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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)


References in periodicals archive ?
We demonstrated that the first intron of the porcine [beta]-casein gene contains a transcriptional promoter/enhancer sequence and that lactogenic expression of the porcine [beta]-casein gene was probably regulated by the first intron.
Even the intron sizes were quite variable among taxa and the taxa where only one intron is present, the size varies from 329 bp (C.
17) Furthermore, group II introns are mobile genetic elements that can transpose from one location within a genome to another and require an intron-encoded reverse transcriptase/ maturase protein for transposition and proper splicing.
Enzon designed PEG-INTRON to allow for less frequent dosing and to yield greater efficacy, as compared to INTRON A.
REBETOL had been previously approved in the United States for this indication only as a component of REBETRON(TM) Combination Therapy, which contains REBETOL Capsules and INTRON A Injection in a single package.
In addition to making this technology readily available within the Indian market, INTRON will provide expert applications assistance to design engineers to help them improve the performance of end products with VFR Bulk Metal([R]) Foil resistors.
But we saw the RNA molecule flexing on its own, with the intron folding and unfolding in a way that brings the exons closer together, suggesting a more active role for introns," Nature quoted Walter as saying.
The size of CD36 gene in each taxa and relative size of exon and intron are shown in Fig.
Psychiatric adverse events, which include insomnia, were common (57%) with PEG-INTRON, but similar to INTRON A (58%).
Geron Corporation (Menlo Park, CA) and University of Rochester (Rochester, NY) have patented a method of inhibiting the self-splicing of a Group I intron.
In its promoter region, three linked single-nucleotide polymorphisms (SNPs; -1468T [right arrow] A, -922A [right arrow] G, and -786T [right arrow] C) have been detected (9-11), and within the gene, SNPs have been identified in intron 2 (IVS2 + 43G [right arrow] A), intron 11 (IVS11 + 174A [right arrow] G), intron 12 (IVS12 + 52G [right arrow] T), intron 18 (IVS18 + 27A [right arrow] C), intron 22 (IVS22 + 15A [right arrow] G), and intron 23 (IVS23 + 11G [right arrow] T) (11).