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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 ?
Amplification of fragments containing intron 20 of OGT by PCR revealed a variation between the European and Chinese pig breeds.
In conclusion, we suggested that the intron 3 VNTR variant in the XRCC4 gene may be associated with the etiopathogenesis of RA as a marker of immune aging.
For HLA-A and HLA-C, intron 1 and 4 were found to have the second highest average sequence variability respectively.
The intron 22 inversion was detected in three patients, while point mutations were identified in five patients: two nonsense (p.
Polymorphism in intron 2 of FCGRT gene in neonatal buffalo calves and its association with serum IgG concentration in National symposium on "Policy Planning for Livelihood Security through Domestic Animal Biodiversity" on 11-12 Feb, 2016.
A detailed analysis of that deletion reveals that it causes a shift of the reading frame and expands over the GT donor splice site of intron 5.
Under this agreement builders of synthetic turf systems will be certified by INTRON on quality control.
The ratio of exon to intron varies across taxa, minimum in M.
The G79A polymorphism of intron F of the PZ gene was analyzed according to a previously reported method [8] from the DNA bank of the Pediatric Molecular Genetics Department of Ankara University.