Dynamic Replication In all organisms, DNA replication involves a multiprotein complex called the replisome. Active Escherichia coli replisomes contain three copies of DNA polymerase III (Pol III). Lia et al. (p. 328 , published online 22 December) used single-molecule spectroscopy to probe the dynamics of single proteins during E. coli replication in vivo. The results confirmed the presence of three Pol III molecules in the active replisome, with regular exchange of one of these Pol III's with one from the pool. Coordination with single-stranded DNA content suggests the Pol III that performs lagging-strand synthesis is exchanged so that a new Pol III is used for the synthesis of each Okazaki fragment.