In this video, we're going to talk about the steps of DNA replication. DNA replication in prokaryotes can be simplified into just 7 steps that we have numbered down below 1 through 7. These numbers correspond with the numbers in our image for the steps of DNA replication. Notice that below our image right here, we actually have this key, which is helpful for labeling each of the enzymes and proteins, as well as the DNA molecules and the RNA primers.
Starting with the very first step of DNA replication, the enzyme topoisomerase binds to the origin of replication, and recall that the topoisomerase's function is to relieve or remove the strain due to DNA supercoiling. Topoisomerases help to remove DNA supercoiling which can inhibit DNA replication. Below in step number 1, we've got this green oval representing the structure of the topoisomerase, referred to as DNA gyrase sometimes in prokaryotes. This enzyme is in front of the replication fork, moving towards the left, and helps to relieve any DNA supercoiling that the replication fork may encounter.
In step number 2 of DNA replication, the helicase enzyme gets involved. It binds to the origin of replication and helps unwind the two strands of the template DNA by breaking the hydrogen bonds between them, thus creating single-stranded DNA. Our image shows the helicase as a yellow triangle, its function being to break the hydrogen bonds between the 2 DNA strands to unwind the DNA and create single-stranded DNA.
In the third step, now that the single-stranded DNA has been created, single-stranded binding proteins (SSBs) get involved. SSBs bind to the single-stranded DNA to ensure it does not reanneal to create double-stranded DNA and protect the DNA from degradation from other enzymes. The image refers to these as orange little circles, representing the SSBs binding to each of the single-stranded DNA molecules.
Step number 4 involves the primase enzyme, important for adding RNA primers. Primers are a requirement for DNA polymerases, acting as the starting point by providing the free three prime hydroxyl group needed. Primase adds RNA primers to the template DNA for DNA polymerases to start replicating the DNA. In our image, it shows the primase enzyme responsible for building these short RNA primers. On the leading strand, only one RNA primer is required since it is built in the same direction as the replication fork movement. However, on the lagging strand, which is built in the opposite direction, RNA primers need to be continuously built, creating Okazaki fragments.
In step number 5, DNA polymerase, now equipped with a primer to act as the starting point, begins to extend the DNA. DNA polymerase 3 adds nucleotides to the 3 prime end of the primers, continuously elongating the DNA on both the leading and the lagging strands. This enzyme operates on both strands to extend the DNA.
Step number 6 sees DNA polymerase 1 removing the RNA primers built by primase and replacing them with DNA nucleotides. Another DNA polymerase, DNA polymerase 1, performs this function of replacing these primers with DNA.
In the 7th and final step of DNA replication, the enzyme DNA ligase joins Okazaki fragments together covalently on the lagging strand to help create a single new strand. The DNA ligase, shown in our key, is responsible for ligating or covalently joining these Okazaki fragments.
This concludes our introduction to the steps of DNA replication, a complex process that is better understood by watching animations on YouTube. I advise searching for "steps of DNA replication" and watching several videos on this process. 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.
