An intron is a DNA region within a gene that is not translated into protein. These non-coding sections are transcribed to precursor mRNA (pre-mRNA) and some other RNAs (such as long noncoding RNAs), and subsequently removed by a process called splicing during the processing to mature RNA. After intron splicing (i.e. removal), the mRNA consists only of exon derived sequences, which are translated into a protein.
The word intron is derived from the term intragenic region and also called intervening sequence (IVS)
Introns are common in eukaryotic pre-mRNA, but in prokaryotes they are only found in tRNA and rRNA; introns have variable length and alternate with exons in intron-containing genes.
The number and length of introns varies widely among species, and among genes within the same species. Some eukaryotes, e.g. sac fungi, have evolved genomes with few introns, while the genomes of many other eukaryote groups are rich in introns (several per gene).
Alternative splicing of introns within a gene may introduce greater variability of protein sequences translated from a single gene. The control of mRNA splicing is performed by a wide variety of signaling molecules.
Introns may also contain "old code", or sections of a gene that were once translated into a protein, but have since become inactive. It was generally assumed that the sequence of any given intron is junk DNA with no biological function. More recently, however, this is being disputed. For example, a point mutation in intron 7 of the human gene TPH1 is highly correlated to the development of the psychiatric disorder schizophrenia.
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