Introduction to DNA Replication - Video Tutorials & Practice Problems
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Introduction to DNA Replication
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3m
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Video transcript
In this video, we're going to begin our introduction to DNA replication. And so it's important to keep in mind as we move forward in our course and continue to talk more and more about DNA replication that there's actually much more information that is known about prokaryotic DNA replication than there is information known about eukaryotic DNA replication. And so for that reason, as we move forward in our course, we're mainly going to be focusing on prokaryotic DNA replication. However, it is also important to keep in mind as we move forward in our course that scientists believe that most of the DNA replication process is fundamentally similar in both prokaryotes and eukaryotes. And so even though moving forward in our course we're mainly gonna be focusing on prokaryotic DNA replication, it's important to remember that most of the process is fundamentally similar in both of them. And as we move forward in our course, we'll try to, point out some of the key differences between prokaryote prokaryotic and eukaryotic DNA replication. But in both prokaryotic and eukaryotic DNA replication, DNA replication occurs via a semi conservative process, which, again, we introduced what semi conservative DNA replication was in our last lesson video. And so, again, semiconservative DNA replication suggests that the old parental strands in the original DNA molecule are going to separate from one another, and they're each going to act as templates in the synthesis of new DNA strands that are complementary to the old parental strands. And so if we take a look at our image down below, what we're showing you is another image of semiconservative DNA replication. So notice over here on the far left, we start with just one DNA molecule. This is the original DNA molecule. And by the end of this DNA replication process, we end up with 2 DNA molecules, 1 here and one here. And so the way that this DNA replication works is that the DNA strand and the old, parental DNA molecule are going to separate from one another. And so you can see the separation here is beginning. And so as these old original DNA strands separate from one another, they act as templates to build new DNA that's complementary to it. And so you can see the new DNA here is this yellow strand that's being built using the old blue strand as a template. And so this process will continue and continue, and, ultimately, it results in 2 DNA strands that are identical to each other and identical to the original since it's building via complementary base pairing. And, again, we'll be able to talk more and more about the the mechanism of this semiconservative DNA replication as we move forward in our course. And we'll start off by introducing the DNA replication components. And so I'll see you all in our next lesson video to talk about that.
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Components of DNA Replication
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4m
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In this video, we're going to briefly introduce some of the most important components of DNA replication. And so it turns out that DNA replication is actually a relatively complex process that requires a host of multiple enzymes and proteins working together. And so notice down below we have this table that shows you some of the most important enzymes and proteins involved in DNA replication along with their functions. Now it's important to note here in this video we're only going to briefly introduce these enzymes and proteins and their functions. But as we move forward in our course in their own separate videos, we're going talk about each of these enzymes and proteins in more detail along with the details of their functions. And so again, keep in mind that this is just a brief introduction and so you can use this table down below as a reference moving forward as we cover these different enzymes and proteins in more detail. And so what you'll notice is over here in this column what we're going to show you is the shape that we're going to use here in clutch prep to represent each of the individual enzymes and proteins. And then, of course we have the name of the enzyme and protein in this column and then we have the function of the enzyme or in this column here. And so the first enzyme, enzyme that we're showing you here is going to be topoisomerase, And topoisomerase is also sometimes referred to as DNA gyrase in prokaryotes. And so, you may see your textbooks or your professor refer to it as that. And its function is to relieve DNA supercoiling ahead of the replication fork. And again, we'll talk more about what DNA supercoiling is and the function of topoisomerase and DNA iris in their own separate video as we move forward. And that again goes for all of these enzymes and proteins. And so the next enzyme that we're showing you is represented with this yellow triangle here, here at Clutch Prep, and it is a helicase. And helicases function is to unwind DNA double helix at the replication fork. Next, what we have are these little orange circles and these represent our single stranded binding proteins and as their name implies these single stranded binding proteins are going to bind to and stabilize single stranded DNA. Now next what we have is this pink structure which is going to be the Primase And as the Primase name implies, it is going to create RNA primers, which act as a starting point for DNA synthesis. Next, what we have is this light blue circle here, which represents DNA Polymerase 3 in prokaryotic organisms. And this is the main DNA polymerase that's responsible for building a new strand of DNA using the old strand of DNA as a template. But there are multiple DNA polymerases, and this DNA polymerase is, going to be DNA polymerase 1 in prokaryotes, and its job is to replace RNA primers with DNA. Now notice that DNA Polymerase 2 is not mentioned here on this list, and that's because DNA Polymerase 2 is not one of the most important components of DNA replication and it has a slightly different function that we're not really going to cover in this course. Now the last protein, the last component that's, very important for DNA replication is going to be DNA ligase. And DNA ligase, its function is to covalently join together what are known as Okazaki fragments in the lagging DNA strand. And again, this will make a lot more sense, these functions, and, these enzymes and proteins will, be cleared up as we move forward and talk about them in more detail in their own separate videos. But again, this here concludes our brief introduction to some of the most important components of DNA replication that we're going to encounter moving forward. And again, we'll be able to talk more about these as we move forward in our course. So I'll see you all in our next video.
