Replication slippage involves DNA polymerase pausing and dissociation

Genome rearrangements can take place by a process known as replication slip page or copy-choice recombination. The slippage occurs between repeated seq uences in both prokaryotes and eukaryotes, and is invoked to explain micros atellite instability, which is related to several human diseases. We analys ed the molecular mechanism of slippage between short direct repeats, using in vitro replication of a single-stranded DNA template that mimics the lagg ing strand synthesis. We show that slippage involves DNA polymerase pausing , which must take place within the direct repeat, and that the pausing poly merase dissociates from the DNA, We also present evidence that, upon polyme rase dissociation, only the terminal portion of the newly synthesized stran d separates from the template and anneals to another direct repeat. Resumpt ion of DNA replication then completes the slippage process.