TWISTED LADDER OF DNA (above left, seen in side view and top view) may not be the only macromolecule capable of storing the blueprints for living organisms. Scientists are experimenting with semiartificial nucleic acids, such as xDNA (at right), that are more stable and thus less likely to suffer mutations. Image: REPRINTED WITH PERMISSION FROM H. LIU ET AL. IN SCIENCE, VOL. 302, PAGES 868¿871; 2003. ©2003 AAAS
Life on earth has taken a tremendous range of forms, but all species arise from the same molecular ingredients: five nucleotides that form the building blocks for DNA and RNA, and 20 amino acids that serve as building blocks for proteins. (At least two additional natural amino acids are made by a few odd species.) These ingredients limit the chemical reactions that can happen inside cells and so constrain what life can do.
That constraint was eased in 2001, probably for the first time in more than three billion years. After years of trying, Lei Wang, Peter G. Schultz and their co-workers at the Scripps Research Institute in La Jolla, Calif., at last succeeded in adding to Escherichia coli bacteria all the genetic components the cells need to decode the three-nucleotide DNA sequence TAG into unnatural amino acids of various kinds.