ABSTRACT
I f we think about the origin of genes, we almost inevitably imagine the primitive genetic system passing through an evolutionary stage characterised by genes just long enough to encode a few dozen amino acids, before the genes went on to reach greater lengths. This viewpoint is in keeping with the exon theory of genes, which suggests that the discontinuous structure of eukaryotic genes, consisting of exons alternated with introns, reflects the way in which genes have evolved: the exons might actually be minigenes specify ing primarily modular structures that were assembled, i.e., joined, by introns to form the discontinuous genes found in the genome of eukaryotes. According to this theory, every exon in eukaryotic gene would correspond to an ancestral minigene and the introns would constitute points of suture between these minigenes.3 There is a great deal of evidence in favor of this theory.4
