What is a true statement about the leading and lagging strands during DNA synthesis?
The leading strand only utilizes DNA polymerase III during synthesis, while the lagging strand only utilizes DNA polymerase I
The lagging strand is created due to DNA polymerase III only being able to synthesize in the 5′ 3′ direction, despite the template DNA strands being in antiparallel directions
Unlike the Okazaki fragments in the lagging strand, the leading strand does not require RNA primers to undergo synthesis
Synthesis of the lagging strand does not require the function of topoisomerase, since the replication machinery is moving away from the replication fork
None of the above
The Correct Answer and Explanation is :
The correct answer is:
The lagging strand is created due to DNA polymerase III only being able to synthesize in the 5′ → 3′ direction, despite the template DNA strands being in antiparallel directions.
Explanation:
DNA replication occurs in the 5′ to 3′ direction, meaning that DNA polymerase can only add nucleotides to the 3′ end of the growing strand. Because the two strands of DNA are antiparallel, one of the strands, called the leading strand, can be synthesized continuously in the same direction as the replication fork is moving. However, the other strand, known as the lagging strand, is oriented in the opposite direction and cannot be synthesized continuously.
This is where the challenge arises. As the replication fork unwinds, the lagging strand is being synthesized in the opposite direction to the direction of fork movement. Since DNA polymerase III can only synthesize DNA in the 5′ → 3′ direction, it cannot continuously add nucleotides to the lagging strand. Instead, it synthesizes short fragments called Okazaki fragments in the 5′ → 3′ direction, each requiring an RNA primer to initiate synthesis. After each fragment is synthesized, the primer is removed by DNA polymerase I, and the gap is filled with DNA. Finally, DNA ligase joins the Okazaki fragments together to form a continuous strand.
Why the other options are incorrect:
- “The leading strand only utilizes DNA polymerase III during synthesis, while the lagging strand only utilizes DNA polymerase I”: This is incorrect because both DNA polymerase III and DNA polymerase I are involved in synthesizing both strands. DNA polymerase III is primarily responsible for elongating the DNA strand, while polymerase I removes RNA primers and fills in gaps.
- “Unlike the Okazaki fragments in the lagging strand, the leading strand does not require RNA primers to undergo synthesis”: This is false because RNA primers are required for both the leading and lagging strands. The leading strand requires an RNA primer to start the synthesis, just as the lagging strand does.
- “Synthesis of the lagging strand does not require the function of topoisomerase, since the replication machinery is moving away from the replication fork”: This is incorrect. Topoisomerase is required in both strands of replication because it alleviates the tension caused by the unwinding of DNA ahead of the replication fork. It is crucial for both the leading and lagging strands.
Thus, the most accurate statement is that the lagging strand is synthesized in short fragments due to the antiparallel nature of DNA and the fact that DNA polymerase III can only synthesize in the 5′ → 3′ direction.