DNA Replication - Online Tutor, Practice Problems & Exam Prep

DNA replication is a semi-conservative process where each new DNA molecule consists of one old and one new strand. It begins at specific origins rich in adenine and thymine, forming replication bubbles with two replication forks. The leading strand is synthesized continuously, while the lagging strand is made in fragments called Okazaki fragments, requiring multiple RNA primers. Key proteins include DNA polymerase, which synthesizes DNA in the 5' to 3' direction, and telomerase, which extends telomeres to protect chromosome ends. Proofreading mechanisms ensure high fidelity during replication, correcting errors through exonuclease activity.

Overview

Hello, everyone. In this lesson, we are going to be talking about an overview of DNA replication. And in our later lessons, we are going to go over that process in more detail, but this is just going to be an overview and a refresher. Okay. So, I know that you have probably talked about DNA replication in previous biology classes, but it is a very important and crucial mechanism for life on our planet. So, we are going to go over it and this is just going to be an overview and the background information. Okay. So, the way that DNA replicates is going to be via semi-conservative replication, and that means that the old strands of DNA are going to be utilized as templates for the new strands of DNA. So, every one of your double helix DNA molecules in your cells has one old strand and one new strand. So, it is semi-conservative because one old strand is conserved. Now, this whole process of DNA replication is going to begin at the replication origins. These are going to be very specific DNA sequences where replication begins. And, generally, these origin sequences are going to be composed of adenines and thymines, and that is because As and Ts, where they bind together, they are going to have fewer hydrogen bonds between them than Gs and Cs, which makes them easier to pull apart. So, generally, replication origins and transcription origins, where the DNA is commonly pulled apart and those two strands are pulled apart, they are going to be composed of As and Ts because there are fewer hydrogen bonds and it is easier to pull them apart. So, what is an origin actually going to look like? Well, let's say that this is the DNA and, remember, to replicate DNA, we have to split those two strands apart, so that those two old strands can be utilized as templates for the new strand. And let's say that this is the origin spot and this is where the machinery is actually going to pull apart these two strands of DNA. So what's going to happen is we are going to form what is called a replication bubble. This replication bubble is going to be where the two strands of DNA are actually being separated. So, here you can see that this area that I'm drawing in right now is still conjoined, but then inside of the bubble, these strands have been separated from one another. This is what an origin is going to look like. Where the origin of replication begins, those two strands of DNA must be pulled apart. So you're going to have these two separate single-stranded DNA strands. Now, there are some important terms to know here. Where the DNA is actively being forked. So, there are two replication forks for each replication bubble. So, there is one here, that's where those strands are actively being pulled apart, and there is one right here. You can kind of think of it as a fork in the road. If you're going down a road and then the road splits into two roads, that's a fork in the road. The same thing happens with the DNA. The two strands are connected then they are split apart. That is a replication fork and just remember that this is the replication bubble. Because the two strands of DNA actually kind of bubble apart and form that bubble-like structure. And at these replication forks and in this bubble, the initiation proteins and molecules that begin the process of DNA replication are going to bind here. We will talk more about that machinery in our next lesson. This is just an overview. So, we have talked about our replication forks that are formed at each origin. Now, DNA replication is bidirectional due to the fact that each of these DNA strands that are separated above me are going to be utilized for templates. So that means that there are two sets of replication processes happening at the same time for both template strands. Also, it is important to realize that DNA replication is also bidirectional because it is occurring in this direction and it is occurring in this direction. It is going in opposite directions from the origin location at the same time. So, it is bidirectional in that process as well. Now, what is going to be the protein that is incredibly important for this whole process? That is going to be DNA Polymerase. This is going to be a protein that catalyzes the replication of DNA. Basically, what it is going to do is it is going to build that new DNA strand by connecting the DNA nucleotides together. Now, there is a drawback to DNA Polymerase because it only adds Nucleotides in one direction. It adds Nucleotides at the 3' end. Excuse me. It only adds Nucleotides to the 3' end of the growing DNA strand. Remember, DNA strands have 5' ends and 3' ends. And remember, they are going to look like this. If this is our DNA strand, Remember, they are antiparallel. So, if this is the 5' end, and this is the 3' end, then the other DNA strand is going to be the opposite. And recall, the DNA polymerase can only add new nucleotides to a growing strand on the 3' end. So, that is going to cause some issues, which we will talk about later when we talk about the lagging strand. So that means it forms a new strand in the 5' to 3' direction. This is very important. This will absolutely be asked of you on your test, or your quiz, or your homework, which direction does DNA Polymerase synthesize DNA? And, that's from 5' to 3' direction. So now, let's just go over some just a really quick what this would actually look like. So remember our replication bubble up here. So now let's just take one side of that replication bubble and look at how DNA is actually replicated