Gen Tech 8 New

8/6/2019 Gen Tech 8 New

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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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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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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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An R plasmid may contain several transposons carrying(resistance genes)


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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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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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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 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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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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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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Can affect genes (delection, insertion, exchange), regulatory elements


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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

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Gen Tech 8 New