It has been well established that the fundamental building blocks for life are proteins, fats, carbohydrates and nucleic acids (DNA and RNA). The elements that are required to assemble these constituents are carbon, hydrogen, nitrogen, oxygen, sulfur and phosphorus. In addition there are trace metallic elements that are required for metabolic activity through the agency of enzymes, oxygen transport and other biological functions; examples of these are iron, zinc and molybdenum. These are usually present in trace amounts and function as necessary co-factors in a host of enzyme activities. There have been many reported instances of substitutions in regard to these co-factors; examples include the substitution of tungsten for molybdenum and cadmium for zinc. There has also been reported the substitution of copper for iron as an oxygen carrier in some arthropods and mollusks.
Doctor Felisa Wolfe-Simon and her colleagues at the NASA Astrobiology Institute have recently made quite an astounding discovery. They discovered a bacterium, strain GFAJ-1, isolated from Mono Lake in California that is capable of growing in the presence of Arsenic (As). Furthermore, they were able to show in the laboratory that As was incorporated into the structure of nucleic acids and proteins. As is a chemical analog of phosphorus (P); it resides directly below P on the periodic table. Its similarity to P in terms of its chemical properties explains why it is a deadly poison for complex multi-cellular organisms such as humans.
This is quite a significant discovery; it dramatizes just how adaptable living organisms can be. Furthermore, it may broaden our perspective regarding the possibility of extra-terrestrial life.