Segregation of Bacillus subtilis chromosomes radioactively labeled during the first round of replication after germination of spores

Spores of Bacillus subtilis W23 thy his were allowed to incorporate [3H]thymine for short periods of time either continuously from, or soon after, the start of the first round of replication after germination. They were then transferred to nonradioactive medium to allow growth into microcolonies (up to 12 cells), which were autoradiographed. The relative numbers of various types (major versus minor) of grain clusters associated with individual microcolonies throughout the populations were scored. Analysis of the results showed clearly that, in the majority of spores at least, only one chromosome was undergoing replication soon after the start of deoxyribonucleic acid synthesis. Furthermore, under the conditions used, no evidence for initiation of replication of a second chromosome within 25 min after the first could be obtained. Accepting that B. subtilis spores are essentially homogenous in deoxyribonucleic acid content, the results support the conclusion that the spore contains only one copy of the chromosome, not two.