(redirected from Introns)
Also found in: Dictionary, Thesaurus, Medical.
Related to Introns: Exons


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 ?
Co-author Jose Luis Gomez-Skarmeta, PhD, a geneticist at the Andalusian Center of Developmental Biology in Sevilla, Spain, said that instead they found that the promoter for IRX3, a gene several hundred thousand base pairs away, did interact with these introns, as well as a large number of other elements across the vast genetic distance we studied.
However, transcriptional enhancer elements present in the intron increase the basal activity of the [beta]-casein promoter.
Intron gain and loss in organisms are highly discussed (24) and appears that recombination causes the widely observed but poorly understood phenomenon of internal intron loss and that DNA repeat expansion can create new introns (25).
This whole process should start with computational approach, first in characterization of genes in a particular species of importance and looking at homologous DNA sequences that are conserved across different taxa, followed by comparison of length of genes and introns and exons (in nucleotides), number of introns and exons, distribution of CpG islands, etc.
We report here on the development of a set of nuclear DNA markers based on introns in the Manila and the grooved carpet-shell clams.
Modern eukaryotic genomes are loaded with intervening sequences called introns that interrupt coding sequences.
Many of the first introns discovered were in genes underlying DNA metabolism, but the introns don't discernibly influence the process, says Marlene Belfort of the Wadsworth Center of the New York State Department of Health in Albany.
Once the introns are removed, the spliceosome can stitch together exons in various combinations.
In a previous study, we reported that the G allele frequency of the G10-T polymorphism in intron 23 was significantly higher in CAD patients (15), although other polymorphisms showed no differences in terms of their allele frequencies between CAD patients and healthy controls.
They found that all of the chickens' sequences were shorter because they contain shorter introns, the regions of genes that contain so-called nonsense, or noncoding, DNA.
The transcript subsequently becomes messenger RNA through a natural splicing process in which selected introns are removed and exons joined.
7) have shown that sequence diversity within introns adjacent to exon 2 is also group-specific and can be used to separate alleles for unequivocal typing (8).