The Origin and Function of Intervening Sequences in DNA: A Review

The discovery, in 1977, of intervening sequences (introns) in the protein-coding genes of eukaryotes was a surprise. The origin and function of introns were explained by two major theories: (1) introns were introduced into the genomes of eukaryotes after the eukaryotes arose from within the prokaryotes, and introns have served to "speed" evolution; and (2) introns are relicts of precellular genome-assembly processes and have since been eliminated from prokaryotes. Current data support the second view, suggesting that although introns have played an important role in the evolution of new functional proteins, this role cannot be seen as an adaptation, at least at the level of organisms within a species. A scheme for the origin of genomes is suggested by the potential for self-replication shown by some contemporary introns.