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How arsenic has integrated itself into DNA

Simple recipe: remove phosphate and add arsenic!

DNA, the founding thing of all living organisms, is a fascinating and complex molecule. Since the discovery of its structure by Watson and Crick in 1953, scientists have done their best to understand its ins and outs.

How its multiplication and division gives us our children, how slight modifications in its pattern can change everything about its carrier, how its mutations can lead us down the path of evolution and how the damage it incurs can lead to cancer and other diseases.

Its structure is well known: a double strand of long molecules composed of 4 different nucleotides (A, T, C and G) which match up in pairs (A-T and C-G) creating a double helix shape. Each nucleotide is composed of a base (A, T, C or G) and a five-carbon sugar which are linked together by a phosphate group. This creates what is known as a phosphate backbone.

The only known alteration to this motif is RNA, a single stranded version of DNA. While it has some differences, the main atoms used in its structure are the same found in all other molecules of living things. These same atoms (oxygen, carbon, nitrogen, hydrogen, sulphur, phosphate) are the basis of all bio-molecules and hardly any others are found in their structure. Until now.

A group of scientists lead by evolutionary geochemist Felisa Wolfe-Simon have found a strain of bacteria that contradicts everything we know about the genetic make-up of living organisms. The bacterium GFAJ-1 lives under very special conditions in the arsenic infested waters of Mono Lake in California. Now there are plenty of bacteria that survive under undeniably harsh conditions, like the archea bacteria who can live within acidic hot springs or the methanotrophic bacteria who are known for feeding of the poisonous gas methane.

But what makes GFAJ-1 so special is that it has started to integrate arsenic into its genetic make-up. The findings of this group of scientist show that this bacterium is assimilating arsenic and using it as a phosphate substitute in the structure of their DNA and proteins, ultimately creating an arsenic backbone.

So how is this possible? Arsenic is similar in its structure to phosphorous, found in the form of phosphate (PO43-) in bio-molecules. Arsenic is so toxic because its biological form (AsO43-) can be mistaken for phosphate under physiological conditions and lead to huge disruptions of molecular pathways essential to survival. This is mainly due to the fact that arsenic is very instable.

The scientists postulated that if a bacterium was able to overcome this instability, it might be capable of exchanging its phosphate for arsenic in biological pathways. After a series of experiments, it seems that GFAJ-1 was not only able to deal with the harsh arsenic infested environment, but used it to change its genetic make-up and proteins, creating a whole new form of life.

These findings are extremely challenging to biology as we know it today, not only from a biochemistry aspect but also from an evolutionary point of view. Is now seems possible that life forms can arise from different atoms than the ones that favoured our evolution.

If different conditions can allow the formation of a new life form, can life be found on planets that do not favour the same elements as on Earth? NASA seems to think so, announcing these results as extremely relevant to the search for extraterrestrial life. Further research still needs to be undertaken, but already our horizons could be broadening.

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