Editing Exon (section)

==Structure and function==
[[File:Gene structure.svg|frame|right|Exons in a messenger RNA precursor (pre-mRNA). Exons can include both sequences that code for amino acids (red) and untranslated sequences (grey). Introns — those parts of the pre-mRNA that are not in the mRNA — (blue) are removed, and the exons are joined (spliced) to form the final functional mRNA. The 5′ and 3′ ends of the mRNA are marked to differentiate the two untranslated regions (grey).]]

In protein-coding genes, the exons include both the protein-coding sequence and the 5′- and 3′-[[untranslated region]]s (UTR). Often the first exon includes both the 5′-UTR and the first part of the coding sequence, but exons containing only regions of 5′-UTR or (more rarely) 3′-UTR occur in some genes, i.e. the UTRs may contain introns.<ref>{{cite journal|last1=Bicknell|first1=AA|s2cid=5808466|title=Introns in UTRs: Why we should stop ignoring them.|journal=BioEssays|date=December 2012|volume=34|issue=12|pages=1025–1034|doi=10.1002/bies.201200073|pmid=23108796|doi-access=free}}</ref> Some  [[non-coding RNA]] transcripts also have exons and introns.

Mature mRNAs originating from the same gene need not include the same exons, since different introns in the pre-mRNA can be removed by the process of [[alternative splicing]].

Exonization is the creation of a new exon, as a result of mutations in [[introns]].<ref>{{cite journal |author=Sorek R |title=The birth of new exons: mechanisms and evolutionary consequences |journal=RNA |volume=13 |issue=10 |pages=1603–8 |date=October 2007 |pmid=17709368 |pmc=1986822 |doi=10.1261/rna.682507}}</ref>