Intron
Introns are non-expressed sequences within genes removed during RNA processing.
An intron is a segment of DNA or RNA within a gene that does not appear in the final, functional RNA molecule. The name comes from "intragenic region," meaning a sequence inside a gene. The parts of the gene that are kept and joined together after introns are removed are called exons. Introns are common in most eukaryotes and many eukaryotic viruses, and they occur in both protein-coding genes and noncoding RNA genes. They are extremely rare in bacteria and archaea.
Introns were first found in protein-coding genes of adenovirus, and later in genes for transfer RNA and ribosomal RNA. The discovery that genes are split by introns was made independently in several labs in 1977, including those of Phillip Sharp and Richard Roberts, who shared a Nobel Prize in 1993, as well as the labs of Louise Chow and Thomas Broker. Much of the work in Sharp’s lab was done by Susan Berget. The term "intron" was coined by Walter Gilbert in 1978, who suggested replacing the idea of a cistron with a transcription unit containing introns (lost from the mature messenger) alternating with exons (expressed regions). The term also refers to an intracistron, an extra piece of DNA inside a cistron. While introns are sometimes called intervening sequences, that term can also include inteins, untranslated regions, and nucleotides removed by RNA editing.
The number of introns varies greatly across organisms. They are very common in the nuclear genomes of jawed vertebrates like humans, mice, and pufferfish, where protein-coding genes usually have multiple introns. They are rare in some eukaryotic microbes, such as baker’s yeast. Vertebrate mitochondrial genomes lack introns entirely, while those of some eukaryotic microbes may have many. An extreme case is the *Drosophila* DhDhc7 gene, which contains an intron over 3.6 million base pairs long, taking about three days to transcribe. The shortest known metazoan intron is 30 base pairs in the human MST1L gene, while in heterotrich ciliates like *Stentor coeruleus*, over 95% of introns are just 15 or 16 base pairs long.
At least four main types of introns exist, identified by their structure and splicing mechanisms: spliceosomal introns (removed by spliceosomes in nuclear protein-coding genes), tRNA introns (removed by proteins in nuclear and archaeal tRNA genes), group I introns (self-splicing via RNA catalysis), and group II introns (also
- discovered
- 1977
- discoverers
- Phillip Allen Sharp, Richard J. Roberts, Louise Chow, Thomas Broker, Susan Berget
- term_coined_by
- Walter Gilbert
- types
- tRNA introns, group I introns, group II introns, spliceosomal introns
- distribution
- Common in jawed vertebrates; rare in some eukaryotic microorganisms; absent in vertebrate mitochondrial genomes
- shortest_known_metazoan_intron
- 30 base pairs (human MST1L gene)
- shortest_known_introns
- 15 or 16 base pairs (heterotrich ciliates like Stentor coeruleus)
Lore & Background
Introns were first discovered in protein-coding genes of adenovirus, and subsequently identified in genes encoding transfer RNA and ribosomal RNA. The fact that genes were split or interrupted by introns was discovered independently in several labs in 1977, including those run by Phillip Allen Sharp and Richard J. Roberts, for which they shared the Nobel Prize in Physiology or Medicine in 1993. Other labs included those of Louise Chow and Thomas Broker, and much of the work in the Sharp lab was done by postdoctoral fellow Susan Berget. The term intron was introduced by American biochemist Walter Gilbert in 1978, who wrote: 'The notion of the cistron ... must be replaced by that of a transcription unit containing regions which will be lost from the mature messenger – which I suggest we call introns (for intragenic regions) – alternating with regions which will be expressed – exons.'
Reader's Guide
Introns are fundamental to eukaryotic gene structure and RNA processing. Their discovery in 1977 overturned the classical view of genes as continuous sequences, revealing that genes are split into expressed exons and intervening introns. Introns are removed from RNA transcripts by splicing, which can be catalyzed by spliceosomes (for nuclear pre-mRNA introns), by proteins (for tRNA introns), or by self-splicing mechanisms (group I and group II introns). The frequency and length of introns vary widely across organisms: jawed vertebrates have many long introns, while some eukaryotic microorganisms have few, and vertebrate mitochondrial genomes lack introns entirely. Splicing accuracy is not perfect; error rates can be as high as 2–3% per gene, and most splice variants are rapidly degraded by nonsense-mediated decay. The persistence of suboptimal splice sites in genomes is attributed to the large number of possible slightly deleterious mutations, especially in species with small long-term effective population sizes like humans.
Did You Know?
- The term 'intron' was coined by Walter Gilbert as a contraction of 'intragenic region'.
- The shortest known metazoan intron is 30 base pairs long, found in the human MST1L gene.
- Spliceosomal introns are often much longer than their surrounding exons.
- Group I introns use a non-encoded guanosine nucleotide (typically GTP) to initiate splicing.
More in Genetics And Genomics 1-24
Spotted an error? Know more?
This is a living reference — every entry is fact-audited, and reader corrections feed straight into our audit queue. Suggest an edit · See this site's audit record