in this process. Now, there are going to be two strands that we are going to be talking about in more detail: the leading strand and the lagging strand. This one on top is the leading strand, and the one down here on the bottom is going to be the lagging strand. The leading strand, pretty simple, straightforward in its creation. The lagging strand's got to be a little bit more complicated. It is a little bit more annoying to create and it is going to have more steps. So basically, what's happening here, remember that this is the replication fork. And DNA is being actively separated in this process. So what is going to happen with the leading strand? Which is pretty much the easiest one to understand. So, let's say that an RNA primer is formed. We'll talk more about this later, but just know that DNA polymerase cannot begin the process of DNA replication without an RNA primer. It cannot start from scratch. It has to have something to build off of. So this is the RNA primer. And, in the leading strand, this only has to be made once. So, the RNA primer is made and then, we are going to have DNA Polymerase, which I'm going to draw here in green. DNA Polymerase is going to come up and it is going to begin forming the new strand of DNA. I'm just going to write DNA poly, but this is DNA polymerase. So then, what it is going to do is it is going to begin creating this new strand of DNA, which I am coloring here in green. So, this one is the new strand. And, it is going to continue along in this direction until it reaches the end of the chromosome and the DNA polymerase simply falls off. Pretty simple. Right? RNA primer is placed down. DNA polymerase then sits on top of the primer and then begins to form the new strand of DNA. And here in black, this is the template strand of DNA or the old strand. And the new strand is going to be made off of that template. And in the leading strand, it's pretty simple. Pretty easy. It just, the DNA polymerase just continues on until it finishes its process. And the reason that this is so easy is because of the orientation of the new strand. As you can see, the new strand is 5' to 3'. And that's perfect because remember that is exactly how DNA polymerase builds new DNA. It adds from 5' to 3'. Now, that's an issue with this lagging strand here. Now, for the lagging strand, I'm going to draw it, a little bit differently. This diagram was great for the leading strand, but it's not my favorite for the lagging strand, so I'm going to draw it for you guys, so you can know what is happening. Okay. So, I'm going to draw this one from scratch. Okay? So let's say that we have our old strand of DNA, and this is going to be utilized to build the lagging strand. So basically, you can think of it as this black line right here. So, what is going to be the orientation of this black line? So, we have our 3' and we have our 5'. Now, the overall direction of the process of building the lagging strand is going to be the opposite direction of the way that DNA polymerase actually works, which is confusing. I know. It's going to be kind of annoying for a little bit, but it'll be fine. Okay. So what is going to happen? Well, remember, we have to lay down that RNA primer first. So, we're first going to lay down the RNA primer, and we are going to put it right here. So this is our primer. And this is going to be what the DNA polymerase is going to build off of. Now remember, the DNA polymerase can only add to the 3' end. It only adds nucleotides in the 5' to 3' direction. So what is going to be the orientation of this new strand? Well, this end down here is going to be the 5', and this end down here is going to be the 3'. So that means that the new DNA strand can only be created in this direction. That's the only way the DNA polymerase will move. But, overall, the process of building this new strand is actually going to go in the other direction, which is going to make it a little difficult. So, first off, the primer is laid down. RNA Polymerase comes in, and it begins to create this section of DNA. So, that's great. So, it is going to move in this direction in the 3' direction. So, for our little strand here, we're going to have the 5' end is going to be down here, and the 3' end is going to be down here. So now, what's going to happen? Well, overall, the movement of this strand creation, DNA replication, is going to happen in this direction. So, what's going to have to happen is a new RNA primer is going to be built. And then, the DNA polymerase is going to have to jump backward, and place itself on that new primer and then begin forming this other new strand of DNA. And as you can see, this is a little bit more complicated than the leading strand. So, you're going to see this happening. The DNA polymerase is going to build a fragment, then jump back. Build a fragment, then jump back. Build a fragment, jump back. That's why it's called the lagging strand because it takes a while. It lags behind because it's jumping back and it's replacing all these primers and DNA Polymerase has to move every single time. It's kind of annoying. Right? So this process is a little bit more annoying. And the reason that this diagram isn't the greatest is because up here, this black line is representing the direction that DNA polymerase adds DNA to the new strand of DNA, but overall, remember that DNA, this DNA strand is growing in this direction, which can be kind of confusing. I know, I completely understand. So, now that we've talked about the overview of how this process works, we are going to talk.clone on and talk about the replication machinery, which is going to be the specific molecules and proteins needed to build these new strands of DNA. Now if that confused you, I completely understand this is kind of a weird topic to wrap your brain around. After you watch this, and then after you watch the different names and jobs of the replication machinery, I definitely recommend looking up an animation of this process. That always greatly helped me to watch a video of, actually, the machinery moving around and building this new strand of DNA, especially for the lagging strand because it can be pretty confusing. Okay, everyone, let's go on to our next topic.

