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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)

McGraw-Hill Concise Encyclopedia of Bioscience. © 2002 by The McGraw-Hill Companies, Inc.


McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
References in periodicals archive ?
Complete androgen insensitivity syndrome caused by a deep intronic pseudoexon-activating mutation in the androgen receptor gene.
Signature intronic mutations of Tp53 in different tumors in Canis familiaris.
Exonic and intronic circular RNAs (circRNAs) in 886 circRNAs identified.
Like Gnas, the Grb10 DHMR is intronic, meaning any functional effects of differential 5-hmC at this region would have to be through long-distance contacts.
By conducting several GWASs in AS patients over the past few years, researchers have identified a number of potentially functional intronic variants (14,18) .
A1 B1 C1 A12 B12 C12 Exonic 94.02% 95.22% 94.44% 95.16% 94.21% 94.69% Intergenic 1.92% 1.64% 1.78% 1.64% 1.79% 1.71% Intronic 4.06% 3.14% 3.78% 3.20% 4.00% 3.60% Note: Table made from pie chart.
Molto et al., "Friedreich's ataxia: autosomal recessive disease caused by an intronic GAA triplet repeat expansion," Science, vol.
Intronic variant rs3772130 in the FBXO40 gene was reported as associated with a cognitive performance in GWAS [6].
These 4 patients were tested for the known pathogenic deep intronic mutations in introns 30 and 36 [16] and, in addition, screened for large deletions or duplications by MLPA analysis, but no second mutation was identified in them.
Once the transcripts for each gene have been selected, coding exons with flanking intronic regions are used to define the region of interest.
Furthermore, they have demonstrated the significance of intronic sequences in regulating NIS expression: the first intron of NIS contains both DR2-type elements and one overlapping DR10 element, the latter directly interacting with RAR and acting as an enhancer of the gene.
In the MSH3 gene, missense, silent, and intronic variations have been mainly identified; these mutations are associated with a severe phenotype in the case they are inherited in combination with each other or associated with variants in the MSH2 gene [51].