Folding up a transfer RNA molecule is not simple

For much of its history, molecular biology has concerned itself with the implications and consequences of the "Central Dogma" (Crick, 1958): DNA leads to RNA leads to protein. This "pathway" is of course atypical in that it describes, not chemical interconversions, but rather the flow of information from gene to product. It has been only recently appreciated, however, that in those cases where the final gene product is an RNA molecule, this pathway is not simply truncated. To the contrary, in virtually every instance for which sufficient data are now available, it is found that the RNA transcript is subject to a series of reactions in which the primary gene product undergoes nucleotide additions, deletions, and modifications. The set of biosynthetic reactions that intervene between transcription of the gene and production of the mature functional product is collectively referred to as RNA processing. It now appears that the existence of these complex biosynthetic events cannot always be adequately explained by the necessity to overcome otherwise insurmountable topological or energetic constraints: the final product can, at least in some cases, "self-assemble." The genesis of transfer RNA by this indirect route appears to insure the delivery of a functional product, at the right rate, at the right time. We suggest, moreover, that this process is primarily determined by recognition of some of the same structural elements in the precursor that are required for the function of the mature tRNA molecule.