Final answer:
The most radiation-resistant organism known is Deinococcus radiodurans, a polyextremophile with robust DNA repair mechanisms allowing it to survive extreme levels of ionizing radiation and other harsh conditions.
Step-by-step explanation:
The most radiation-resistant organism known is Deinococcus radiodurans, a prokaryote that is capable of surviving exposure to extremely high doses of ionizing radiation. This microbe, sometimes nicknamed 'Conan the Bacterium,' is considered a polyextremophile due to its ability to endure a variety of harsh conditions such as extreme heat, drought, vacuum, acidity, and particularly high levels of radiation. It can withstand levels of ionizing radiation a thousand times greater than what humans can tolerate. Deinococcus radiodurans owes its extraordinary survivability to efficient DNA repair mechanisms that can reconstruct its chromosome even if it has been shattered into hundreds of pieces by radiation or heat.