The puzzling origin of the genetic code.

Recent results add to the mystery of the origin of the genetic code. In spite of early doubts, RNA can discriminate between hydrophobic amino acids under certain contexts. Moreover, codon reassignment, which has taken place in several organisms and mitochondria, is not a random process. Finally, phylogenies of some aminoacyl-tRNA synthetases suggest that the entire code was not completely assigned at the time of the divergence of bacteria from nucleated cells.