Abstract
In nuclear mRNA genes, exon/intron junctions (both exon/intron and intron/exon junctions in this paper) possess the specific duplex pattern with the corresponding ends (3′ to 3′, 5′ to 5′) of exons and introns more or less identical. In genes with group I or group II introns, overall analyses indicate there are also related patterns in their exon/intron junctions. From the analysis of these specific regions of split genes and the study of the composition of primitive genomes, it is proposed that the sequences of primitive exons and introns are identical at least in their corresponding boundary regions. And more fundamentally, it may be concluded that exon/intron junctions were originally related to tandem repeated sequences in the earliest genomes. Results from a preliminary analysis of specific motifs in modern repeated sequences support such a view on the origin of exon/intron junctions. As for the evolution of exon/intron junctions, there have been multiple rather than single paths.
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