Biology Reference
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(a)
Exon 1
Exon 2
Exon 3
Exon 4
Gene
Protein
(b)
Exon 1
Exon 2
Exon 3
Exon 4
Gene
Figure 3.4.
Possible
relationships between the
arrangement of exons in a
hypothetical gene and the
structural domains of the
protein it encodes (from Li
1997), (a) each exon
corresponds exactly to a
structural domain, (b) the
correspondence is only
approximate, (c) an exon
encodes two or more domains,
(d) a single structural domain
is encoded by two or more
exons, (e) lack of
correspondence between exons
and domains. The four
structural domains of the
hypothetical protein are
designated by different boxes.
Protein
(c)
Exon 1
Exon 2
Gene
Protein
(d)
Exon 1
Exon 2
Exon 3
Exon 4
Exon 5
Exon 6
Gene
Protein
(e)
Exon 1
Exon 2
Exon 3
Exon 4
Exon 5
Gene
Protein
that 30-40% of the present day intron locations correspond to phase zero
introns originally present in the progenote. The remainder, they suggest, corre-
spond to introns either added or moved and appear equally in all three phases.
In the next section, the evidence for both the insertion or deletion of introns
will be examined.
3.5 Mechanisms of intron insertion and deletion
As we have seen, the introns-late theory both assumes and requires that introns
have been inserted and deleted during evolution. One example of the gain of an
exon during evolution comes from the ten exon human renin (
REN
; 1q32) gene
which contains three amino acids encoded by a sixth 9 bp exon not present in the
mouse gene (Miyazaki
et al
., 1984). This exon must have been acquired in the lin-
eage leading to humans rather than lost in the murine lineage because this exon is
absent from the evolutionarily related pepsin genes in both human (
PGA3
,
PGA4
,
PGA5
; 11q13) and pig. We may surmise that the sixth exon in the human
REN
gene was acquired by mutational change in intron 5 at some stage in the last
