How DNA uses these amino acids is described by Paul Davies in his article Are Aliens among Us? published in Scientific American, Dec 2007. He states, "The genetic code is based on triplets of nucleotides, with different triplets spelling out the names of different amino acids. The sequence of triples in a gene dictates the sequence of amino acids that must be strung together to build a particular protein."
Alien lifeforms may use exotic amino acids that are far different than the ones used by life on Earth. It is possible that the set of amino acids we use may not even appear on other worlds. Evidence by scientific study suggests that there are many other forms of amino acids that may be useful (or at least available) for other types of life. Evidence of exotic amino acids on other worlds has come from meteorites. Also, others have been synthesized in the lab.
Other lifeforms from other worlds may be completely different from us while still using the same basic DNA structure we use. A question remains, would life formed by exotic amino acids be all that much different than Terran life in appearance? Would the exotic amino acids lead life to evolve along completely different paths that we as yet have not conceived?
Reference: “Are Aliens Among Us?” by Paul Davies, Scientific American December 2007