I know all the headlines have been saying it, but the bacteria discovered in Mono lake are not "arsenic-based" life forms. It's just that they can use arsenic in place of phosphorus when the latter element is scarce. The reason this is possible is that arsenic is chemically similar to phosphorus -- this is why it's located in the same column, one row down in the periodic table. The differences between the two are enough to make it toxic to every other life form known.
If you remember your high school biology, cells on Earth use a molecule called ATP (adenosine triphosphate) to transport energy. This bacteria can substitute arsenic for the phosphate in this molecule, which results in adenosine triarsenate (ATA), and it can also use arsenic in its DNA.
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