Steps of DNA Replication - Online Tutor, Practice Problems & Exam Prep

DNA replication occurs in six key steps: helicase unwinds the DNA strands by breaking hydrogen bonds, followed by stabilizing proteins that keep the strands apart. Primase adds RNA primers for DNA polymerase to initiate replication in the 5' to 3' direction. DNA polymerase also replaces RNA primers with DNA. Finally, DNA ligase connects Okazaki fragments on the lagging strand, forming a continuous DNA strand. This process is essential for accurate genetic information transfer during cell division.

Steps of DNA Replication Concept 1

Now, DNA replication can be simplified into six steps. Step 1 involves the enzyme helicase. Helicase binds and unwinds the two strands of the template DNA by breaking hydrogen bonds. If you come down here in the image, we're first going to label the 5' and 3' ends of our DNA template strands. We're going to say up here is our 5' and if we follow this, here, this will be 3'. If this is 5', this would have to be 3'. This would have to be 5'. Then, coming down here, if we follow this, we can see that this trend here would be 5' and therefore, this would have to be 3'. So these are the ends of our DNA strands.

We're talking about Step 1 where helicase is going to cut through our hydrogen bonds between the nitrogenous bases. So, we can see here that these hydrogen bonds are broken with a bang by the helicase enzyme. It slices through them, opening up the DNA double helix so that replication can commence.

Now, here for Step 2, we have our stabilizing proteins. Here, they're going to bind to and stabilize the single-stranded DNA. DNA wants to maintain these hydrogen bonds, but we have these stabilizing proteins which come in and force it to stay open long enough so that DNA replication can proceed. Remember, they're represented by these orange spheres. So, this would be Step 2.

Next, we have Step 3, which deals with primase, our enzyme. Primase adds the RNA primer to the template DNA so that polymerase can start replicating. Now, if we come down here, we see primase adding down these primers. We have one here, here, here, and here. And it's important here that we're going to continuously add primers to the lagging strand to make several Okazaki fragments. Remember, the lagging strand goes in the opposite direction of the replication fork movement. Because of this, we're going to have to continuously add primers to make our new DNA strands, and this is just the way it has to be.

Now we have DNA polymerase, which has two functions. First, it adds new DNA nucleotides in the 5' to 3' direction. So here, this would be Step 4. We're adding in the 5' to 3' direction. So, since this is the 3' end, and the strands have to be antiparallel in order to be complementary, we're going to say here that this is the 5' to 3' end in which we lay down our new DNA.

The second function of DNA polymerase is that it removes RNA primers and replaces them with DNA. So, we're going to say here, Step 5, there's this primer here; this polymerase will also replace that primer in order to add in new DNA. These pink portions and all other pink portions are eventually going to be removed by DNA Polymerase.

And finally, Step 6, we have DNA ligase which links Okazaki fragments together on the lagging strand to create a single new DNA strand. Remember, there are gaps that we have between our Okazaki fragments. Step 6, our ligase comes in; it kind of fills in these gaps here. So, we have one strand of new DNA just like we have our leading strand up here that's all one strand of DNA.

Right, so when we talk about DNA replication, we can talk about it in reference to these six major steps, which incorporate different enzymes and proteins in order for DNA replication to be successful. So, again, just remember, from helicase all the way down to ligase, they all have a job in order to get new DNA, new daughter strands to be created from our DNA template strands.

Steps of DNA Replication Example 1

In this example question, it says, DNA Polymerase can join new nucleotides only to the end of a pre-existing strand. So we're talking about the primer, and the strands are antiparallel. This will provide the basis of, well, think about what DNA polymerase does. It's going to help add new DNA strands. These new DNA strands can come in the form of leading and lagging strands. Now, here if we take a look, it does more than just leading strands. Yes. It can have the formation of Okazaki fragments, which are the individual parts of the lagging strand. It does more than that. The formation of the Ori? No. The origin of replication? That's not what DNA Polymerase does. Both leading and lagging strands of DNA? Yes. Again, Polymerase has a function of adding new DNA. This comes in the form of either leading strands or lagging strands. Remember, if we're looking at one individual lagging strand, that represents an Okazaki fragment. So really by saying lagging strands, we're including Okazaki fragments as well. So this is our answer. If we look at the last one, it says DNA ligase and stabilizing proteins. Those are other enzymes and other proteins. They work independently; they work in conjunction with DNA polymerase. They are not responsible for what polymerase does. They have their own separate functions. So this would not be a good example, would not be a good answer. So here the answer is option D.

