Tuesday, July 10, 2012


Last year  NASA made a sensational claim about discovery of  DNA in which the phosphorous is replaced by arsenic.  The bacterial strain GFAJ-1 was isolated from the arsenic-rich sediments of California’s Mono Lake by its ability to grow in the artificial Mono Lake medium without phosphate but with high concentrations of arsenate. It was claimed that GFAJ-1 grew in -P medium only when arsenate was provided, and  significant amounts of arsenate were detected in subcellular fractions.  This was interpreted as “ growth due to  use of arsenate in place of phosphate”. Wolfe-Simon et al. further reported that arsenic was incorporated into the DNA backbone of GFAJ-1 in place of phosphorus, with an  estimated 4% replacement of P by As based on the As:P ratio measured in DNA samples embedded in agarose gel slices.

This discovery claim by NASA  Scientists was sensational as
it changes the basic rules of life and prompted many to think that life is  much more diverse than we know before so we can expect life forms with different basic components in different planets. But many Scientists were skeptical about this  and predicted possible phosphate contamination in the media used. Two recent studies have found that  media contamination may have contributed attributed to the bacterial growth and not arsenic replacement of phosphorous .

In recent paper published ahead of print in Science journal  Erb et al (2012) have shown that , although the bacterium is able to grow under high-arsenate and low-phosphate conditions, it needs to be supplied with some amount of phosphate in order to grow at all. They found no evidence of arsenic substitution for phosphorus in the bacterium's metabolism. Phosphorus remained essential for the organisms to grow, they conclude, although that particular strain of bacterium is able to grow at much lower phosphate concentrations than other arsenate-resistant strains. In another independent study published simultaneously,  Reaves  et al (2012) have shown  that arsenic does not contribute to the bacteria's growth and the faulty findings is attributed to  trace contamination with phosphorous that  let the bacteria grow. 
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