Evidence that F' lac replicates asynchronously during the cell cycle of Escherichia coli B/r

The replication of F' lac was studied in exponentially growing cultures of E. coli B/r. The cells were pulse induced for the synthesis of beta-galactosidase and their DNA pulse labelled with 3H thymidine. The cells were then separated into age classes by centrifugation through a sucrose gradient in a zonal rotor. Plasmide replication was measured in each age fraction by three methods: the rate at which beta-galactosidase could be induced, the amount of label incorporated into CCC plasmid DNA which had been separated from chromosomal DNA on agarose gels, and the amount of label incorporated into plasmid DNA which had been separated from chromosomal DNA by ultracentrifugation through CsCl-EtBr gradients. All these methods gave the same result, that replication of F' lac occurs in cells of all ages and is not confined to a part of the cell cycle.