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