Intron

An intron is the intervening, non-coding sequence of nucleic acid that is between the expressed sequences (exons) in a gene. It is removed from the primary RNA transcript by splicing and is a common feature of eucaryotic genes.

There is no single mechanism for splicing an intron from the primary RNA transcript. Introns are classified into four groups based on the mechanism of splicing. In general splicing can be either autocatalytic or be catalyzed by a large complex of proteins know as the spliceosome. Splicing can also be viewed as a form of gene regulation since alternative splicing can result in different combinations of exons in the mature RNA transcript. In this way one gene can code for distinct open reading frames and hence has the potential to code for more than one protein.

Sometimes the DNA that codes for introns is classified as junk DNA but this is an oversimplification since introns can contain functional RNA or DNA sequences; these include transfer RNA (tRNA) or microRNA (miRNA) sequences. In the chromosome, DNA sequences that code for an intron can also include enhancers that are important for gene expression.

The true role of introns is unclear but one hypothesis is that introns allow gene shuffling to occur, resulting in the creation of new exon combinations and novel proteins. The presence of an intron can also enhance gene expression since the process of RNA splicing seems to facilitate the trafficking of the mRNA out of the eucaryotic nucleus.