DNA Replication Proteins

Okay, everyone. In this lesson, we are going to be talking about the specific molecules and proteins that are utilized to build new strands of DNA, and these are going to be collectively called replication machinery. Okay. So, DNA replication machinery, like I said, is going to be composed of all of the things needed to build a new strand of DNA during the process of DNA replication. Now, DNA replication is characterized by bidirectional replication, which we talked about in our last lesson. Remember, that means that two strands of DNA are being used as templates, and two strands of DNA are being created at the same time. Also, remember that this is also referring to the fact that DNA replication is continuing in two opposite directions at the same time as well. So, bidirectional kind of has two meanings here. Now, both strands are replicated at the same time. This is going to include the leading strand and the lagging strand and they are going to happen at the exact same time, but not in the exact same way. Remember, we talked about the main differences between the leading and the lagging strand in our last lesson. Now, remember something very important. The DNA Polymerase can only add new nucleotides onto the 3' end of a new strand of DNA. But also realize the DNA polymerase reads the template DNA strand in the 3' to 5' direction and synthesizes the new DNA in the 5' to 3' direction. This confused me when I was a student and I know that it does confuse many students. So, what is this referring to? So, let's say that this is going to be our template strand. And, our template strand is going to be utilized as, basically, a template to build this new strand of DNA. And, remember that we're going to have one 5' end and one 3' end for each strand of DNA. Now, remember that DNA polymerase can only synthesize from the 5' to 3' direction. So, what's going to happen is you're going to have this new strand of DNA being built in that particular direction. And, we're gonna have DNA Polymerase here actively building new segments of this DNA strand. So this one in green here is NUP. And, we're going to see that it's building from the 5' to the 3' end. It's only adding nucleotides to that 3' end of that green strand. But realize that while it is synthesizing from the 5' to 3' direction, which direction is it moving along the template strand or the black strand? It's moving in the 3' to 5' direction, so it reads the template from 3' to 5' and it creates the new strand in the 5' to 3'. I hope that makes sense. I know that can be kind of hard to wrap your brain around, but it is important to know, and I have seen this on a lot of different test questions. Okay. So now again, we're gonna go over the difference between leading and the lagging strand because it is very important to know. The leading strand is pretty easy to create, and that's because it is continuously synthesized in the 5' to 3' direction. And that's because it's really easy to synthesize, and it just goes in this particular direction. Now, the lagging strand is going to be a little bit more annoying, a little bit more complicated, and it is discontinuously synthesized, which I showed you in the last lesson. But again, it is still synthesized in the 5' to 3' direction because that's the only way that DNA Polymerase can move. Now, the cool thing about the lagging strand is that it is composed of many different fragments of DNA. As we learned in our last lesson, DNA polymerase creates a segment of DNA then jumps back, creates another segment of DNA, and jumps back. What are those segments of DNA called? These are called Okazaki fragments. Now, their really cool name actually comes from the couple who actually discovered this process of the lagging strand, and they gave it their name. So, the Okazaki fragments are these small fragments of replicated DNA. Now, these small fragments of replicated DNA are going to be bound together because we can't have the lagging strand just be all chopped up into these different fragments. It has to be its own whole DNA strand. So they're gonna be bound together, and the way that they're gonna be bound together is via the DNA ligase protein, which I'll talk more about in just a second. So, the leading strand is just continuously made. It doesn't have any breaks. The lagging strand is discontinuously made in fragments. Okazaki fragments. And then those Okazaki fragments are all joined together to make a continuous new strand of DNA. So, now that we have that background information, let's just go over some of the DNA replication machinery. Now, this is not all of the DNA replication machinery, just some of it, and I'll go into the rest of it in just a second. So, remember in our last lesson, I talked about the RNA primer. I told you the DNA polymerase cannot begin the process of DNA synthesis on its own. It needs something to bind to first. It basically needs something to build off of. It can't go from scratch. So the RNA primer is utilized for the jumping-off point. The RNA primer is composed of about 10 RNA nucleotides, and they are going to be complementary to the template strand of the RNA. Now, RNA primers, there's only gonna be one found in the leading strand, but there are gonna be many, many found in the lagging strand. Now, what creates these RNA primers is gonna be this very important protein called Primase. Its name is pretty self-explanatory. It makes those primers. So, it is going to be utilized to build these RNA primers using the template DNA strand. Cool thing about Primase is that it doesn't need a primer, but DNA polymerase does. So, it fills the job of beginning the process of DNA replication. Now, you may be thinking if the lagging strand is full of all of these primers, what do we do? Because our DNA is not full of RNA. What happens? Well, there are actually special RNA or they're actually special polymerases, DNA polymerases, they come along after the process has finished, and remove those RNA primers and fill them in with DNA, which is pretty cool. Those are other specialized types of DNA polymerases. Not the one we are going to be talking about. Now, remember, I told you that those Okazaki fragments have to be joined together to create a full strand. So once the primers have been removed, DNA ligase comes in and just does its job, and it joins those Okazaki