DNA segregation in bacteria

Segregation of DNA in bacterial cells is an efficient process that assures that every daughter cell receives a copy of genomic and plasmid DNA. In thi s review, we focus primarily on observations in recent years, including the visualization of DNA and proteins at the subcellular level, that have begu n to define the events that separate DNA molecules. Unlike the process of c hromosome segregation in higher cells, segregation of the bacterial chromos ome is a continuous process in which chromosomes are separated as they are replicated. Essential to separation is the initial movement of sister origi ns to opposite ends of the cell. Subsequent replication and controlled cond ensation of DNA are the driving forces that move sister chromosomes toward their respective origins, which establishes the polarity required for segre gation. Final steps in the resolution and separation of sister chromosomes occur at the replication terminus, which is localized at the cell center. In contrast to the chromosome, segregation of low-copy plasmids, such as Es cherichia coli F, P1, and R1, is by mechanisms that resemble those used in eukaryotic cells. Each plasmid has a centromere-like site to which plasmid- specified partition proteins bind to promote segregation. Replication of pl asmid DNA, which occurs at the cell center, is followed by rapid partition protein-mediated separation of sister plasmids, which become localized at d istinct sites on either side of the division plane. The fundamental similarity between chromosome and plasmid segregation-place ment of DNA to specific cell sites-implies an underlying cellular architect ure to which both DNA and proteins refer.