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concept
Origin of Replication
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In this video, we're going to introduce the origin of DNA replication or just the origin of replication. And so DNA replication actually begins at a specific set of DNA sequences that are collectively referred to as the Origin of Replication, or just the ori for short. And so it turns out that prokaryotic cells have a small circular chromosome, And these small circular chromosomes of prokaryotic organisms have just one single origin of replication on their chromosome. Whereas eukaryotic organisms have larger linear chromosomes, and these larger linear chromosomes of eukaryotes actually have multiple origins of replication. In some cases they can have up to 100 of origins of replications or more. And so if we take a look at our image down below, notice on the left hand side over here, what we're showing you is prokaryotic DNA replication, And on the right hand side over here, what we're showing you is eukaryotic DNA replication. And what you'll notice is that the chromosome of prokaryotes is circular and the circular chromosome of prokaryotes only has just one single origin of replication. One origin of replication. Now if we take a look over here at the eukaryotic chromosome, notice that its chromosome is linear, and the linear chromosome actually has 2 origins of replication in this image, but again they can have many origins of of replication, so multiple origins of replication. And so what you'll see here is that the darker blue molecules represent the old original parental DNA strands, and, as DNA replication proceeds, it's going to occur and begin at this origin of replication. And so what you'll notice here in this image is that the new DNA strands are, in the light blue color, And of course, these little pink, structures are representing RNA primers, which we're going to talk more about the functions of these RNA primers, as we move forward in our course. But the point here is that the origin of replication is where DNA replication is going to begin the process. And so, you can see over here, in the is going to begin and proceed. And so, this here concludes our brief introduction to the origin of replication, this, set of DNA sequences that marks the beginning of DNA replication. And we'll be able to talk more and more about DNA replication as we move forward in our course. So I'll see you all in our next video.
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concept
Replication Forks
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4m
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In this video, we're going to introduce replication forks. And so recall from our last lesson video that DNA replication begins at a specific set of DNA sequences called the origin of replication or just the ori for short. And so it turns out that many different proteins are going to bind to this origin of replication, and those proteins are going to separate the 2 strands of DNA, unwinding the DNA so that DNA replication can begin. And this unwinding forms a replication fork, which is sometimes also referred to as a replication bubble. And so replication forks are really just these y shaped regions that are at the end of the bubble where DNA is unwound. And so we'll be able to see the replication forks down below here in our image. And so if we take a look at this image, what you'll notice is here, over here on this side, we're showing you this replication force, and so you can see that the DNA strands have unwound, so we have the old original DNA strand are these darker blue color, and what you'll see is that they're starting to be separated here. There's a separation of these two strands. And the separation of these two strands creates the replication forks or the replication bubbles. And so what you'll notice is that at each of these positions on each side of this unwinding is a replication fork. So the replication forks are indicated with these yellow backgrounds that you see here and over here. And you'll notice that they are these y shaped regions that they kinda create these this sideways, y right here and this sideways y is the replication fork and you'll also see the sideways y over here. And so that's why we call these replication forks these y shaped regions at the end of the bubble where DNA is unwound. And you'll notice that it does kinda create somewhat of a bubble, so from here to here there's a little bit of a bubble and so, sometimes it's referred to as the replication bubble. And so, what you'll notice here is that DNA replication is actually going to proceed bidirectionally. And bidirectionally just means that it's going to occur in both directions, at each replication fork. And so you'll see that the direction of DNA replication, and the replication fork is gonna be indicated by these, red arrows. And so DNA replication is going to proceed in this direction at this replication fork, and it will also proceed in this direction for this replication fork as well. And so that's important to keep in mind moving forward because we're going to talk about DNA replication at just one replication fork, but we have to keep in mind that DNA replication occurs at the other replication fork in the same in the opposite direction. And so DNA replication proceeds bidirectionally in both directions at each replication fork. And so once again, we're gonna talk more about of DNA replication, involving these RNA primers and, these different strands as we move forward in our course. But for now, this here, concludes our video introducing the terms replication forks, 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.
