Prokaryotic DNA Replication : Direction of DNA replication

The DNA polymerases (DNA pols) responsible for copying the DNA templates are only able to read the parental nucleotide sequences in the 3′→5′ direction, and they synthesize the new DNA strands only in the 5′→3′ (antiparallel) direction. Therefore, beginning with one parental double helix, the two newly synthesized stretches of nucleotide chains must grow in opposite directions, one in the 5′→3′ direction toward the replication fork and one in the 5′→3′ direction away from the replication fork (Fig. 1). This feat is accomplished by a slightly different mechanism on each strand.

Figure 1: Semidiscontinuous synthesis of DNA. Black arrows = continuous synthesis; white arrows = discontinuous.
1. Leading strand: The strand that is being copied in the direction of the advancing replication fork is synthesized continuously and is called the leading strand.
2. Lagging strand: The strand that is being copied in the direction away from the replication fork is synthesized discontinuously, with small fragments of DNA being copied near the replication fork. These short stretches of discontinuous DNA, termed Okazaki fragments, are eventually joined (ligated) by ligase to become a single, continuous strand. The new strand of DNA produced by this mechanism is termed the lagging strand.