intron (intervening sequence) A nucleotide sequence in a gene that does not code for the gene product (compare exon). Introns, which occur principally in eukaryotes, are transcribed into messenger RNA but are subsequently removed from the transcript before translation (see RNA processing). In certain cases, removal of the introns is an autocatalytic process – self-splicing – whereby the RNA itself has the properties of an enzyme (see ribozyme). Self-splicing occurs in primary transcripts of some single-celled organisms, such as Tetrahymena, as well as chloroplasts, mitochondria, and some viruses. However, splicing of primary transcripts produced in the nucleus generally requires the participation of a spliceosome, a complex of proteins and RNAs. The function of introns is still subject to lively debate. They may simply be sequences of selfish DNA, able to move between different loci within the genome (see transposon) with no benefit to the host. On the other hand, introns may act as ‘spacers’ for exons and facilitate exon shuffling – recombination or rearrangement of exons – which permits rapid evolution of proteins with novel permutations of functional groups. Introns have also been found in certain archaebacteria and cyanobacteria and in some viruses.

intron (silent DNA) In eukaryotes and some prokaryotes, part of the DNA of genes that is not expressed in the polypeptide chains or in m-RNA. Compare EXON.

intron A DNA segment of a transcribed gene which is removed during transcription and, therefore, does not appear in the mature messenger-RNA.