Transposable ElementsTransposable Elements

DNA Sequences That Change DNA Sequences That Change Positions in the GenomePositions in the Genome

Types of Transposable ElementsTypes of Transposable Elements

Type Type TranspositionTransposition ExamplesExamples

TransposonTransposon(Class I)(Class I)

Use a DNA Use a DNA intermediateintermediate

Corn: Ds elementCorn: Ds element

DrosophilaDrosophila: P : P element element

RetrotransposonsRetrotransposons(Class II)(Class II)

Use an RNA Use an RNA intermediateintermediate

Yeast: TyYeast: Ty

DrosophilaDrosophila: Copia : Copia

Human: Alu Human: Alu Human: L1Human: L1

Transposition: movement of a transposable elementTransposition: movement of a transposable element

Characteristics of Transposable ElementsCharacteristics of Transposable Elements

• All elements have All elements have direct repeatsdirect repeats: : short repeated short repeated sequences sequences flanking the flanking the element, arise by element, arise by transpositiontransposition

Characteristics of Transposable ElementsCharacteristics of Transposable Elements

• Some elements have Some elements have terminal terminal inverted repeatsinverted repeats

Characteristics of Transposable ElementsCharacteristics of Transposable Elements

• Carry gene for enzyme that catalyzes Carry gene for enzyme that catalyzes transpositiontransposition– transposasetransposase for elements that use a DNA for elements that use a DNA

intermediateintermediate– reverse transcriptasereverse transcriptase for elements that for elements that

use an RNA intermediateuse an RNA intermediate

• May contain other genes May contain other genes

Mechanisms of TranspositionMechanisms of Transposition• Use of a DNA IntermediateUse of a DNA Intermediate

– Replicative- new copy in new location, old Replicative- new copy in new location, old copy retained at original site, element is used copy retained at original site, element is used as template to produce the new copyas template to produce the new copy

Mechanisms of TranspositionMechanisms of Transposition• Use of a DNA IntermediateUse of a DNA Intermediate

– Non-replicative: moves to another site Non-replicative: moves to another site without replication of the elementwithout replication of the element

Mechanisms of TranspositionMechanisms of Transposition

• Use of an RNA Use of an RNA IntermediateIntermediate– element is element is

transcribedtranscribed– reverse transcriptase reverse transcriptase

produces a double-produces a double-stranded DNA copy stranded DNA copy for insertion at for insertion at another siteanother site

Types of RetrotransposonsTypes of Retrotransposons

• Viral Retrotransposons Viral Retrotransposons – resemble retroviruses = viruses with an resemble retroviruses = viruses with an

RNA genomeRNA genome• Long terminal direct repeat at each endLong terminal direct repeat at each end• Carry genes for enzymes usually found in Carry genes for enzymes usually found in

RNA virusesRNA viruses

Retrovirus Retrovirus CharacteristicsCharacteristics

Types of RetrotransposonsTypes of Retrotransposons

• Non-viral RetrotransposonsNon-viral Retrotransposons– do not resemble retrovirusesdo not resemble retroviruses

– two types in humanstwo types in humans• LINES = long interspersed elementsLINES = long interspersed elements

– 6-7 kb long6-7 kb long– example: L1 has 600,000 copies, represents example: L1 has 600,000 copies, represents

15% of human DNA 15% of human DNA

• SINES = short interspersed elementsSINES = short interspersed elements– 300 bp long 300 bp long – example: Alu has 1 million copies, represents example: Alu has 1 million copies, represents

10% of human DNA10% of human DNA

Applying Your KnowledgeApplying Your Knowledge

Which type of transposable elementWhich type of transposable element• Uses a DNA intermediate for transposition?Uses a DNA intermediate for transposition?• Contains long terminal repeats on its ends? Contains long terminal repeats on its ends? • Generates direct repeats as a result of Generates direct repeats as a result of

transposition?transposition?• Carries a gene for reverse transcriptase?Carries a gene for reverse transcriptase?• Can insert a copy in a new location while leaving Can insert a copy in a new location while leaving

the old copy at the original site? the old copy at the original site?