fragments together. It basically allows the DNA backbone of these Okazaki fragments to bind of DNA replication machinery, so we're just gonna go in chunks. So, of DNA replication machinery, so we're just gonna go in chunks. So, I talked about the RNA primer, And you can see the RNA primer here, and it is being built. And this is gonna be something that DNA Polymerase can build off of. Now, you can see DNA Polymerase up here, and you can see DNA Polymerase up here. And that's because, or down there. Excuse me. And that's because remember, two strands are being duplicated at the same time. The lagging. Excuse me. The lagging strand and the leading strand. So the leading strand is continuous, and this process of DNA replication really doesn't hit any roadblocks, and it just continues on this process. The lagging strainer, remember, has to do all that jumping and the creation of the Okazaki fragments. So you can see the DNA polymerases are the kind of box-looking thing. Now, once the DNA polymerase is done, remember, we're going to have the DNA ligase, which is here. This little guy here is going to come in to the lagging strand and bind all of those Okazaki fragments together. Also, I forgot to mention, but here is the DNA Primase protein. It is used once in the leading strand and many many times in the lagging strand. So those are gonna be some of the components that you're definitely going to need to know. Okay, so now, let's go down because that is definitely not all of the DNA replication machinery, and let's go down and talk about some other really cool proteins and molecules that are utilized in this process. Okay. One of my favorites, just because it's really cool, DNA helicases. Helicases are going to be the enzymes or proteins that actually cleave or pry apart the two strands of DNA. This is going to break apart that double helix. It's gonna break the hydrogen bonds between the the bases. Bases complementarily bind via hydrogen bonding, and helicase is just gonna come in and push them all apart. So, once helicase does that and it separates the double helix into two single strands, we want to keep it single-stranded so that we can duplicate our DNA. But DNA doesn't want to be single-stranded, so we're gonna have to have some helper proteins that come in and help with that. And these are going to be single-stranded DNA binding proteins. They're commonly abbreviated SSB. Single-stranded binding proteins. And these are going to be here to basically just ensure that the double helix does not reform before the process of DNA replication has occurred. Now another really cool named protein that is very important, DNA topoisomerase. It's also very commonly called DNA gyrase. And we have talked about topoisomerase before. This is going to be utilized to ensure that supercoiling does not happen. So, DNA helicase is actually running down the DNA and separating it. But because DNA is a double helix, this is creating a lot of tension on the coils in the helix, and DNA topoisomerase is actively cutting the DNA and relieving that pressure, that super coiling. So this is also very important. Now, the sliding clamp and the clamp loader are going to be something that is going to be specific to the DNA polymerase. The DNA polymerase isn't perfect. Remember, it needs a primer. It's also gonna need something to hold it onto the DNA, so it doesn't fall off. So the sliding clamp or beta clamp or DNA polymerase clamp, it has many names, actually just holds the DNA polymerase onto the template strand of DNA so it can continuously build new strands of DNA. Now, the sliding clamp actually cannot get onto the DNA on its own, and it is going to utilize the clamp loader which uses ATP to clamp onto the DNA and allow the DNA polymerase to bind to the DNA. Now, this is only gonna happen once in the leading strand but can happen many, many, many times in the lagging strand. This is another reason why the lagging strand takes longer. It's called the lagging strand and is kind of annoying to build. Okay. So now let's look at the same picture. Same picture as I showed you before, but we're gonna go over the different parts. Just not to overwhelm you. Okay. So, helicase. Really cool protein right here. This blue triangle thing. It's basically just cleaving through, bulldozing through those hydrogen bonds and splitting the DNA apart into single strands. But because it does this, we're gonna need the single-stranded binding proteins, which you can see here in this purple color. And they're actively holding the single-stranded, DNA. And they're holding it in such a way that it can't bind with another strand of DNA. So it keeps it single-stranded. Now, I'm gonna go out of the picture because I'm in the way of the topoisomerases. So this big green guy here in front of the DNA helicase is going to be utilized to relieve that supercoiling. You can see that the DNA is actively coiling here, but not here. That's because of this topoisomerase is really actually cutting the backbone of the DNA to ensure that it does not supercoil. Now, what else did we talk about? I don't want to forget. Oh, the sliding clamp and the clamp loader are not actually shown on this particular diagram, but if they were, they would be right here, kind of holding the DNA polymerase onto the DNA. And they would be holding that DNA on, and the clamp loader would be waiting for the clamp to dissociate and then actually it would be utilized to put that clamp back on if a new Okazaki fragment needed to be made. So, just so you guys know what we're talking about, this is the DNA clamp. And the clamp loader doesn't actually clamp onto the DNA. It is just utilized to help the DNA clamp get onto the DNA. So, I know I went over a ton of replication machinery, but all these proteins and molecules are very important to understand. I do want you to understand these, and I want you guys to be able to look at this diagram without labels and be able to label this particular diagram because I have seen that a lot in homework and quizzes and things like that. You are gonna need to know these different machinery pieces, why they're important, what they do, and their function in both of the strands because their functions can be a little different, and their importance can be a little different in each of these strands. Okay, everyone. Let's go on to our next topic.