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Problem
Problem
Which of the following is incorrect regarding DNA replication forks?
A
DNA replication forks begin forming at the origin of replication (ORI).
B
DNA replication forks are caused by helicase separating two complementary strands of DNA.
C
There are two replication forks found in each replicating prokaryotic chromosome.
D
DNA replication forks are found at both ends of the replication 'bubble'.
E
None of the above are incorrect.
6
concept
Unwinding the DNA: Topoisomerase, Helicase & SSBs
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5m
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Video transcript
In this video, we're going to talk about some of the enzymes and proteins involved with the unwinding of the DNA during DNA replication, and these include the enzymes topoisomerase and helicase, as well as the protein single stranded binding proteins or SSBs. And so it turns out that several different proteins participate in the unwinding of the DNA during DNA replication. And of course the unwinding of the DNA is just referring to the separation of the two strands of DNA. And so these include the proteins, these 3 proteins that are listed down below, 1, 2, and 3, and, the first of these 3 is topoisomerase, which is also sometimes referred to as DNA gyrase and prokaryotes. And topoisomerase, or DNA gyrase, its function is to cut and rejoin the DNA in order to relieve strain that's caused by DNA supercoiling. And so DNA supercoiling or just supercoiling can actually inhibit DNA replication and therefore supercoiling must be relieved in order for DNA replication to proceed normally. And so if we take a look at our image down below, over here on the left, we're showing you, some DNA supercoiling here. So you can see that we're showing you a DNA molecule, but the DNA molecule here is supercoiling, and so this structure that you see here is a DNA supercoil. And so, the DNA supercoil is going to, again, cause strain and it can inhibit DNA replication, and so this DNA supercoil must be relieved in order for DNA replication to proceed, and that is the function of the DNA gyrase or the topoisomerase which is right here ahead of the replication fork over here. And so the DNA guy raise, its job is to again eliminate or remove the DNA super coils. And the DNA supercoiling kinda represent can be better understood by thinking about the supercoiling of one of these old telephone wires. And so this old telephone wire will supercoil just like the DNA molecule has a tendency to supercoil. And so, the topoisomerase is going to help with getting rid and eliminating the DNA supercoils. Now the second protein involved with the unwinding of the DNA is helicase, and helicase unwinds the DNA by breaking hydrogen bonds. And the hydrogen bonds are forming between the 2 DNA strands. And so when helicase breaks these hydrogen bonds between the two strands, it separates the strand. So helicase is going to help to create single stranded DNA. And so, if we take a look at our image down below, what you'll notice is that the helicase is represented as this yellow triangle and its job is to break the hydrogen bonds that hold the 2 DNA strands together, and so it separates the 2 DNA strands creating single stranded DNA. And this is where the 3rd proteins come into play, the single stranded binding proteins, which are abbreviated as just SSBs. And so these single stranded binding proteins, or SSBs, are going to bind to the single stranded DNA to prevent the reannealing of the DNA, to prevent those hydrogen bonds from reforming. And they're also going to prevent the degradation of each of these single stranded DNA molecules since single stranded DNA within cells tends to be degraded. And so these single stranded binding proteins will bind bind to the single stranded DNA, prevent reannealing and degradation of each of the separated DNA strands. And so when we take a look at our image down below, notice that the single stranded binding proteins are these orange circles here that are binding to the single stranded DNA that's being separated here. And so these are the proteins that are going to be important for the unwinding of the DNA and the separation of the 2 strands of DNA so that they can serve as templates for building the brand new DNA strands. And so this here concludes our lesson on the unwinding of the DNA, and we'll be able to some practice applying the concepts that we've talked about here as we move forward in our course. So I'll see you all in our next video.
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Problem
Problem
What is the function of the enzyme topoisomerase in DNA replication?
A
Relieving strain in the DNA ahead of the replication fork caused by the untwisting of the double helix.
B
Elongating new DNA at a replication fork by adding nucleotides to the existing chain.
C
Reattaching the hydrogen bonds between the base pairs in the double helix.
D
Building RNA primers using the parental DNA strand as a template.
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