Download Transposons: Mobile DNA DNA

Transposons: Mobile DNA
Transposons are segments of DNA that can move around to
different positions in the genome of a single cell.
In the process, they may
•cause mutations
•increase (or decrease) the amount of DNA in the genome
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These mobile segments of DNA are sometimes called
"jumping genes".
There are two distinct types of transposons:
• DNA transposons - transposons consisting only of DNA that
moves directly from place to place.
•Retrotransposons that
~first transcribe the DNA into RNA and then
~use reverse transcriptase to make a DNA copy of the RNA to
insert in a new location.
Classification of
transposons into two
classes
In both cases ds
DNA intermediate
is integrated into
the target site in
DNA to complete
movement
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DNA transposons are able to transpose in direct,
DNA-DNA manner and are present in
prokaryotes and eukaryotes
Two distinct mechanisms of transposition:
•Replicative transposition – direct interaction between the donor
transposon and the target site, resulting in copying of the donor
element
•Conservative transposition – involving excision of the element
and reintegration at a new site.
DNA transposons of prokaryotes
ITR
Insertion sequence
Composite transposon
Tn3-type transposon
AB resistance gene
IS
~ 0.3-up to 2kb
2.5-10kb
IS
ITR
ITR
transposase
ITR
Transposable phage
ITR
transposase
Integration and
replication genes
resolvase AB resistance gene
Lysis
genes
~5kb
ITR
Protein coat genes
~38kb
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Non-replicative transposition
of bacterial insertion
sequence
Enzyme that catalyses
transposition – transposase.
Bacterial transposon – composite transposon
Note – internal inverted repeats of IS1 abutting the resistance
gene are so mutated, that transposase does not recognize them.
IS element transposase makes cuts at positions of red arrows and
entire transposon is moved to the donor site.
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Transposition via RNA intermediate
Retrotransposons
Retrotransposons move by a "copy and paste" mechanism but in
contrast to the transposons described above, the copy is made of
RNA, not DNA.
40% of the entire human genome consists of retrotransposons.
Retroelements
LTR
gag
pol
env
LTR
Retrovirus
LTR
gag
pol
LTR
Ty1/copia retroelement
gag?
pol
poly(A)
LINE
SINE
poly(A)
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RNA transposons or retroelements are features of eukaryotic
genome, they are not found in prokaryotes.
Great deal of attention:
attention because of similarities to retroviruses.
General structure of eukaryotic viral retrotransposons
LTrs – are also present in retroviral DNA.
Like other transposons, they have short target –site direct repeats.
The basic mechanism – three steps:
1. An RNA copy of transposon is synthesized by the normal
process of transcription.
2. The RNA transcript is copied into DNA, which initially
exists as an independent molecule outside of the genome. This
conversion of RNA to DNA, the reverse of normal transcription
process required reverse transcriptase. It is often coded by a
gene within the transposon and is translated from the RNA
copy synthesized instep 1.
3. The DNA copy of the transposon integrates into the genome,
possibly back into the same chromosome occupied by original
unit, or by another chromosome.
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Generation of retroviral genomic RNA from integrated retroviral DNA
Left LTR functions
as a promoter that
directs host cell
RNA polymerase II
to initiate
transcription at 5’
nt of R.
After all is
transcribed, the
right sequence
directs host cell
processing enzymes
to cleave primary
transcript and add
polyA tail to the
end of R sequence.
The resulting retroviral RNA genome lacks complete LTRs. However, after virus
enters the cell genome, reverse transcription by virus encoded RT yields a a
dsDNA with LTRs. Intergase, coded by retrovirus, inserts the ds retroviral DNA
into genome. And short target sequence repeats are generated. Like retroviruses, Ty
and copia encode RT and integrase.
General structure of an L1 LINE element – a common type
of non-viral retrotransposon
ORF1 – encodes and RNA binding protein; ORF2 – protein
similar to reverse transcriptase.
NO LRTs, but AT reach region is thought to function in
transposition.
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Transposable elements and genome dynamics
Transposition and host genome
•Mutation
•Accumulation
•Sequence duplication
•Horizontal transfer
•Host defense
Safe-haven hypothesis
Transposable elements select integration sites that minimize
genetic damage to the host
References
Lodish et al., Molecular Cell Biology, 5th edition.
T. A. Brown, Genomes, 1999, Wiley-Liss, New-York.
Nature Reviews Molecular Cell Biology 2; 151-155 (2001).
Weaver, Molecular Biology, 2005, McGraw Hill.
Peterson-Burch, B.D., Wright, D.A., Laten, H.M. &.Voytas, D.F.
2000. “Retroviruses in Plants?” Trends Genet. 16: 151-152
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