Telomeres

Hello, everyone. In this lesson, we are going to be talking about telomeres. Now, remember, whenever you have DNA like ours, we have linear DNA. And the ends of the DNA are going to be called telomeres. These are going to be very specialized regions of the DNA that are going to help protect those chromosomes, those linear chromosomes from degradation at their ends. And the whole process of DNA replication that we have talked about is going to occur differently at the ends of the chromosomes and we're going to talk about how this process works in this lesson. So the reason that DNA replication is going to occur differently at the ends of the chromosomes is going to be because of the way that the leading strand and the lagging strand are going to be. Remember, we learned that the process of replication for the leading strand and the lagging strand are actually very different. It's pretty simplified to copy the leading strand but it's a lot more complicated to create the lagging strand. So the leading strand really has no issue replicating the end of the chromosome, but the lagging strand cannot replicate the end of the chromosome because there's no real place to put an RNA primer. So telomeres are going to be utilized to solve this issue. And I'll talk more about the specifics of how this works in just a second. So just know that telomeres are going to be the ends of the DNA, linear DNA.

So what that would look like is if you have a linear chromosome that is not replicated like this stick drawing here, the ends of the chromosome are going to be unique and these are going to be telomeres. And, their job is to protect the coding region of the DNA. So, this is the coding region in black. And we obviously don't want to lose any of our coding region. We don't want to lose those genes that code for specific proteins and specific molecules that we need to survive. So, basically, we do is we put caps on the ends of our linear chromosomes called telomeres to protect our coding region from degradation. So telomeres are going to be long repetitive nucleotide sequences at the ends of chromosomes and they're non-coding sequences and they're highly repetitive. So that means that if they're lost a little bit, it doesn't really matter because it's not affecting the coding region of the DNA. And in fact, in human beings, the repetition of nucleotides is the sequence AGGGGTT. I don't believe that your professors probably need you to know this. They might. I just thought it was interesting that this is going to be the repeated sequence. And in humans, I believe, when cells are created, the repetition is about 25 to 100 times. So this sequence of bases is repeated in a telomere 2,500 times at the end of your chromosome and that is to protect it from degradation. So if you lose a couple of bases at the ends of your chromosomes, it's not a big deal because it's part of the telomere. It's a non-coding repetitive sequence that you don't need to create your proteins and your molecules you need to survive.

