Bacterium uses arsenic in DNA

NASA's big press release today was that a new form of DNA was discovered in a type of bacteria living in Mono Lake, California. DNA usually requires phosphorus to hold together the different "rungs" of the "ladder". On the periodic table of the elements, phosphorus falls directly above arsenic, meaning they have the same number of electrons in their outer shells, and therefore act similarly in forming molecules. This is the very reason that arsenic is well-known as a poison: it is easily incorporated into human (or animal, or plant) chemistry, it replaces the phosphorus, but it does a crappier job than phosphorus and even though it can form similar molecules they easily fall apart.

Apparently this bacterium has not only overcome that - there are many bacteria that can live in an arsenic-rich environment - but it even uses that fact. This bacterium can apparently switch between using phosphorus, and using arsenic, depending upon which is available in its environment.

NASA press release

A very slightly more technical article, including a description of tests used to determine that the arsenic is actually incorporated into the DNA.

And a couple blog posts, one from a science writer Ed Yong, one from astronomer-turned-science-writer Phil Plait.