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I'll have to go dig out my old chem textbooks to further understand the similarities and differences, but at the very least they are not in the same series (Phosphorous=non-metal, Arsenic=metalloid).
Metalloids share properties with non-metals and metals, but they also differ from non-metals in that they are good conductors. The main thing with metalloids is that their properties are highly dependent on the other elements with which they are reacting.
It will be interesting to see what the structure of the alternate DNA is. Is it an arsenate deoxyribose backbone?
Correct that they are not in the same series (or row) of the periodic table, but they are in the same column (group), so they are able to react similarly. As you say, because they are in different series they will have different properties, but the key is that arsenic can mimic phosphorous because it has the same # of valence electrons (like forming arsenate, AsO4 3- just like phosphate PO4 3-). Yes I would expect the structure to be arsenate in place of phosphate.
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Looks like there are questions about the experimental methods used.
Some have pointed out that many other bacterium are able to survive on incredibly low levels of phosphate, similar to what may have been present in these samples.
Norman Pace a microbiologist at the University of Colorado was scathing. 'Low levels of phosphate in growth media, naive investigators and bad reviewers are the stories here', he said.
Professor Redfield wrote in her blog: ‘Lots of flim-flam, but very little reliable information. The mass spec measurements may be very well done (I lack expertise here), but their value is severely compromised by the poor quality of the inputs.
‘If this data was presented by a PhD student at their committee meeting, I'd send them back to the bench to do more cleanup and controls.
'There's a difference between controls done to genuinely test your hypothesis and those done when you just want to show that your hypothesis is true. The authors have done some of the latter, but not the former.
To understand why, we need to back up a bit. One thing that everyone agrees on is that all things being equal, DNA with an arsenate backbone will hydrolyze quickly in water, while DNA with a phosphate backbone will not. Steve Benner has pointed out that the half-life of the hydrolysis reaction is about 10 minutes.
But here's the relevant question: Is 3 micromolar phosphate a lot? Or a little? One point of comparison is the Sargasso Sea, where plenty of microbes survive and make normal DNA. Here, the phosphate concentrations are less than 10 nanomolar - or 300 times less phosphate than the "phosphate-free" media in the GFAJ-1 experiment. At such low phosphate concentrations, some bacteria compensate by removing phosphorus from their lipids - but not from their DNA.
So the Sargasso Sea tells us that some bacteria are capable of making DNA at very low phosphate concentrations. The most plausible explanation is that the bacterium GFAJ-1 can make normal DNA at micromolar phosphate concentrations, and that it also has the ability to tolerate very high arsenate concentrations.
There are numerous other aspects of this study that don't make sense. Why would bacteria from Mono Lake need the ability to substitute arsenate for phosphate in their DNA? Yes, arsenic concentrations are high in Mono Lake. But so are phosphate concentrations, which approach 1 millimolar - or 100,000 times higher than in the Sargasso Sea. Mono Lake has more phosphate available than nearly any other environment on Earth. There is no selective pressure for the evolution of what would surely be a massively complex switch in nucleic acid chemistry from phosphate to arsenate. I can only begin to imagine the structural problems that would be imposed on DNA by this switch, which would change bond lengths between nucleotides, and cause secondary problems with transcription, etc. Then there is the radical suggestion that nucleotide chemistry is stable because might occur in a 'non-aqueous' environment. It's not clear how that could work.
A research article published December 2, 2010 by the journal Science provided several lines of evidence, collectively suggesting that a bacterium isolated from California's Mono Lake can substitute arsenic for a small percentage of its phosphorus and sustain its growth.