As Gropper, Smitth and Groff tell it, arsenic "conjures an image of toxicity" unlike any other ultratrace mineral (1), but there are good things that come of arsenic and its story is worth discussion.
Without arsenic, DNA synthesis couldn't happen. This is because arsenic is needed for normal metabolism. Specifically, the mineral is required for forming and using methyl groups to S-adenosylmethionine (SAM) (1). SAM is used for methylation to form DNA compounds (1).
Arsenic, in fact, may have once been part of DNA itself. The mineral is very similar to phosphorus, which is currently the backbone of nucleic acids. Because this is is so it has been suggested that arsenic may have served as an alternate element early on, although not possible in modern biochemistry (3).
According to scientists Felisa Wolfe-Simon, Paul Davies and Ariel Anbar, there may even be possibility that organisms may still be using arsenic in their DNA today, but simply not yet found (3).
1. Gropper SS, Smith JL, Groff JL. Advanced Nutrition and Human Metabolism. Belmont, CA: Thomson Wadsworth, 2009.2. Emsley J. Nature's Building Blocks: An A-Z Guide to the Elements. Oxford University Press, 2003.
3. Wolfe-Simon F, Davies PCW, Anbar A. Did nature also choose arsenic? Nature Precedings, Jan 2008.