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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 ?
Twenty-six SINE and 16 long interspersed nuclear elements (LINE) are scattered within the introns of OGT in European pigs (http://genome.
Considering that HLA typing by traditional legacy methods provides sequencing information for only a few of the exons, we were previously unaware of the extent of polymorphisms within introns.
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.
Using data from 153 brain samples from individuals of European ancestry, they discovered that the mutations to FTO introns that affected body weight are associated with IRX3 expression, but not FTO.
578_579 + 5delAAGTATG mutation results from a deletion of 2 nucleotides at the 3' end position of exon 5 and the first 5 nucleotides of intron 5 including the donor splice site.
informativeness of STR based linkage analysis at introns 44,45,49 and 50 of DMD gene, for carrier detection and prenatal diagnosis in families having children affected with Duchenne Muscular Dystrophy.
Evolutionarily conserved genes preferentially accumulate introns.
Specifically, we determined various aspects of introns particularly the length, number and relationship to CpG islands across different taxa, etc.
Introns show often moderate to high levels of length polymorphisms, and levels of nucleotide variability that are usually higher than in exons, which are more constrained.
Modern eukaryotic genomes are loaded with intervening sequences called introns that interrupt coding sequences.
Binding sites for HIP-1 have been found within the introns of the mammalian glycolytic genes phosphofructokinase and aldolase [21], although in the case of aldolase the intronic site may not contribute to hypoxic regulation of this gene [22].