Rolling circle replication

Rolling circle replication is the unidirectional mode of DNA replication employed by circular DNA molecules, such as plasmids and the genomes of bacteriophages and some eukaryotic viruses. In viruses with linear genomes, the ability to circularise once inside a cell is a crucial prerequisite for rolling circle replication. By replicating in this fashion, the virus can ensure that no genetic material is lost from its genome as a consequence of successive rounds of replication. Circularisation of linear phage genomes occurs by the interaction between cos sites (cohesive sites) in the viral genome.

The process begins with a plasmid or phage-encoded enzyme called relaxase, which creates a nick in the circular DNA at a site called the double-strand origin (DSO); the relaxase remains bound to the 5' phosphate at the site of this nick, so the 3' OH group is available as a primer for DNA synthesis by DNA polymerase III. The polymerase moves along the nicked strand, using the un-nicked strand as a template for replication, and a helicase displaces the nicked strand behind polymerase as a single-stranded DNA molecule. This procedure can be repeated multiple times to create numerous linear copies in a continuous head-to-tail series called a concatemer.

To make these linear strands double-stranded and circular again, an initiator protein makes another nick to terminate DNA synthesis. DNA polymerase III and RNA polymerase then work in conjunction to replicate the single-strand origin (SSO) of a linear strand to make it double-stranded. Finally, DNA polymerase I removes the primer, replacing it with DNA, and DNA ligase covalently binds the strands end-to-end to make the final circular structure.