Which of the following statements correctly describes the difference between the leading and the lagging strands of DNA during DNA replication?

• A. The leading strand is synthesized by adding nucleotides to the 3' end of the growing strand, and the lagging strand is synthesized by adding nucleotides to the 5' end.
• B. The leading strand is synthesized in the same direction as the movement of the replication fork, and the lagging strand is synthesized in the opposite direction.
• C. The lagging strand is synthesized continuously, whereas the leading strand is synthesized in short fragments that are ultimately stitched together.
• D. The leading strand slows its rate of replication so that the lagging strand can catch up.

Below is a close-up of the portion of a DNA replication bubble.

Helicase is shown as a yellow triangle currently moving from left to right. Based on what you know about the creation of new DNA during replication, which is the lagging strand and why?

• A. A is the lagging strand, as DNA is always synthesized in the 5' to 3' manner.
• B. B is the lagging strand, as DNA is always synthesized in the 5' to 3' manner.
• C. A is the lagging strand, as DNA is always synthesized in the 3' to 5' manner.
• D. B is the lagging strand, as DNA is always synthesized in the 3' to 5' manner.
• E. It is impossible to tell, with the information provided.

Here’s what students ask on this topic:

What are the six steps of DNA replication?

The six steps of DNA replication are:

1. Helicase: This enzyme unwinds the DNA double helix by breaking hydrogen bonds between the nitrogenous bases.

2. Stabilizing Proteins: These proteins bind to the single-stranded DNA to keep it open and stable for replication.

3. Primase: This enzyme adds RNA primers to the template DNA, providing a starting point for DNA polymerase.

4. DNA Polymerase: This enzyme adds new DNA nucleotides in the 5' to 3' direction and also replaces RNA primers with DNA.

5. Removal of RNA Primers: DNA polymerase removes RNA primers and replaces them with DNA.

6. DNA Ligase: This enzyme links Okazaki fragments on the lagging strand, forming a continuous DNA strand.

What role does helicase play in DNA replication?

Helicase is an essential enzyme in DNA replication. Its primary role is to unwind the DNA double helix by breaking the hydrogen bonds between the nitrogenous bases. This action separates the two strands of the DNA molecule, creating a replication fork and allowing the replication machinery to access the template strands for the synthesis of new DNA. Without helicase, the DNA strands would remain tightly coiled, preventing the replication process from proceeding.

How do stabilizing proteins function during DNA replication?

Stabilizing proteins, also known as single-strand binding proteins (SSBs), play a crucial role during DNA replication. Once helicase unwinds the DNA double helix, these proteins bind to the single-stranded DNA to prevent it from re-annealing or forming secondary structures. By keeping the strands separated and stable, stabilizing proteins ensure that the replication machinery can efficiently access and replicate the DNA template strands.

What is the function of primase in DNA replication?

Primase is an enzyme that synthesizes short RNA primers on the DNA template strands during replication. These RNA primers provide a starting point for DNA polymerase to begin adding DNA nucleotides. Primase is particularly important for the lagging strand, where multiple primers are needed to create Okazaki fragments. Without primase, DNA polymerase would not be able to initiate the synthesis of new DNA strands.

What are Okazaki fragments and how are they connected?

Okazaki fragments are short segments of DNA synthesized on the lagging strand during DNA replication. Because the lagging strand is oriented in the opposite direction of the replication fork movement, DNA polymerase must synthesize it in short, discontinuous segments. These fragments are later connected by the enzyme DNA ligase, which seals the gaps between them, forming a continuous DNA strand.