Abstract
Replication of the chromosome of E. coli B/r has been studied by measuring the change with cell age of the rates of reversion to wild type of auxotrophic markers induced by nitrosoguanidine. Synchronous populations of bacteria were obtained by the procedure of Helmstetter and Cummings. The origin of DNA synthesis was determined to occur at 49 minutes +/- 5 minutes on the genetic map, and the direction of synthesis was found to be clockwise.