Now, it's very important to understand that there is a specialized protein that actually creates these telomeres, and this is going to be called telomerase. Now, telomerase is very important because it's a different type of DNA polymerase and it is going to be the one that specializes in building those telomeres. Remember, we talked about the fact that DNA replication occurs differently at the ends of chromosomes at telomeres, and that replication is going to be done by telomerase not by DNA. It's going to be started by telomerase and then finished by DNA polymerase, which I'll show you in just a second. And the cool thing about telomerase is that within itself, it has an RNA template. So it doesn't really need a primer because it has the primer within itself. So that allows it to kind of build from scratch more than DNA polymerase. So telomerase is going to be responsible for adding the short repetitive DNA sequences onto the end of the lagging strand so that DNA polymerase can finish the replication of the lagging strand end.

Now, I'm going to draw this out for you. So we will come back to this image in just a second. I'm going to talk about it in just a second, but we're going to skip down here so I can show you what's going on because I can understand that it might be a little confusing as to why the leading strand can perfectly create the ends of the chromosomes but the lagging strand can't. What's going on? Why can't the lagging strand actually replicate the ends of the chromosomes?

Well, I've drawn these 2 template strands. So imagine that your DNA has been separated into single strands and now it is going to be replicated."

Proofreading

Hello, everyone. In this lesson, we are going to be talking about DNA proofreading and the fixing of errors in DNA, and we are going to be talking about how new nucleotides are added onto the end of a growing DNA strand. Okay. So, we've talked about DNA replication and we've talked about that whole process, but we haven't really talked about the fact that nothing's perfect. We make mistakes and so does the DNA polymerase. These are going to be things called mutations. Errors in the genetic code whenever the incorrect base is put down. Now mutations can be made in many ways, radiation, different mutagens, but this is one of the main ways that mutations are created. And that is due to the fact the DNA polymerase isn't always perfect and it doesn't always correctly match the correct base pairs. But overall DNA replication is highly accurate. It's extremely accurate when you think about it. There's really only one error per 10,000,000 base pairs which is pretty good if you think about it. You're like wow only one error per 10,000,000 base pairs? That's pretty great. But realize that inside each cell, there are 3,000,000,000 base pairs. In the human genome, there are 3,000,000,000 base pairs. So if DNA polymerase makes a mistake every 10,000,000 base pairs, you're kind of likely you're going to get a mistake, right? So even though it's rare, it does happen. So what does DNA Polymerase do? How does it fix this issue? Well, the cell doesn't want mutations, so it has this ability inside of the DNA Polymerase called Proofreading. And Proofreading is the ability of Polymerase to double-check and correct its mismatched bases. And it does this by kind of putting down a base and then looking back at it and making sure that it's right and then moving on to the next base. Now the DNA polymerase that is actively building the new strand of DNA can do this. There are other mechanisms by other DNA polymerases that come along after this fact to fix mutations, but we're not really going to talk about that in this lesson. Right now, we're just going to talk about DNA polymerases proofreading ability and fixing the mistakes as it's building the new strand of DNA. So like I said, proofreading occurs before the next nucleotide is added. So like I said, basically, DNA polymerase puts down a base that it believes is complementary to the template strand, looks at it, and says is this correct? If it is, it moves on. If it's not, it is going to remove that incorrect base and then put the correct one in its place. How does it do this? It has this really neat ability called the Exonuclease Activity. Specifically the 3' to 5' Exonuclease Activity.

