To fit in the bacterial cell, the genetic material is packed into tightly wound structures - the chromosomes. During replication, the chromosome is unwound and the DNA is copied so that when the bacterium divides, each cell will contain a complete genome. The molecular complex responsible for this crucial process is called the replisome. The replisome contains several proteins including enzymes for melting the chromosome structures and synthesizing new DNA, but the dynamics of this elaborate machine are debated - do the complex remain stationary, pulling the DNA through as it is replicated, or does the replisome follow the chromosome, like a tiny cart on a track (or a baby dragon running on spaghetti)?

Our latest study sheds new light on this debated question. Using high-throughput fluorescence microscopy of individual E. coli cells, we determine the location and movement of the replisome and different chromosomal DNA loci throughout the cell cycle. We find that during replication, the replisomes are confined to a small region, a few hundred nanometers wide, favoring the stationary replisome model. In line with this model, the loci appear to move to and through this region sequentially based on their distance from the origin of replication. When a locus is replicated, its short-time-scale movement decreases.

This cartoon illustrates the paths of selected DNA loci and replisomes throughout the cell cycle. Ori and Ter are the loci where replication starts and finishes, respectively, and L3 and L4 are loci in between.

Read more in Nature Communications!