Back To CourseBiology 101: Intro to Biology
22 chapters | 151 lessons | 12 flashcard sets
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April teaches high school science and holds a master's degree in education.
The replication of DNA is a complex process that took scientists many years and lots of hard work to understand. Previously, we discussed the fine details of semi-conservative replication: how the DNA double helix is unwound and how parent DNA is copied to produce daughter strands. We learned about a cast of helpful enzymes that make all the molecular movements possible. But, understanding how all the pieces fit together can be a challenge. So, in this lesson, we'll review all the parts of DNA replication and add some new bits of information that will help to complete the picture.
Let's start by reminding ourselves of the basics of semi-conservative replication. We begin with the original DNA molecule, and both strands in that molecule are referred to as parent strands. The goal of DNA replication is to make a second DNA molecule, using the parent strands as a template to create two new daughter strands. The term semi-conservative refers to the fact that both parent strands are conserved, or saved, in each of the new molecules. So, in semi-conservative replication, the parent strands split apart, but each remain whole, while new daughter strands are added onto them. The end result is two DNA molecules, each containing one parent strand and one daughter strand.
Now, let's go over all the steps of DNA replication. Along the way, we'll check in with each essential enzyme and discuss how it helps in completing each step. Since the names and functions of the enzymes can get confusing, we'll make an enzyme chart before we begin. As we discuss the steps of DNA replication, we'll fill in our enzyme chart to keep all our information organized.
The first step occurs when DNA helicase unwinds the double helix by breaking the hydrogen bonds between the parent strands of DNA. This splitting and unwinding process opens up the DNA molecule into a Y-shape, which we call the replication fork. Already we have our first enzyme, so let's fill that in. DNA helicase is the enzyme that unwinds the DNA double helix.
Before new daughter strands can be added on, the parent strands are first made ready by an RNA primer. The primer is built by the enzyme RNA primase. So, let's go to the chart: RNA primase is the enzyme that builds an RNA primer on the parent strand to initiate DNA replication.
Once the RNA primer is built, then the next enzyme, DNA polymerase, is free to do its job. DNA polymerase slides into the replication fork and positions itself behind the RNA primer. It begins to add DNA nucleotides onto each parent strand. DNA polymerase always works by starting at the 3' end of DNA and moving toward the 5' end. This means that on the leading strand, it works continuously as it follows DNA helicase, which is constantly opening the fork more and more.
But on the lagging strand, DNA polymerase works discontinuously, making Okazaki fragments in the opposite direction. So, now that we understand what DNA polymerase does, let's go back to our chart. DNA polymerase is the enzyme that matches and lays down nucleotides to build the daughter DNA strand along each parent DNA strand.
Now we're left with all these Okazaki fragments that are separate from each other, so they need to be joined together by the enzyme DNA ligase. DNA ligase binds the fragments end to end, forming a continuous daughter strand on top of the lagging parent strand. So, that was our last enzyme: DNA ligase joins the adjacent Okazaki fragments on the lagging strand of DNA.
Let's back up for just a minute. Have you noticed a couple of elements that don't quite seem to fit? If you have, then you're on the right track. Until now, we haven't had a chance to put the concept of the RNA primer together with the concept of Okazaki fragments. In earlier lessons, we talked about how the RNA primer must be present before DNA polymerase can begin to do its job. Later, when we talked about Okazaki fragments, we mentioned that DNA polymerase has to keep going back and restart its work at the beginning of every fragment. So, you may be wondering about this. Doesn't there have to be a new RNA primer to begin every new Okazaki fragment?
Well, the answer is yes. Every time DNA polymerase restarts its work, it needs an RNA primer. Of course, in the leading strand, only one primer is needed. This primer is built upon the 3' end, the free end, before replication begins moving toward the fork. But on the lagging strand, we need an RNA primer for each Okazaki fragment! Every time DNA polymerase finishes out its track and moves back for another go, it has to have a primer waiting there to help it get started again. So, on the lagging strand, there are lots of RNA primers scattered all down the line - one for every Okazaki fragment that is to be made.
At this point, you can probably see why the two strands are named as they are. The leading strand is always growing just as quickly as the replication fork opens up. It's always ahead of the other strand, so it leads the other in completing DNA replication. The lagging strand is always behind because of the discontinuous replication. It takes more time for it to complete its daughter strand because it has to wait for DNA polymerase to restart its work over and over again. And then it has to wait for DNA ligase to join the fragments. So, it lags behind the leading strand.
Now, all this talk about the leading and lagging strands may have you wondering something else: What about the replication bubbles? Remember, DNA replication begins at multiple places along the parent molecule. DNA helicase splits it open and creates replication bubbles that grow and merge together. Replication begins at the center of the bubble and moves in both directions, outward. But strangely, whenever we've talked about the leading and lagging strands, we've looked at it like everything just starts from one end. We've only been looking at half the bubble. So, if DNA replication really does happen inside all these bubbles, then how do the leading and lagging strands fit in?
The truth is, we've only been looking at half the bubble to make the leading and lagging strands easier to see. So, now let's zoom out and look at the whole bubble. The leading and lagging strands actually join up with similar structures in the other half. For every replication bubble, there are two replication forks, one at either end. So, at the top of the bubble, the leading strand from one replication fork joins the lagging strand from the other replication fork. And the same is true at the bottom of the bubble.
So, as you can see, we end up with an intricate layout. On the upper left and the lower right, we have leading strands being replicated continuously, needing only one RNA primer each. On the upper right and lower left, we have lagging strands being replicated discontinuously, with multiple RNA primers initiating all the Okazaki fragments.
So, now you've learned a couple of new things that tie together all the details of DNA replication. As complicated as it may seem, the end result is the same as we've always predicted by the model of semi-conservative replication: a new daughter strand is built upon each of the separated parent strands, so that every parent strand is conserved in each of the new DNA molecules.
One of the parent strands is the leading strand, which is replicated continuously. The other one is the lagging strand, and it's replicated by way of short Okazaki fragments. Each fragment requires that an RNA primer is built first upon the parent DNA strand. The RNA primers are made by RNA primase, and the Okazaki fragments are joined by DNA ligase.
DNA polymerase is the enzyme that carries in the daughter nucleotides, and DNA helicase is the one that unwinds the double helix to open the replication fork. While every replication fork consists of a leading and lagging strand, each fork is actually joined to another fork in the form of a replication bubble. Once all of the bubbles grow and merge together, the process of DNA replication is finally complete.
By the end of this lesson, you will be able to explain the process of DNA replication, using the semi-conservative model and identifying the important steps and enzymes involved.
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Back To CourseBiology 101: Intro to Biology
22 chapters | 151 lessons | 12 flashcard sets