Given a long, linear chromosome, there are three ways that a hypothetical DNA polymerase could proceed. Two copies of the polymerase could start, one at each end, and work their way to the center. Or a single copy of polymerase could start somewhere in the middle and move in a single direction, copying as it goes. Or you could start in the middle with two copies of polymerase and move out in both directions. This bidirectional replication seems to be at work in eukaryotes and most prokaryotes, involving the the formation of expanding replication bubbles (Figure 13).

Figure 13. Replication Bubble. The bubble has a growing fork at each end, where DNA replication is carried out by the DNA polymerase complex. The bubble grows in both directions (right and left) as replication proceeds.