Multiple In the context of DNA replication, how can the growing fork proceed in one direction, when both parental strands must be replicated and run in opposite directions?
A Replication is continuous on both strands.
B Replication is discontinuous on one strand.
C kazaki fragments are formed on the leading strand.
D Phosphase aids in replication.
The correct answer and explanation is:
The correct answer is B: Replication is discontinuous on one strand.
DNA replication is a semi-conservative process, meaning that each of the two new double-stranded DNA molecules contains one strand from the original DNA molecule. The replication process occurs in both directions at the replication fork, but because the two parental strands are antiparallel (running in opposite directions), the replication mechanisms differ for each strand.
DNA polymerase, the enzyme responsible for adding nucleotides to the growing DNA strand, can only add nucleotides in the 5′ to 3′ direction. This presents a challenge because the two parental strands run in opposite directions. On one strand, called the leading strand, DNA polymerase can replicate the strand continuously, moving in the same direction as the replication fork. This continuous synthesis happens because the DNA polymerase is moving along the template strand in the 3′ to 5′ direction, allowing the new strand to be synthesized in the 5′ to 3′ direction.
On the other hand, the lagging strand is synthesized discontinuously. As the replication fork opens, the lagging strand runs in the opposite direction of the replication fork. DNA polymerase cannot synthesize continuously in this direction, so it synthesizes short segments called Okazaki fragments, which are later joined together by the enzyme DNA ligase. This process ensures that the lagging strand is also copied in the 5′ to 3′ direction, despite the opposite orientation of the template strand.
Thus, replication is continuous on the leading strand but discontinuous on the lagging strand, which is why the growing fork can proceed in one direction even when both strands are being replicated in opposite directions.