Introns Louis J. Sheehan, Esquire 200115

Introns are common in eukaryotic pre-mRNA, but in prokaryotes they are only found in tRNA and rRNA. Introns, which are non-coding sections of a gene that are removed, are the opposite of exons which remain in the mRNA sequence after processing.

The number and length of introns varies widely among species, and among genes within the same species. Genes of higher organisms, such as mammals and flowering plants, have numerous introns, which can be much longer than the nearby exons. Some less advanced organisms, such as fungus Saccharomyces cerevisiae, and protists, have very few introns. In humans, the gene with the greatest number of introns is the gene for the protein Titin, with 362 introns.[1]
Simple illustration of a pre-mRNA, with introns (top). After the introns have been removed via splicing, the mature mRNA sequence is ready for translation (bottom).
Simple illustration of a pre-mRNA, with introns (top). After the introns have been removed via splicing, the mature mRNA sequence is ready for translation (bottom).

Introns sometimes allow for alternative splicing of a gene, so that several different proteins which share some sequences in common can be 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 been discarded. It was generally assumed that the sequence of any given intron is junk DNA with no function. More recently, however, this is being disputed.[2]

Introns contain several short sequences that are important for efficient splicing. The exact mechanism for these intronic splicing enhancers is not well understood, but it is thought that they serve as binding sites on the transcript for proteins which stabilize the spliceosome. It is also possible that RNA secondary structure formed by intronic sequences may have an effect on splicing.