Some organisms have more copies of their genome than others do. At first, you may think it would be good to have extra copies of your own personal blueprints, just in case you lose one of the instructions. Although having an extra copy has obvious advantages, it also may have some costs, including the additional time it takes to replicate two copies of your genome before cell division. Biologists think that these costs must outweigh the benefits for most bacteria, which is why many bacteria have only a single copy of their genome.
Sometimes, when the benefits of having a backup copy outweigh the costs of growing more slowly, an organism has an extra copy of its genome. These cases include organisms that really need multiple spare genomes to fix errors. One such organism is the bacteria species Deinococcus radiodurans. This little critter has not one extra copy of its genome, but several copies, probably due to the fact that it lives in extremely harsh environments where DNA damage is more likely to occur from such factors as extreme drying. As a result of having these extra copies of its genetic material, Deinococcus radiodurans is the most radiation-resistant organism known. This little fellow can handle 500 times more radiation than you can.
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