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Piece of DNA that can move from place to place in
an organisms genome
Structurally and functionally distinct
Excised from one site and inserted at another site
Alternate names
› “jumping genes”
› Mobile genetic elements
› Transposable elements
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Exist in all organisms
More prevalent in eukaryotic than bacterial genomes
e.g.. 25-40% mammalian chromosomes consist of
transposable elements
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Conservative transposition
› Element is moved and not replicated
› “cut and paste” transposition
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Non-conservative (replicative)
transposition
› Element is replicated during transposition
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Retrotransposition
› Transposition mediated by an RNA
intermediate
Classified based on mechanism of
transposition
 Class I elements (Retroelements)
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› Transpose via RT of RNA intermediate
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Class II elements (DNA elements)
› Transpose directly as DNA using cut and
paste mechanism
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Copia
elements
(Drosophila)
Ty elements
(Yeast)
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Single stranded RNA animal viruses
Use ds DNA as an intermediate for replication
Not similar to retroviruses
 Short (<500bp) interspersed elements
 E.g.. Alu sequence
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› +10% of human DNA
› 300 bp sequence
› Modified from processed retrogene (7SL RNA)
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Found between genes in introns
Do not encode their own transposition machinery
Retrotransposed by enzymes coded for elsewhere
Long (>5kb) interspersed elements
 E.g.. L1 and L2 elements in humans
 21% of human genome
 Usually encode enzymes necessary for the
transposition
 Consist of two ORFs + promoter
 RT often prematurely terminated
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Virtually identical to
structure of retrovirus
 Flanked by two identical
LTR
 1-3 ORFs encoding
structural proteins and
enzymes needed for
transposition
 Completely autonomous
 Only found in eukaryotes
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Terminal inverted repeat sequences flank ORF for
transposase
 DNA sequence recognised by transposase
 Cut out and inserted at new position
 Conservative transposition
 No replication of transposon
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≈ 5900 np
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340 TyA = gag
np
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TyB = pol
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340
np
LTR - retrotransposon
Ty1 35 copies in Saccharomyces cerevisiae
330 bp delta (d) sequences are found in
about 100 copies
d sequences show significant sequence
divergence
Direct repeat sequences - not Inverted
repeats (IRs) like bacteria
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Generate a repeated sequence of target DNA
(5bp) during transposition
 Caused mutation by insertion
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Boeke and Fink (1985)
 Ty element altered by
adding galactose
promoter
 Galactose increased
frequency of
transposition
 An intron was added
 Newly transposed
elements lacked intron
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