A model has been proposed suggesting that the tRNA molecule must have originated by direct duplication of an RNA hairpin structure. A non-monophyletic origin of this molecule has also been theorized. In other words, the tRNA genes evolved only after the evolutionary stage of the last universal common ancestor (LUCA) through the assembly of two mini-genes codifying for different RNA hairpin structures, which is what the exon theory of genes suggests when applied to the model of tRNA origin. Recent observations strongly corroborate this theorization as it has been found that some tRNA genes are completely separate in two mini-genes codifying for the 5' and 3' halves of this molecule. It is shown that these tRNA genes codifying for the 5' and 3' halves of this molecule are the ancestral form from which the tRNA genes continuously codifying for the complete tRNA molecule are thought to have evolved. This, together with the very existence of completely separate tRNA genes codifying for their 5' and 3' halves, proves a non-monophyletic origin for tRNA genes, as a monophyletic origin would exclude the existence of these genes which have, on the contrary, been observed.
Dr. Massimo Di Giulio obtained his PhD in Biology (on bacterial genetics) from the University of Naples in 1976. From 1976 to 1979 he was a Professor of Science at the Italian National High Schools. In this period he became interested in molecular evolution, in particular the origin of the genetic code. From 1980 to 1986 he was a Research Fellow at the International Institute of Genetics and Biophysics, National Council for Research, Naples, where he learned the basic techniques of molecular biology. From 1987 to the present he has been affiliated to the Institute of Genetics and Biophysics 'Adriano Buzzati Traverso' in Naples, where he investigates the origin of translation. His research aims to clarify the relative importance of the theories that explain the organization of the genetic code. Dr. Di Giulio has been a visiting scientist at the Department of Biochemistry, Hong Kong University (1997 and 2005); at the Institute for Theoretical Physics, University of California, Santa Barbara (2000); at the Department of Molecular, Cellular, and Developmental Biology, University of Colorado, Boulder (2004); and at the Department of Crop Sciences, University of Illinois, Urbana-Champaign (2009). In 2007 he was a Fellow at the Collegium Budapest. Selected publications (2008) Permuted tRNA genes of Cyanidioschyzon merolae, the origin of the tRNA molecule and the root of the Eukarya domain. Journal of Theoretical Biology 253: 587-592. (2006) The non-monophyletic origin of the tRNA molecule and the origin of genes only after the evolutionary stage of the Last Universal Common Ancestor (LUCA). Journal of Theoretical Biology 240: 343-352. (1999) The non-monophyletic origin of tRNA molecule. Journal of Theoretical Biology 197: 403-414. (1995) Was it an ancient gene codifyng for a hairpin RNA that, by means of direct duplication, gave rise to the primitive tRNA molecule? Journal of Theoretical Biology 177: 95-101. (1992) On the origin of the transfer RNA molecule. Journal of Theoretical Biology 159: 199-214.