Final answer:
DNA replication is bidirectional, starting from the origin and moving in opposite directions. It is also discontinuous on the lagging strand, where replication takes place in short fragments known as Okazaki fragments.
Step-by-step explanation:
Replication moves out from the origin in bidirectional directions and is said to be bidirectional. The correct answer is b. opposite, bidirectional. In the process of DNA replication, the replication begins at a specific location on the DNA molecule known as the origin of replication. From there, enzymes like DNA helicases unwind the DNA, and replication proceeds in both directions from the origin, creating two replication forks that move outward. This bidirectional nature ensures that both strands of the DNA double helix are replicated efficiently and accurately.
DNA replication is also described as discontinuous, particularly in reference to the lagging strand, where DNA is synthesized in short stretches known as Okazaki fragments. On this strand, replication occurs in a series of segments because the DNA polymerase can only add nucleotides to the 3' end of a growing strand. As replication forks move away from the origin, the lagging strand must be synthesized in these discontinuous stretches, while the leading strand is synthesized continuously in the same direction as the replication fork movement.