The bacterium Deinococcus radiodurans has the remarkable ability to resist doses of ionising radiation many times higher than those that kill other organisms. In the January 10 Science Smadar Levin-Zaidman and colleagues at the Weizmann Institute of Science, Israel, report that the D. radiodurans genome has an unusual ring-like structure that may account for its radioresistance by restricting the diffusion of radiation-generated free DNA ends (Science 2003, 299:254-256). Scanning electron microscopy revealed that D. radiodurans cells have a tetrad morphology with each quarter containing equal amounts of DNA (each contains a single copy of the bacterial genome). This compartmentalization suggests that DNA repair after radiation does not involve homologous recombination. The bacterial nucleoids adopt a toroidal morphology that presumably dictates a rigid structure, facilitating template-independent, error-free, end-joining of DNA breaks.