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In this article we will discuss about the evolution of exon structure.
Average cellular gene is vertebrates contains about eight exons and detail analysis of human gene has yielded a classification scheme for exons according to their transcriptional or translational boundaries. According to this scheme there are 12 different categories which are:
(i) 5′ terminal un-translated exons.
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(ii) 3′ terminal un-translated exons.
(iii) 5′ terminal exons having a 5′ un-translated sequence followed by a coding sequence.
(iv) 3′ terminal exons having a 3′ un-translated sequence followed by a coding sequence.
(v) Internal exons having a 5′ portion of 5′ un-translated sequence followed by a coding sequence.
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(vi) Internal exons having a coding sequence followed by a 3′ portion of 3′ un-translated sequence.
(vii) Internal un-translated exons.
(viii) Internal translated exons.
(ix) Exons containing the complete coding sequence but which do not contain the transcriptional end.
(x) Exons containing the complete coding sequence but which do not contain transcriptional start.
(xi) Exons containing the complete coding sequence and both transcriptional start and transcriptional end.
(xii) Exons containing the complete coding sequence but neither trariscriptional start nor the transcriptional end.
Now the question, were introns used in the assembly of the first genes or were they added only later to previously contiguous coding sequence? The “introns early” theory or simply “the exon theory of genes” proposes that the genes encoding complex extant proteins emerged through the coalescence of primordial minigenes.
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These minigenes are held to have originally encoded protein modules and are now represented as exons whereas the non-coding linker DNA between the minigenes has survived as introns. Introns were then lost and novel exons made by fusing smaller exons together.
By contrast, the “introns late” theory postulates that fully functional genes had introns inserted into them at different stages in their evolution. The best characterized example of the insertion of an intron into a gene is from the sex-determining gene which in humans (SRY; Ypll.3) and other placental mammals is intronless.