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Lecture 8 Chapter 7
Mobile DNA Sequences in the Genome
Barbara McClintock (nobel prizewinner 1983)Found in the late 1940s -> geneticelements in maize can direct their ownmovement within the genome
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Lecture 7 Chapter 7
Mobile DNA Sequences in the Genome
Mobile Elements:
-> can be found in all organisms-> 50% of human genome-> interfere with gene expression-> generating mutations (-> Evolution)-> reorganize genomic structure
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Mutation can be caused by insertions:
Mutation in the gal operon was movedto phage
Insertion
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Mobile elements -> Intergration into genome
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Principle of mobile elements -> Intergration into genome
Transposase-> Enzyme coded on mobile elements-> resposible for excition and integration
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Insertion Element (IS): -> most simple mobile elements-> found mainly in bacteria
IS e
lement is cut out and leaves behind flanking repeats (non replicative)-> orginal target sequence are increased after jumb
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Insertion Element (IS):
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Transposons (TN): -> More complex mobile elements-> found in all organismsTransposons:
Transposons carryfrequently antibioticresistence genes
Transposons can jumpfrom genome to phage orconjugative plasmid -> genetransfere to otherbacteria (problem withantibiotica resistentbacteria)
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Gene TechnologyTransposons
Composite Transposon:
-> Insertion elements on both
end of the transposon (oneactive transposase oneinactive transposase)
Simple Transposons:
-> Inverted repeats on bothends
-> Transposase and Resolvase -> responsible for transfer
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Gene TechnologyTransposons
Simple Transposon(Tn3):
-> Replicative
transposition -> after thejump both have a copy ofthe transposon
Composite Transposons(Tn10):
->non replicative(conservative) -> liketranposition of ISelements
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Transposons
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Gene TechnologyTransposons
Simple Transposon(Tn3):
-> Replicative
transposition -> after thejump both have a copy ofthe transposon
Transposase: responsiblefor excition and transfer
Resolvase: responsible for
resolution -> replicativetransfer
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Gene TechnologyTransposons
An R plasmid may contain several transposons carrying(resistance genes)
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Gene TechnologyTransposons
Composite transposons:
-> non replicative transfer
Ac element (first discovered in
maize)-> Activator (avtive transposon)
Ds element (first discovered inmaize)-> Dissociation-> passive transposon-> needs Ac to
move
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Gene TechnologyTransposons
Composite transposons:
-> non replicative transfer
Ac element (first discovered in
maize)-> Activator (avtive transposon)
Ds element (first discovered inmaize)-> Dissociation-> passive transposon-> needs Ac to
move
Deletions in Ac element -> gives Dselement -> inactive transposase ->needs Ac to move
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Gene TechnologyMechanism of Ac or Ds tranfer
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Gene TechnologyTransposons
Transposition has influenceon phenotype of maize
First recognized byBarbara McKintock (firsttransposon found)
Movement of Transposonduring development of fruit-> pattern in pigmentation
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Gene TechnologyTransposons
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Gene TechnologyRetrotransposons are similar to retroviruses
RNA intermediate + useof reverse transcriptase
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Gene TechnologyRetrotransposons are similar to retroviruses
RNA intermediate + useof reverse transcriptase
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Gene TechnologyTransposons versus Retrotransposon
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Gene TechnologyRetrotransposon
There are 2 groups:
1. LTR (long terminal repeats) retrotransposons (viral like)
2. Non-LTR retrotransposons (polyA)
Carry strong promotersIf transposon inserted 5 to silencedgene -> activates gene expression of
these genesRetrotransposons can cause cancer!!
Promoternecessary fortransposition
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Gene TechnologyRetrotransposon
Carry strong promoters for RNAploymeraseIf transposon inserted 5 to silencedgene -> activates gene expression ofthese genesRetrotransposons can cause cancer!!Retroviruses can also cause cancer!!
Protein coded for :Integrase + Reversetranscriptase
ORF1: RNA binding proteinORF2: Endonuclease + reversetranscriptase
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Gene TechnologyRetrotransposon
Demonstration thatretrotransposon transfersthrough RNA intermediate
Ty is yeast LTR retrotransposon
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Gene TechnologyNon-LTR Retrotransposon
Promoternecessary for
transposition
ORF1: RNA binding proteinORF2: Endonuclease + reversetranscriptase
mRNA transcript with poly A
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Gene TechnologyNon-LTR Retrotransposon
Transposition -> target-primed reverse transcription
Reverse transcription commonly stops before elementhas been fully transcribed-> a lot of truncated L1 elements
Alu elements: 300bp repetitive sequences (11% of human
genome) -> belong to SINEs (do not encode their ownreverse transcriptase -> nonautonomous transposons
SINEs are probably trancated LINEs (share same 3sequence)
Alu elements are activated by LINEs
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Types of transposable elements in the human genome
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Human genes contain many transposable elements
Important for survival -> Exon/Intron concept of a gene
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Transposons are mainly responsible for differences in genome size
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Transposons can cause Mutations
Transposableelements at work insnapdragon
Result of transposition or excision -> not always clean excited -> has an
effect on enhancer -> different expression level
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Transposons can cause Mutations
Most mutations caused by transposons are deleterious -> L1 insertion into hemophilia A
Retrotransposons can be responsible for genetic disease (34 diseases identified) -> hemophilia,muscular dystrophy, Development of genetic disease can happen by transposon jumb in the embryo!!!
Transposons and retrotransposons can induce mutations by inserting near or within genes.-> transposon-induced mutations are relatively stable, because the sequence at theinsertion site is retained as they transpose via the replication mechanism.
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Transposons generate diversity for evolution
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Transposons generate diversity for evolution
Can affect genes (delection, insertion, exchange), regulatory elements
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Transposons generate diversity for evolution
Can affect genes (delection, insertion, exchange), regulatory elements
Mutant receptors -> cannot remove cholesterol from blood
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Transposons can be silenced
Organisms with high transposition (Drosphila) can silence transposition-> P element encodes not only transposase but also an inhibitor to transpostion (Protein altersslicing of transposase -> inactive transposase)
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Transposons can be silenced
If female has P element ->oocytes (eggs) with inhibitorproduced -> normal flies
If female has no P element ->oocytes have burst oftransposition in embryo ->steril offspring