Biology Reference
In-Depth Information
Transcriptional
initiation
Translational
start
Translational
stop
Poly(A) site
5'
3'
An archetypal gene with its flanking region
(1)
(2)
5uexon (5'-untranslated)
3uexon (3'-untranslated)
(3)
(4)
5utexon (5'-untranslated & translated)
3tuexon (3'-translated & untranslated)
(5)
(6)
iutexon (internal-untranslated & translated)
ituexon (internal-translated & untranslated)
(7)
(8)
iuexon (internal-untranslated)
itexon (internal-translated)
(9)
(10)
5utuexon
3utuexon
(11)
(12)
5-3utuexon
iutuexon
Legend
Flank
Intron
UTR
CDS
Figure 3.2.
Exon classification. All exons can be classified into these 12 mutually
exclusive classes. At the top, a schematic gene model is depicted which indicates how
some types of exons may be organized in a gene (redrawn from Zhang, 1998). For
explanation, see text. UTR: Untranslated region. CDS: Coding sequence.
emerged through the coalescence of primordial minigenes (Gilbert, 1987, 1997).
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
