases will help to remove DNA supercoiling which can inhibit DNA replication. In step number 1, you'll see a green oval here, representing the structure of the topoisomerase, which is also referred to as DNA gyrase sometimes in prokaryotes. The topoisomerase enzyme is in front of the replication fork, moving in this direction towards the left, helping to relieve any DNA supercoiling that the replication fork may encounter.
In step number 2 of DNA replication, the helicase enzyme is going to get involved. It is going to bind to this origin of replication, and it's going to help unwind the two strands of the template DNA. By doing so, it breaks the hydrogen bonds that exist between the two strands, creating single-stranded DNA. At number 2, the helicase is represented with a yellow triangle, and its function is to break the hydrogen bonds that exist between the 2 DNA strands to separate those 2 DNA strands, unwinding the DNA, and creating single-stranded DNA. One single-stranded DNA is here and another single-stranded DNA is here.
Now in the third step of DNA replication, now that the single-stranded DNA has been created, the single-stranded binding proteins can get involved. Recall the single-stranded binding proteins are abbreviated as just SSBs. The single-stranded binding proteins, or the SSBs, are going to bind, as their name implies, to the single-stranded DNA. When it binds to the single-stranded DNA, it helps to make sure that that single-stranded DNA does not reanneal to create double-stranded DNA, and it helps protect the DNA from degradation from other enzymes that might degrade single-stranded DNA. In step number 3, it's referring to these orange little circles here, which are the single-stranded binding proteins binding to the single-stranded DNA for each of these single-stranded DNA molecules.
In step number 4, the primase enzyme is going to get involved, and recall that the primase enzyme is important for being able to add the RNA primers. The primers are a requirement for DNA polymerases. They act as the starting point for DNA polymerase to provide the free three prime hydroxyl group that's needed for DNA polymerases. The primase enzyme is going to add the RNA primers to the template DNA so that those DNA polymerases can actually start replicating the DNA. The primase only needs to add 1 primer to the leading strand, but it needs to continuously add primers to the lagging DNA strand, and that is going to make several Okazaki fragments. At step 4, we're showing you the primase enzyme here, which is responsible for building these short RNA primers. On the leading strand, there's only one RNA primer that's required since the leading strand is being built in the same direction as the replication fork movement. But on the lagging strand, which is being built in the opposite direction of the replication fork movement, there need to be RNA primers continuously built, and so the lagging strand is built in these small fragments called Okazaki fragments.
In step number 5, now that the DNA polymerase has the primer to act as the starting point and provide the free 3 prime hydroxyl group, the DNA polymerase can now begin to extend the DNA, and specifically, DNA polymerase 3 is going to add nucleotides specifically to the 3 prime end of the primers, which provide the 3 prime hydroxyl group needed by the DNA polymerase 3. At number 5, the DNA polymerase is going to be extending in the 5 prime to 3 prime direction, adding nucleotides to the 3 prime end of the growing DNA strand. We also have a DNA polymerase on the lagging strand extending the primer here and elongating the DNA in the opposite direction on the lagging strand. The DNA polymerase 3 is going to be operating on both the leading and lagging strand to extend the DNA.
In step number 6, the DNA polymerase 1 is going to remove those RNA primers that were built by the primase and replace those RNA primers with DNA nucleotides. At step 6, notice that we have yet another DNA polymerase, but this one is DNA polymerase 1. DNA polymerase 1's job is to remove these RNA primers at these positions to replace them with DNA.
In the 7th and final step that we have here for DNA replication, the enzyme DNA ligase is going to join or link Okazaki fragments together covalently on the lagging strand to help create a single new strand. The DNA ligase, enzyme, which is here in our key, is going to be responsible for ligating or covalently joining or sealing or linking these Okazaki fragments so that we have one single strand, on the lagging strand.
This concludes our introduction to the steps of DNA replication. This is a pretty complex process and a very helpful way to be able to better understand the steps of DNA replication is to watch YouTube videos on DNA replication since they show the moving pieces. It's helpful at times to be able to watch an animation on YouTube for DNA replication. I would advise you to YouTube, steps of DNA replication and watch a bunch of YouTube videos on this process. But for now, this concludes our video on the steps of DNA replication and we'll be able to get some practice applying these concepts as we move forward in our course. I'll see you all in our next video.
