Schematic representation of the replication termination fork and fork traps of the E. coli and B. subtilis chromosomes.
Replication forks proceed from the origin of the circular chromosome in opposite directions, creating a characteristic theta structure . Although the use of two active polymerase complexes is clearly faster than one, this mechanism generates difficulty when it comes to terminating replication. Among other factors, the presence of DNA-associated proteins causes the two replication complexes to proceed at different rates and they therefore do not necessarily meet directly opposite the origin. However, the position of termination is not random, and is defined by the presence of termination sites which associate protein factors capable of halting the advance of the replication fork from only one direction. From their positions within the chromosome and their directionality, you can see that this occurs only once the complex has traversed more than half of the total DNA.
