DNA replication can proceed in only one direction, from the top of the DNA strand to the bottom. Because the strands that form the DNA double helix align in an antiparallel fashion with the top of one strand juxtaposed to the bottom of the other strand, only one strand at each replication fork has the proper orientation (bottom-to-top) to direct the assembly of a new strand in the top-to-bottom direction. For this leading strand, DNA replication proceeds continuously in the direction of the advancing replication fork.
DNA replication cannot proceed along the lagging strand, i.e. the strand with the top-to-bottom orientation, until the replication bubble expands enough to expose a sizeable stretch of DNA. DNA replication then moves away from the advancing replication fork. It can proceed only a short distance along the ‘top-to-bottom’ oriented strand before the replication process must stop and wait for more of the parent DNA strand to be unwound.