1.1. RetrotransposonRetrotransposon2.2. TransposonTransposon3.3. Both retrotransposons and transposonsBoth retrotransposons and transposons4.4. Neither retrotransposons nor transposonsNeither retrotransposons nor transposons

Effects of TranspositionEffects of Transposition

Transposable elements can:Transposable elements can:

• Cause mutations in adjacent genesCause mutations in adjacent genes

• Cause chromosomal rearrangementsCause chromosomal rearrangements

• Relocate genes Relocate genes

Possible Advantages of Transposable ElementsPossible Advantages of Transposable Elements

Transposable elements may:Transposable elements may:• Create genetic diversity Create genetic diversity • Act as promotersAct as promoters• Allow recombination between plasmid Allow recombination between plasmid

and genomic DNA when multiple and genomic DNA when multiple copies of the element are presentcopies of the element are present

• Carry antibiotic resistance genes, Carry antibiotic resistance genes, conferring an advantage on bacterial conferring an advantage on bacterial cellscells

• Increase the number of copies of an Increase the number of copies of an exon or gene exon or gene

Examples of Transposable ElementsExamples of Transposable Elements

• Bacterial Insertion Sequences and Bacterial Insertion Sequences and more Complex Transposonsmore Complex Transposons

• Ac-Ds Elements in CornAc-Ds Elements in Corn

• P elements in Fruit FliesP elements in Fruit Flies

Transposable Elements in BacteriaTransposable Elements in BacteriaInsertion SequencesInsertion Sequences contain only the contain only the elements needed for transpositionelements needed for transposition

Composite TransposonsComposite Transposons contain DNA that contain DNA that has insertion sequences on both sideshas insertion sequences on both sides

Antibiotic resistance genes are often included

Ac and Ds Elements Ac and Ds Elements in Cornin Corn

• Ac stands for activator element Ac stands for activator element • Ds stands for dissociative element Ds stands for dissociative element • Barbara McClintock showed thatBarbara McClintock showed that

----transposition of the Ds element transposition of the Ds element altered kernel coloration altered kernel coloration

--movement of the Ds element --movement of the Ds element required the activity of Ac element required the activity of Ac element

Animation available at http://www.dnalc.orgAnimation available at http://www.dnalc.org

Transposition of Ds Element Disrupts Transposition of Ds Element Disrupts Gene Controlling Kernel ColorGene Controlling Kernel Color

Excision of Ds Element Leads to Excision of Ds Element Leads to Variegated Kernels Variegated Kernels

Relatedness of Ac and Ds ElementsRelatedness of Ac and Ds Elements

For transposition, Ds elements require the For transposition, Ds elements require the transposase produced by the Ac element.transposase produced by the Ac element.

Autonomous and Non-autonomous Autonomous and Non-autonomous ElementsElements

TypeType PropertiesProperties ExampleExample

AutonomousAutonomous •Can transpose withoutCan transpose without the presence of the presence of another element another element

Non-Non-autonomousautonomous

•Requires the Requires the presence of another presence of another functional element functional element to move to move•Autonomous element Autonomous element provides transposase provides transposase or reverse transriptase or reverse transriptase

AcAc

DsDs

The P Element in The P Element in DrosophilaDrosophila Codes for a Codes for a Transposase and a Repressor of TranspositionTransposase and a Repressor of Transposition

No repressor

P element inserts in multiple

locations

Repressor produced

Transposition is repressed

Use of the P Element As a Vector in Use of the P Element As a Vector in DrosophilaDrosophila

P element codes for transposaseP element codes for transposase

P element with gene of interest can insert into chromosomesP element with gene of interest can insert into chromosomeswith help of plasmid containing only transposase. with help of plasmid containing only transposase.

Applying Your KnowledgeApplying Your Knowledge

Which type of transposable elementWhich type of transposable element• Contains only the sequences needed for Contains only the sequences needed for

transposition in bacteria?transposition in bacteria?• Represents a SINE found in humans? Represents a SINE found in humans? • Is used to insert genes into fruit fly Is used to insert genes into fruit fly

chromosomes?chromosomes?• Causes reversible alterations for kernel color Causes reversible alterations for kernel color

in corn?in corn?

1.1. Ac-Ds ElementsAc-Ds Elements2.2. Alu ElementAlu Element3.3. Insertion SequenceInsertion Sequence4.4. P elementP element