Spatial organization of a replicating bacterial chromosome.

Emerging evidence indicates that the global organization of the bacterial chromosome is defined by its physical map. This architectural understanding has been gained mainly by observing the localization and dynamics of specific chromosomal loci. However, the spatial and temporal organization of the entire mass of newly synthesized DNA remains elusive. To visualize replicated DNA within living cells, we developed an experimental system in the bacterium Bacillus subtilis whereby fluorescently labeled nucleotides are incorporated into the chromosome as it is being replicated. Here, we present the first visualization of replication morphologies exhibited by the bacterial chromosome. At the start of replication, newly synthesized DNA is translocated via a helical structure from midcell toward the poles, where it accumulates. Next, additionally synthesized DNA forms a second, visually distinct helix that interweaves with the original one. In the final stage of replication, the space between the two helices is filled up with the very last synthesized DNA. This striking geometry provides insight into the three-dimensional conformation of the replicating chromosome.