Biology Reference
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functional genes through fusion (
Gilbert et al. 1997
). It is likely that there has
been an average of two or three acts of such fusions of minigenes into the
larger exons of today (
Gilbert et al. 1997
). Some introns have been inherited
for millions of years, making it possible to find a consistent location for the
introns when homologous genes from different organisms are examined. The
actual sequences of the introns in these homologous genes may have diverged
through mutation to the point that they seem to have no sequence similarity.
Trotman (1998)
suggests that this consistent location of introns is evidence that
introns may have been integral to the development of primordial genes, leading
to the hypothesis that novel genes could arise from new combinations of exons,
and thereby generate novel proteins and functions.
The
introns-late
hypothesis assumes that mechanisms for splicing introns out
were not present in the common ancestor of life but that these mechanisms
arose and spread within eukaryotes during their evolution. According to this
hypothesis, introns could not have played a role in ancient gene and protein
assembly. As is often the case with many “either/or” debates, the truth may be a
combination of the two hypotheses (
Tyshenko and Walker 1997
). Both concepts
may be correct; the introns in the
triosephosphate isomerase
genes of insects
may be the result of the insertion of a transposable element relatively recently,
whereas other introns may have been present for a very long time (
Logsdon
et al. 1995
).
DeSouza et al. (1998)
suggest that 30-40% of the present-day
intron positions in ancient genes correspond to the introns originally present in
the ancestral gene. The rest of the intron positions are due to the movement or
addition of introns over evolutionary time. Thus, introns may be both early and
late, with
≈
65% of the introns having been added to preexisting genes.
Introns are present in low frequency in prokaryotes and are rare in some
eukaryotes, such as yeast. The number of introns and their lengths vary from
species to species and from gene to gene. Some genes in eukaryotic organisms
lack introns, whereas other genes in the same species may have as many as 50
introns. Introns may interrupt a coding region, or they may occur in the untrans-
lated regions of the gene. Some eukaryotic genes contain numerous and very
large introns, but introns typically range from 100 to 10,000bp in length. A
few introns contain genes themselves; how the genes got into the middle of an
intron of another gene remains a mystery.
The presence of introns within many eukaryotic protein-coding genes requires
that an additional step take place between transcription and translation in
eukaryotes. Thus, when the DNA is transcribed into RNA, the initial RNA tran-
script is
not
mRNA. The synthesized RNA is a precursor to mRNA and is called
