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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 ?
Alternatively, ICAM1 rs281432 and VCAM1 rs3176867, which are both intronic SNPs, may perform a functional role by exerting a direct effect on gene splicing (Wang and Cooper 2007) and expression of noncoding RNAs (Ragvin et al.
Noteworthy, all SNPs genotyped in the present study and shown to be associated with altering SF risk are localized in intronic, non-coding regions within candidate genes.
Results: In intronic region of HAND1 gene, we identified a C>G substitution both in patients and controls.
These rearrangements are promoted by two elements, the intronic enhancer (iE[micro]) in V(D)J recombination and the Igh3' RR in class switch recombination.
It is possible, however, that there could be mutations in the regulatory or intronic regions of the gene, which could contribute to the cause of the disease.
Intronic microRNA precursors that bypass Drosha processing.
12) The promoter and the 18 exons of the LDLR gene (including about 50 bp of the intronic boundaries) and APOB gene regions (exons 26 and 29) were mutations causing FDB have been described were analyzed.
Mutation analysis: Patient DNAs were investigated for mutations in the genes TCIRGl (ATP6V0A3) and CLCN7 by amplifying all exons and flanking intronic regions by PCR using genomic DNA as a template.
However, SMN2 doesn't produce normal protein because of the presence of a specific intronic sequence in the gene or DNA.
In a previous study, we examined the entire beta-actin gene and found that the majority of the intronic differences between L.
For example, a few years ago, researchers in Germany found that two genes might be involved in the vulnerability for suicidality: intronic polymorphisms of the tryptophan hydroxylase 1 (TPH 1) gene, and the insertion/deletion polymorphism of the serotonin transporter gene (5-HTTLPR) (Mol.