16 TRANSPOSONS (Full Edition)
1 Introduction
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- A transposon (transposable element)
is a DNA sequence able to insert itself (or a copy of itself) at a new location
in the genome, without having any sequence relationship with the target locus.
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Genomes evolve both by acquiring new sequences and by rearranging existing sequences.
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The sudden introduction of new sequences results from the ability of vectors to
carry information between genomes. Extrachromosomal elements move information horizontally
by mediating the transfer of (usually rather short) lengths of genetic material.
In bacteria, plasmids move by conjugation (see Conjugation transfers single-stranded
DNA), while phages spread by infection (see Phage strategies).
Both plasmids and phages occasionally transfer host genes along with their own replicon.
Direct transfer of DNA occurs between some bacteria by means of transformation (see
DNA is the genetic material of bacteria). In eukaryotes, some viruses,
notably the retroviruses, can transfer genetic information during an infective cycle
(see Retroviruses may transduce cellular sequences).
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Figure 16.1
A major cause of sequence change within a genome is the movement of a transposon
to a new site. This may have direct consequences on gene expression. Unequal crossing-over
between related sequences causes rearrangements. Copies of transposons can provide
targets for such events.
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Rearrangements are sponsored by processes internal to the genome. Two of the major
causes are summarized in Figure 16.1.
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>Unequal recombination results from mispairing by the cellular systems for homologous
recombination. Nonreciprocal recombination results in duplication or rearrangement
of loci (see Unequal crossing-over rearranges gene clusters). Duplication
of sequences within a genome provides a major source of new sequences. One copy
of the sequence can retain its original function, while the other may evolve into
a new function. Furthermore, significant differences between individual genomes
are found at the molecular level because of polymorphic variations caused by recombination.
We saw in Minisatellites are useful for genetic mapping that recombination
between minisatellites adjusts their lengths so that every individual genome is
distinct.
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Another major cause of variation is provided by transposable elements
or transposons: these are discrete sequences in the genome that
are mobile — they are able to transport themselves to other locations within the
genome. The mark of a transposon is that it does not utilize an independent form
of the element (such as phage or plasmid DNA), but moves directly from one site
in the genome to another. Unlike most other processes involved in genome restructuring,
transposition does not rely on any relationship between the sequences at the donor
and recipient sites. Transposons are restricted to moving themselves, and sometimes
additional sequences, to new sites elsewhere within the same genome; they are therefore
an internal counterpart to the vectors that can transport sequences from one genome
to another. They may provide the major source of mutations in the genome.
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Transposons fall into two general classes. The groups of transposons reviewed in
this chapter exist as sequences of DNA coding for proteins that are able directly
to manipulate DNA so as to propagate themselves within the genome. The transposons
reviewed in Retroviruses and retroposons are related to retroviruses, and
the source of their mobility is the ability to make DNA copies of their RNA transcripts;
the DNA copies then become integrated at new sites in the genome.
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Transposons that mobilize via DNA are found in both prokaryotes and eukaryotes.
Each bacterial transposon carries gene(s) that code for the enzyme activities required
for its own transposition, although it may also require ancillary functions of the
genome in which it resides (such as DNA polymerase or DNA gyrase). Comparable systems
exist in eukaryotes, although their enzymatic functions are not so well characterized.
A genome may contain both functional and nonfunctional (defective) elements. Often
the majority of elements in a eukaryotic genome are defective, and have lost the
ability to transpose independently, although they may still be recognized as substrates
for transposition by the enzymes produced by functional transposons (for review
see 164). A eukaryotic genome contains a large number and variety of transposons.
The fly genome has >50 types of transposon, with a total of several hundred individual
elements.
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Transposable elements can promote rearrangements of the genome, directly or indirectly:
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- The transposition event itself may
cause deletions or inversions or lead to the movement of a host sequence to a new
location.
- Transposons serve as substrates for
cellular recombination systems by functioning as "portable regions of homology";
two copies of a transposon at different locations (even on different chromosomes)
may provide sites for reciprocal recombination. Such exchanges result in deletions,
insertions, inversions, or translocations.
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The intermittent activities of a transposon seem to provide a somewhat nebulous
target for natural selection. This concern has prompted suggestions that (at least
some) transposable elements confer neither advantage nor disadvantage on the phenotype,
but could constitute "selfish DNA," concerned only with their own propagation. Indeed,
in considering transposition as an event that is distinct from other cellular recombination
systems, we tacitly accept the view that the transposon is an independent entity
that resides in the genome.
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Such a relationship of the transposon to the genome would resemble that of a parasite
with its host. Presumably the propagation of an element by transposition is balanced
by the harm done if a transposition event inactivates a necessary gene, or if the
number of transposons becomes a burden on cellular systems. Yet we must remember
that any transposition event conferring a selective advantage — for example, a genetic
rearrangement — will lead to preferential survival of the genome carrying the active
transposon (for review see 146).
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- 146 Campbell, A. (1981). Evolutionary significance
of accessory DNA elements in bacteria. Annu. Rev. Immunol. 35, 55-83.
PubMed Journal
- 164 Finnegan, D. J. (1985). Transposable
elements in eukaryotes. Int. Rev. Cytol. 93, 281-326. PubMed
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©Jones and Bartlett Publishers (2007)
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