Dinococcus radiodurans is quite the interesting little bug. Its ability to resist radiation is thought to stem from its high genome copy number (aka it has more than one of its full genome per cell).

When ionizing radiation hits DNA it breaks it into fragments, these fragments are repaired by a variety of pathways with the most common being one called homologous recombination. This process works through complementarity between the broken DNA and intact DNA, and is particularly effective at repairing breaks during chromosomal replication (when there is often two genomes present, the parental copy and the newly forming daughter copy). In dinococcus, it has at minimum 2 copies of its genome, and during active replication phases of its cell cycle it can have up to 10 copies! This means that if large amounts of radiation blast at it there will be a lot of homologous intact regions that the repair machinery of the cell can use as a template to stitch the broken pieces back together in the appropriate locations.

Fascinating! :)