Steps of DNA Replication - Video Tutorials & Practice Problems
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Steps of DNA Replication
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In this video, we're going to talk about the steps of DNA replication. And so DNA replication in prokaryotes can actually be simplified into just 7 steps that we have numbered down below 1 through 7. And, of course, these numbers 1 through 7 here correspond with the numbers 1 through 7 that you see down below in our image for the steps of DNA replication. Now notice that below our image right here, we actually have, this key, and, this key here 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. And so starting with the very first step of DNA replication, what we have here is that the enzyme topoisomerase is going to bind to the origin of replication or the aurei, and recall that the topoisomerase's function is going to be to relieve or remove the strain that's due to DNA supercoiling, and so topoisomerases will help to remove DNA supercoiling which can inhibit DNA replication. And so what you'll notice is down below in step number 1, what you'll notice is we got this green oval here, which is representing the structure of the topoisomerase, which is also referred to as DNA gyrase sometimes in prokaryotes, And so, what you'll notice is that this topoisomerase enzyme is in front of the replication fork. This replication fork here is moving in this direction towards the left. And so this topoisomerase is in front of the replication fork, helping to relieve any DNA supercoiling that the replication fork may encounter. And so in step number 2 of DNA replication, what we have is 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. And when it unwinds those two strands of the template DNA, it's doing so by breaking the hydrogen bonds that exist between the two strands, again, creating single stranded DNA. And so when we take a look at our image down below at number 2, notice that the helicase is represented with this 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 strand 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. And recall the single stranded binding proteins are abbreviated as just SSBs. And so the single stranded binding proteins, or the SSBs, are going to bind, as their name implies, to the single stranded DNA. And 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. And so when we take a look at our image down below at step number 3 here, notice that 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. Now in step number 4, what we have is the primase enzyme is going to get involved, and recall that the primase enzyme is important for being able to add the RNA primers. And so the primers, recall, 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. And so 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. Now recall that 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. And so if we take a look at our image down below at step 4, what you'll notice is we're showing you the primase enzyme here, which is responsible for building these short RNA primers. Now 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 needs to be RNA primers continuously built, and so the lagging strand is built in these small fragments called Okazaki fragments. Now in step number 5, what we have is, the DNA polymerase now that it 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 build the DNA, and specifically, DNA polymerase 3 that 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. And so when DNA polymerase 3 adds nucleotides to the 3 prime end of the primer, it's going to be continuously elongating the DNA on both the leading and the lagging strands. And so if we take a look at step number 5 down below in our image, notice that here we have 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. And we also have a DNA polymerase down below here on the lagging strand extending the, the the primer here and elongating the DNA in the opposite direction on the lagging strand. And so the DNA polymerase 3 is gonna be operating on both the leading and lagging strand to extend the DNA. Now in step number 6, what we have is 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. And so if we take a look at our image down below, step 6, notice that we have yet another DNA polymerase, but this one is DNA polymerase 1. And DNA Polymerases 1's job is to remove these RNA primers at these positions to replace them with DNA. Now in the 7th and final step that we have here for DNA replication, we have 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. And so what you'll see here is that the DNA ligase, enzyme, which is here in our key, it's 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. And so this here concludes our introduction to the steps of DNA replication. And again, this is a pretty, complex process and a very, very helpful way to be able to better understand the steps of DNA replication is to watch YouTube videos on DNA replication since what they do is they show the moving pieces since a lot of these pieces here are moving. And so it's helpful at times to be able to watch an animation on YouTube for DNA replication. And so I would advise to to YouTube, steps of DNA replication and watch a bunch of YouTube videos on this process. But for now, this here 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. So I'll see you all in our next video.
2
Problem
Problem
During DNA replication, the enzyme ___________, catalyzes the elongation of new DNA by adding, to the 3' end of the previous nucleotide, new nucleotides that are complementary to a DNA template.
A
Helicase.
B
DNA polymerase.
C
DNA ligase.
D
ATP synthase.
3
Problem
Problem
Which of the following enzymes breaks the hydrogen bonds between the DNA strands?
A
Primase.
B
Helicase.
C
Topoisomerase.
D
DNA ligase.
E
DNA polymerase.
4
Problem
Problem
Which of the following enzyme-function matches is incorrect?
A
Helicase - relieves tension of supercoiling by breaking and rejoining ahead of fork
B
Primase - provides short stretch of RNA at initiation of strand synthesis
C
Polymerase – synthesizes new strand of DNA while using old strand of DNA as a template.
D
DNA ligase - joins 3'-OH to 5'-phosphate to seal adjacent DNA nucleotides.
5
Problem
Problem
Which of the following enzymes is responsible for removing RNA primers and replacing them with DNA?