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

During DNA replication, two strands are formed: the leading and lagging strands. 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 highlights the limitations of DNA polymerase, which can only synthesize DNA in the 5' to 3' direction, emphasizing the complexity of DNA replication.

Leading & Lagging DNA Strands

In this video, we're going to introduce the leading and lagging DNA strands. It turns out that upon the 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. 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. On the other hand, the lagging DNA strand 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, named after the Japanese scientist whose last name was Okazaki. Each of these small segments, these Okazaki fragments, requires an RNA primer to be built. 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. Now, if we take a look at our image down below, we can better distinguish between the leading and lagging DNA strands. 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. You can also see the origin of replication here, which is the sequence of DNA, where DNA replication begins. 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 a lagging strand, which is built in the opposite direction of the replication fork movement. So recall that this replication fork here is moving in this direction to the left, and this leading strand, with a yellow background, is being built in the same direction as the replication fork movement. 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.

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 needing to build in the 5' to 3' direction. Now, ultimately, remember that these RNA primers are going to be replaced with DNA. Also, these fragments, 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 short segments called Okazaki fragments. Each Okazaki fragment requires its own RNA primer. These fragments are later joined together by the enzyme DNA ligase. This difference arises because DNA polymerase can only synthesize DNA in the 5' to 3' direction, necessitating the discontinuous synthesis of the lagging strand.

Why does the lagging strand require multiple RNA primers?

The lagging strand requires multiple RNA primers because it is synthesized discontinuously in short segments called Okazaki fragments. DNA polymerase can only add nucleotides in the 5' to 3' direction, but the lagging strand is oriented in the opposite direction of the replication fork movement. Therefore, each new Okazaki fragment needs a new RNA primer to initiate synthesis. These RNA primers are later replaced with DNA, and the fragments are joined together by DNA ligase to form a continuous strand.

What role does DNA ligase play in DNA replication?

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

What are Okazaki fragments and why are they important?

Okazaki fragments are short segments of DNA synthesized on the lagging strand during DNA replication. They are important because they allow the lagging strand to be synthesized in the 5' to 3' direction, despite being oriented in the opposite direction of the replication fork movement. Each fragment requires an RNA primer to start synthesis. Once all fragments are synthesized, DNA ligase joins them together to form a continuous strand. This process ensures that the entire DNA molecule is accurately replicated.

How does the direction of replication fork movement affect DNA strand synthesis?

The direction of replication fork movement affects DNA strand synthesis by determining how the leading and lagging strands are synthesized. 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 fragment requires its own RNA primer. This difference is due to the limitation of DNA polymerase, which can only synthesize DNA in the 5' to 3' direction.