The comments are great too. In particular I like what chemDroid had to say:
Hi. I have posted on many places, they should not be
doing standard DNA preps and the experiment we need
to see is to see a caesium chloride density gradient
ultracentrifugation (not a gel)... If indeed the DNA
has arsenic, it should be getting denser and we would
see that as a band shift. Ethidium bromide's mode of
action would not be affected by arsenic substition.
Gels are used to measure the length of a DNA fragment. Here's how they work: Ethidium bromide (EtBr) is an intercalating agent -- think of the DNA as a ladder, it slides between the rungs and gets stuck there. EtBr is also luminescent under UV. Agarose (purified seaweed extract) is used to make a gel doped with EtBr. A DNA sample is placed in a well on one end of the gel and an electric current is applied to the gel. The sample gets dragged through the gel, the longer it is, the more slowly it progresses.
As chemDroid wrote, there is no reason to believe that arsenic would have any impact on this process.
You can extend the density gradient experiemnt and get the DNA out in super pure form and then run mass spec on it to show that arsenic has indeed been incorporated.
With a claim this important, we cannot do enough to try to disprove it. Only after we try everything we know and get consistently convincing evidence that arsenic is indeed incorporated into the DNA can we start believing it.
As chemDroid wrote, there is no reason to believe that arsenic would have any impact on this process.