Escherichia coli chromosomal loci segregate from midcell with universal dynamics

The structure of the \textit{Escherichia coli} chromosome is inherently dynamic over the duration of the cell cycle. Genetic loci undergo both stochastic motion around their initial positions and directed motion to opposite poles of the rod-shaped cell during segregation. We developed a quantitative method to characterize cell-cycle dynamics of the \textit{E.\,coli} chromosome in order to probe the chromosomal steady state mobility and segregation process. By tracking fluorescently-labeled chromosomal loci in thousands of cells throughout the entire cell cycle, our method allows for the statistical analysis of locus position and motion, the step-size distribution for movement during segregation, and the locus drift velocity. The robust statistics of our detailed analysis of the wildtype \textit{E.\,coli} nucleoid allow us to observe loci moving toward midcell prior to segregation, consistent with a replication factory model. Then as segregation initiates, we perform a detailed characterization of the average segregation velocity of loci. Contrary to origin-centric models of segregation, which predict distinct dynamics for \textit{oriC}-proximal versus \textit{oriC}-distal loci, we find that the dynamics of loci were universal and independent of genetic position.