Abstract
Two processes that are required for the initiation of a new round of bacterial DNA synthesis and which are blocked by low and high concentrations of chloramphenicol, respectively, have been examined in order to determine the cell age at which each of these processes occurs as a function of growth rate. The timing of these steps in the life cycle was determined by observing changes in the variation with cell age of the rate of DNA synthesis during growth in medium containing chloramphenicol of the desired concentration. For cell-doubling times ranging from 27 to 49 minutes, it was found that the step which is resistant to low concentrations of chloramphenicol occurs at the cell age at which a new round of DNA synthesis is initiated. The chloramphenicol-sensitive step was found to occur about 21 minutes before the start of rounds, independent of growth rate for doubling times between 27 and 49 minutes. For doubling of times 41 minutes or less, the chloramphenicol-sensitive processes occurred within five minutes after cell division.