Abstract
Synchronized cultures of rapidly growing E. coli B/r cells were starved for a required amino acid at various cell ages to allow ongoing chromosome replication to be completed without initiation of new replication cycles. It has been found that when such synchronized cultures are exposed to the mutagen nitrosoguanidine, genetic markers just in the process of replication show a markedly higher mutation rate than markers elsewhere on the chromosome. The number of growing points on the chromosome at each cell age can then be determined by observing the nitrosoguanidine-induced mutation rates for specific genetic markers on the genome. These experiments indicate that there exist multiple growing points on the genome for about ten minutes during the life cycle when the cells are growing with a doubling time of 28 minutes.