Oh, I'm sorry. I am in the way there guys. Let me scroll down a little bit. Okay. So the exonuclease activity, if you remember proteins that are exonucleases actually remove base pairs at the ends of DNA strands. Proteins that are endonucleases actually cut base pairs out of the middle of the DNA strand. But DNA polymerase is also an exonuclease, meaning that it can look at the end of the strand that it is creating, and if the last base is incorrect it simply cuts it off and puts a new one in. And it's 3' to 5' Exonuclease Activity. This can kind of be kind of confusing. So let me show you real quick. So let's say that this in black is our template strand. Oops, sorry about that guys. This is our template strand. And this is going to be the 3' end and this is the 5' end of the template. Now, what do we know about the process of DNA replication? Remember, we learned that a primer is put down and then the new strand of DNA is gonna be made. And it's gonna be made in this direction. Remember that this direction is the 5' to 3' direction. That is the direction the DNA is created. But which direction would the DNA be removed? It would be the opposite direction. Right? So you create DNA in the 5' to 3' direction. You remove a base from the end of the strand of DNA in the 3' to 5' direction. And that is because that strand or that base of DNA that is being removed is being removed in that direction by the DNA polymerase. Okay? I hope that makes sense. Just know that the direction in which the DNA polymerase removes an incorrect base is the opposite direction in which it adds new bases. So that that kind of makes sense, right? You're adding in one direction and if you mess up you got to take a step back in the backward direction. That's all that this particular DNA polymerase is doing, but it is given a fancy name called the 3' to 5' Exonuclease Activity which is also the proofreading activity. Okay? But you're probably going to need to know, this name, 3' to 5' exonuclease activity. I have seen this on tests and quizzes. It's, it's they're going to ask you to know this. They're also going to ask you to know what proofreading means. Okay?

Alright. So now that I've talked about proofreading, let's talk about why DNA can only build in the 5' to 3' direction. Why can new nucleotides only be added to the 3' end of the DNA? I know that I've told you this over and over and over again, but I haven't really told you why this is the case. So now we're going to go over why this is the process

DNA replication occurs differently at telomeres.

• A. True
• B. False

DNA is replication in which of the following directions?

• A. 5' to 3'
• B. 3' to 5'
• C. 5' to 5'
• D. 3' to 3'

Which of the following proteins are responsible for unwinding the DNA double helix for replication?

• A. DNA helicases
• B. Single stranded binding proteins
• C. DNA Topoisomerases
• D. Sliding Clamp

Only the lagging strand uses telomerase to replicate the ends of the telomeres?

• A. True
• B. False

Here’s what students ask on this topic:

What is the role of DNA polymerase in DNA replication?

DNA polymerase is a crucial enzyme in DNA replication. It catalyzes the synthesis of new DNA strands by adding nucleotides to a pre-existing chain. DNA polymerase can only add nucleotides to the 3' end of the growing DNA strand, synthesizing DNA in the 5' to 3' direction. It also has proofreading abilities, which means it can correct errors by removing incorrectly paired nucleotides through its 3' to 5' exonuclease activity. This ensures high fidelity during DNA replication, minimizing mutations and maintaining genetic stability.

What are Okazaki fragments and why are they important?

Okazaki fragments are short sequences of DNA nucleotides synthesized discontinuously on the lagging strand during DNA replication. They are important because DNA polymerase can only synthesize DNA in the 5' to 3' direction. On the lagging strand, this necessitates the creation of multiple RNA primers, allowing DNA polymerase to synthesize short DNA segments. These fragments are later joined together by DNA ligase to form a continuous strand, ensuring complete replication of the lagging strand.

How does the semi-conservative model of DNA replication work?

The semi-conservative model of DNA replication means that each new DNA molecule consists of one old (parental) strand and one newly synthesized strand. During replication, the double helix is unwound, and each strand serves as a template for the formation of a new complementary strand. This results in two DNA molecules, each with one original and one new strand, ensuring genetic continuity across generations.

What is the function of telomerase in DNA replication?

Telomerase is an enzyme that extends the telomeres, which are repetitive nucleotide sequences at the ends of linear chromosomes. During DNA replication, the lagging strand cannot fully replicate the chromosome ends, leading to gradual shortening. Telomerase solves this problem by adding repetitive sequences to the telomeres, providing a template for DNA polymerase to complete replication. This protects the coding regions of chromosomes from degradation and maintains chromosomal integrity.

Why is DNA replication considered bidirectional?

DNA replication is considered bidirectional because it proceeds in two opposite directions from each origin of replication. When the DNA double helix is unwound at the origin, two replication forks are formed, moving away from the origin in opposite directions. This allows both strands to be used as templates simultaneously, with the leading strand synthesized continuously and the lagging strand synthesized discontinuously in Okazaki fragments.