intron

In genetics, a stretch of DNA that interrupts the coding parts (exons) of a gene but apparently carries no genetic information. Introns, discovered in 1977, are at first transcribed into messenger RNA (mRNA) but removed before translation. While some introns harbour a catalytic activity involving their own excision from the mRNA, in most cases their function is unknown.

Introns are very common in the genomes of many eukaryotes, and particularly in mammals and in flowering plants, where they often account for many more base pairs than the coding exons. In the human genome, for instance, exons make up 1.1% of the total length, and introns 24% (the remainder is referred to as intergenic DNA).

Initially, introns were considered part of the widespread phenomenon of junk DNA, which was believed to be old DNA no longer useful to the cell. With the growing understanding of genomes, however, scientists have increasingly found functional elements within introns and other ‘junk’ elements.

The self-splicing intron of the protozoon Tetrahymena thermophila was one of the first known examples of a ribozyme or RNA enzyme. It was discovered in the 1980s by Thomas Cech, who shared the 1989 Nobel prize in chemistry with Sidney Altman.

Self-splicing introns are divided into class I, II, and III introns. By contrast, the nuclear or spliceosomal introns more widespread in mammals and plants require a cellular apparatus known as the spliceosome for their removal from the mRNA. Editing by the spliceosome can lead to exons being linked in different ways, thus giving rise to different proteins based on the same gene.