ExonThe term ‘exon’ was coined by Walter Gilbert in 1978. It is portion of a gene that holds the code for making a protein. Many proteins are made of several exons which are separated by ‘intervening’ regions of DNA, known as introns, which do not code for proteins. These introns are removed by splicing to produce the final messenger RNA (mRNA) molecule.
Exons comprise three distinct regions of a protein-coding gene. The first is a portion that is not translated into protein, but contains the signal for the beginning of RNA synthesis, and sequences that direct the mRNA to ribosomes for protein synthesis. The second is a set of exons containing information that is translated into the amino acid sequence of a protein. The third region of a gene that becomes part of an mRNA is an untranslated end portion that contains signals for transcription termination and for the addition of a polyadenylate tract at the end of a transcript.
The evolutionary origin of the exon-intron organisation of eukaryotic genes has not been fully resolved. The “intron-early” view defended by Gilbert posits that introns existed before the prokaryote/eukaryote divergence. On the other hand, the “intron-late” view contends that introns have been inserted post-divergence. On the “intron-late” model, intron evolution is an example of convergent evolution, where certain exon spaces have dispositions to become introns.
Web Resources On Exon
Definition of Exon
Wikipedia: Exon
Book Resources On ExonProtein Evolution by Exon-Shuffling by L. Patthy
Positional Cloning by Exon Trapping and CDNA Selection by B. Korn
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