Two conflicting hypotheses have been proposed to explain the origin of introns. The introns early hypothesis assumes that the DNA molecules in which genes originated initially contained random sequences of nucleotides. The random distribution of stop codons permitted only short reading frames to accumulate. Next, a mechanism arose that allowed splicing out regions containing stop codons from the primary message, and so proteins of greater length and with more useful biochemical functions could be translated and selected. The original short reading frames became the exons of present-day genes, while the introns represent segments containing splice junctions originally designed to remove deleterious stop signals. The introns late hypothesis assumes that genes arose from short reading frames that grew larger by duplications and fusions. Introns arose secondarily as a result of insertions of foreign DNA into these genes. Thus, present-day introns are the descendants of ancient transposons (q.v.).
Subjects: Genetics and Genomics.