Last week, NASA announced a press conference that was hyped to the extreme. Some believed NASA would announce an alien life form, or perhaps an organism that used something other than DNA to store it’s genetic information. Sadly, what they announced was neither of these, but instead an interesting little quirk of biology.
Writing in Science, NASA scientists announced that they had discovered an extremophilic bacteria in Lake Mono, California, which has a pH of almost 10, and contains high levels of arsenic. When they cultured this bacteria in the lab, they found that it could grow in growth media which had phosphate replaced with arsenate, conditions that would usually be toxic to any other bacteria.
Then (and this is the fascinating part) they found that the bacteria were actually incorporating the arsenate into their biochemistry, including reaction pathways and their DNA. Usually, arsenic is toxic because it is similar enough to phosphorous that it competes with it, stopping various biochemical apparatus from functioning properly (they are in the same group of the periodic table). However, this organism could substitute the arsenic for phosphorus, and the biochemical pathways remained intact.
This seemed to be where the confusion with DNA comes in. The structure of DNA is well understood, and integral to this structure is a phosphate backbone. As this extremophile can substitute the phosphate for arsenate, it technically has DNA which is different to any other organism. However, this change is cosmetic, the code is exactly the same (as far as we know). It would be like painting your house a different colour then claiming that you had a completely new house.
In fact, this phenomenon was quite predictable. Arsenic is “one below” phosphorous in the periodic table, just as sulphur is “one below” oxygen. Extremophiles have been found living where oxygen is scarce but sulphur is plentiful. These organisms can substitute oxygen with sulphur, so it should come as no surprise that organisms exist that can substitute phosphorous with arsenic.
In conclusion, this is a fascinating discovery, but not earth-shattering. Don’t believe the hype!