Genetic alphabet for RNA gains two letters

Researchers in Switzerland have added two letters to the genetic alphabet, opening the possibility of synthesizing new types of RNA with greater catalytic potential. The genetic alphabet used by RNA molecules consists of only four letters, or bases: the purines adenine (A) and guanine (G), and the pyrimidines cytosine (C) and uracil (U). In double-stranded RNA, these bases, arranged along a sugar-phosphate backbone, hydrogen-bond to a complementary chain of bases so that each adenine is paired with a uracil and each guanine is paired with a cytosine. DNA differs from RNA only in that uracil is replaced by another pyrimidine, thymine (T), and the backbone sugar is deoxyribose instead of ribose. Thus, nature relies on only two types of base pairs (A-U or A-T, and G-C) to carry genetic information. But other types of base pairs are possible, as biological chemist Steven A. Benner and associates at the Swiss Federal Institute of Technology in Zurich have ...