Evolution of Exon Structure | Genetics

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.

(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.

(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.

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.