Which statement is correct about DNA replicatinon. A single DNA strand is either the template for the leading or lagging strand but cannot be both
The Correct Answer and Explanation is:
The statement is correct. A single DNA strand serves as the template for either the leading strand or the lagging strand during DNA replication, but it cannot serve as the template for both simultaneously.
DNA replication is a semi-conservative process in which two new strands are synthesized from an original DNA molecule. The replication occurs in two directions: one strand is synthesized continuously, and the other is synthesized in short, discontinuous segments. This is due to the directionality of the DNA polymerase enzyme, which can only add nucleotides to the 3′ end of a growing strand. The template strand for the synthesis of the leading strand is oriented in such a way that it allows continuous replication, moving toward the replication fork. On the other hand, the template strand for the lagging strand is oriented in the opposite direction, so replication occurs in segments (Okazaki fragments), which are later joined together by DNA ligase.
In more detail:
- Leading strand: The template strand for the leading strand runs in the 3′ to 5′ direction relative to the replication fork. DNA polymerase synthesizes the new strand continuously in the 5′ to 3′ direction toward the replication fork.
- Lagging strand: The template strand for the lagging strand runs in the 5′ to 3′ direction relative to the replication fork. This orientation means that DNA polymerase can only synthesize the new strand in short Okazaki fragments, which are later joined together by DNA ligase.
Therefore, because of the different directions of synthesis, a single DNA strand can only serve as a template for one strand of the replication process—either the leading or lagging strand.
