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In split genes, a portion that is included in ribonucleic acid (RNA) transcripts but is removed from within a transcript during RNA processing and is rapidly degraded. Split genes are those in which portions appearing in messenger RNAs (mRNAs) or in structural RNAs, termed exons, are not contiguous in a gene but are separated by lengths of deoxyribonucleic acid (DNA) encoding parts of transcripts that do not survive the maturation of RNA (introns). Most genes in eukaryotes, and a few in prokaryotes, are split. These include not just a large number of different protein-coding genes but also genes encoding transfer RNAs (tRNAs) in such diverse eukaryotes as yeast and frogs, and genes encoding structural RNAs of ribosomes in some protozoa. Introns are also found in mitochondrial genes of lower eukaryotes and in some chloroplast genes. See Exon

The number of introns in a gene varies greatly, from 1 in the case of structural RNA genes to more than 50 in collagen. The lengths, locations, and compositions of introns also vary greatly among genes. However, in general, sizes and locations—but not DNA sequence—are comparable in homologous genes in different organisms. The implication is that introns became established in genes early in the evolution of eukaryotes, and while their nucleotide sequence is not very important, their existence, positions, and sizes are significant.

Speculation on the roles and the evolution of introns is mostly based on correlations that have been seen between domains of protein structure and the exons of genes that are defined by intervening introns. For example, the enzyme alcohol dehydrogenase (ADH) has two domains, one portion of the protein that binds alcohol, and another that binds the enzyme cofactor nicotinamide adenine dinucleotide (NAD). The ADH gene has an intron that cleanly separates the nucleotide sequences which encode each domain, and gene-sequence arrangements such as this are not uncommon. It has been suggested that introns became established in the genes of eukaryotes (and to a limited extent in bacteria) because they facilitate a genetic shuffling or rearrangement of portions of genes which encode various units of function, thus creating new genes with new combinations of properties. The introns allow genetic recombination to occur between the coding units rather than within them, thus providing a means of genetic evolution via wholesale reassortments of functional subunits or building blocks, rather than by fortuitous recombinations of actual protein-coding DNA sequences. See Gene, Genetic code, Recombination (genetics)

McGraw-Hill Concise Encyclopedia of Bioscience. © 2002 by The McGraw-Hill Companies, Inc.


McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
References in periodicals archive ?
The primer pair (Lep forward, GGCTTACTGCTGTGTGGTCT and rreverse, AATGGCCACGGTTCTACCTC) amplifying the 461 bp DNA fragment (containing the exon 2 and part of the intron 1 and intron 2) was designed using the Primer 3 (NCBI) program (accession AY495587).
Three different mutations were observed in exon 22 and two in each of exons 1, 4, 6, 13,14, 23, 35, and 44 and introns 29 and 40; however, no accumulation in a specific exon was observed.
Amplification of fragments containing intron 20 of OGT by PCR revealed a variation between the European and Chinese pig breeds.
Mager, "Distributions of transposable elements reveal hazardous zones in mammalian introns," PLOS Computational Biology, vol.
By sequencing all the four products in all 221 patients (FET, FPD, and ET-PD) and 100 controls, no exonic variant was identified, while one exon-intron boundary variant (rs2241028) and one intron variant (rs2241027) were detected.
Then the lariat is joined by spliceosome and the introns are removed to form a circRNA.
Average Shannon entropy for each genomic feature [exon, intron, or untranslated region (UTR)] of HLA-A, HLA-B, and HLA-C was calculated by dividing the sum of Shannon entropy over the feature divided by the total length of the genomic feature.
In the study, Fire, Frokjaer-Jensen and colleagues engineered a gene for a fluorescent jellyfish protein with an intron containing PATC watermarks.
The [[beta].sub.2]M, a 6 kb gene (357 bp cds), located on 10th chromosome of bovine, is conserved across species, including mammals and birds containing four exons and three introns. It is located outside of the MHC region.
It is well established that introns results in increased stability and expression of genes (Buchman and Berg, 1988; Okkema et al., 1993).
Molecular analysis identified a novel splice site mutation in DFNA5 intron 8 (IVS8+1 delG).
Moreover, a relevant biological effect can be linked to nucleotide alteration located in introns (Chorev and Carmel, 2012).