Leading & Lagging DNA Strands - Online Tutor, Practice Problems & Exam Prep

During DNA replication, two strands are formed: the leading strand and the lagging strand. The leading strand is synthesized continuously in the same direction as the replication fork, requiring only one RNA primer. In contrast, the lagging strand is synthesized discontinuously in the opposite direction, forming Okazaki fragments, each needing its own RNA primer. These fragments are later joined by DNA ligase. This process is essential for accurate DNA replication, highlighting the roles of DNA polymerase and the importance of the origin of replication.

Leading & Lagging DNA Strands

In this video, we're going to introduce the leading and lagging DNA strands. And so it turns out that upon separation of DNA during DNA replication, at each replication fork, there are going to be 2 single-stranded DNA molecules. The first one will be the leading DNA strand, and the second one will be the lagging DNA strand. Now the leading DNA strand is going to be built with continuous DNA replication in the same direction as the replication fork movement. And because the leading DNA strand is built in the same direction as the replication fork movement, it only requires just one single RNA primer, and so that requirement for one single RNA primer is going to be associated with the leading strand. Now, the lagging DNA strand, on the other hand, is not built with continuous replication. Instead, the lagging DNA strand is going to be built with discontinuous replication in the opposite direction of the replication fork movement. And because it's built in the opposite direction of the replication fork movement, the lagging strand is replicated in multiple small segments that are called Okazaki fragments, and these Okazaki fragments are named after the Japanese scientist whose last name was Okazaki. And each of these small segments, each of these Okazaki fragments, each requires an RNA primer to be built. And so, the lagging DNA strand requires multiple RNA primers and, again the leading strand only requires 1 RNA primer. Now these multiple small segments in the lagging DNA strand, these Okazaki fragments, are eventually going to be covalently annealed to each other by an enzyme called DNA ligase.

And, if we take a look at our image down below, we can better distinguish between the leading and lagging DNA strands. And so, notice that here what we're showing you is a replication bubble with 2 forks. We have one replication fork over here on the left and another replication fork over here on the right. And so, you can also see the origin of replication here, which is, the sequence of DNA, where DNA replication begins. Now, at each of these replication forks, notice that there is a leading strand, which is going to be built in the same direction as the replication fork movement, and there's also going to be a lagging strand, which is built in the opposite direction of the replication fork movement. And so recall that this replication fork here is moving in this direction to the left, and this yellow background, is moving and being built in the same direction as the replication fork movement. And that also applies for this leading strand over here being built in the same direction as the replication fork movement. But the lagging strand, once again, which is highlighted with a greenish background, is being built in the opposite direction of the replication fork movement, and that goes again over here as well, being built in the opposite direction of the replication fork movement. And so, the reason that the lagging strand needs to be built in these small segments called Okazaki fragments is because of the limitations of DNA polymerase and DNA polymerase needing to build in the 5 prime to 3 prime direction. And so you can see that these Okazaki fragments are being labeled as so, and there are multiple RNA primers required for the lagging strands, whereas the leading strands, once again, only require one RNA primer. Now ultimately, remember that these RNA primers are going to be replaced with DNA. And, also these fragments of, these Okazaki fragments are going to be covalently annealed to each other with another enzyme called DNA ligase.

This here is going to conclude our introduction to the leading and lagging DNA strands, and we'll be able to get 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.

Which is involved in replicating the lagging strand of DNA, but is not involved in leading strand replication?

• A. Ribosome.
• B. RNA primer.
• C. DNA polymerase.
• D. Okazaki fragments.

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 in the same direction as the movement of the replication fork, and the lagging strand is synthesized in the opposite direction.
• B. 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.
• 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 is synthesized at twice the rate of the lagging strand.

The mechanisms of DNA synthesis differs between the two new daughter strands during replication. This is due to the fact that:

• A. one RNA primer attaches to the 5' end of the parent strand and the other primer to the 3' end.
• B. Both daughter strands can't extend toward the replication fork because there would not be room for two DNA polymerase enzymes.
• C. Both RNA primers attach to the 3' end of the template strands, which are at opposite ends from each other.
• D. The DNA strands run antiparallel to each other and the DNA polymerase can only add nucleotides to the 3' end of the growing strand.

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 is the difference between the leading and lagging DNA strands?

The leading and lagging DNA strands differ in their synthesis during DNA replication. The leading strand is synthesized continuously in the same direction as the replication fork movement, requiring only one RNA primer. In contrast, the lagging strand is synthesized discontinuously in the opposite direction, forming small segments called Okazaki fragments. Each Okazaki fragment requires its own RNA primer. These fragments are later joined by the enzyme DNA ligase. This difference is due to the DNA polymerase enzyme, which can only synthesize DNA in the 5' to 3' direction.

Why does the lagging strand require multiple RNA primers?

The lagging strand requires multiple RNA primers because it is synthesized discontinuously in small segments called Okazaki fragments. Each fragment needs its own RNA primer to initiate synthesis. This discontinuous synthesis occurs because DNA polymerase can only add nucleotides in the 5' to 3' direction, which is opposite to the direction of the replication fork movement for the lagging strand. As a result, new primers are needed to start each new fragment.

What are Okazaki fragments and how are they joined together?

Okazaki fragments are short segments of DNA synthesized on the lagging strand during DNA replication. These fragments are formed because the lagging strand is synthesized discontinuously in the opposite direction of the replication fork movement. Each fragment requires an RNA primer to start synthesis. Once synthesized, the RNA primers are replaced with DNA, and the enzyme DNA ligase covalently joins the Okazaki fragments together to form a continuous DNA strand.

What role does DNA ligase play in DNA replication?

DNA ligase plays a crucial role in DNA replication by joining Okazaki fragments on the lagging strand. After the RNA primers are replaced with DNA, DNA ligase catalyzes the formation of phosphodiester bonds between the fragments, creating a continuous DNA strand. This enzyme ensures the integrity and continuity of the newly synthesized DNA, which is essential for accurate replication and cell division.

How does the direction of replication fork movement affect the synthesis of leading and lagging strands?

The direction of the replication fork movement affects the synthesis of the leading and lagging strands due to the 5' to 3' synthesis directionality of DNA polymerase. The leading strand is synthesized continuously in the same direction as the replication fork movement, requiring only one RNA primer. In contrast, the lagging strand is synthesized discontinuously in the opposite direction, forming Okazaki fragments, each needing its own RNA primer. This difference ensures that both strands are replicated efficiently despite the unidirectional activity of DNA polymerase.