Transposable Elements DNA Sequences That Change Positions in the Genome

Types of Transposable Elements TypeTranspositionExamples Transposon (Class I) Through DNA Corn: Ds element Drosophila: P element Retrotransposons (Class II) Use an RNA intermediate Yeast: Ty Drosophila: Copia Grapes: Gret1 Human: Alu Human: L1 Transposition: movement of a transposable element

Characteristics of Transposable Elements All elements have flanking direct repeats: short repeated sequences on each side of the element, arise by transpositionAll elements have flanking direct repeats: short repeated sequences on each side of the element, arise by transposition

Characteristics of Transposable Elements Some elements have terminal inverted repeatsSome elements have terminal inverted repeats

Characteristics of Transposable Elements Carry gene for enzyme that catalyzes transpositionCarry gene for enzyme that catalyzes transposition –transposase for elements that transpose as DNA –reverse transcriptase for elements that use an RNA intermediate May contain other genesMay contain other genes

Mechanisms of Transposition Transposition through DNATransposition through DNA –Replicative- new copy in new location, old copy retained at original site, element is used as template to produce the new copy

Mechanisms of Transposition Transposition through DNATransposition through DNA –Non-replicative: moves to another site without replication of the element

Mechanisms of Transposition Use of an RNA IntermediateUse of an RNA Intermediate –element is transcribed –reverse transcriptase produces a double- stranded DNA copy for insertion at another site

Types of Retrotransposons Viral RetrotransposonsViral Retrotransposons –resemble retroviruses = viruses with an RNA genome Long terminal direct repeat at each endLong terminal direct repeat at each end Carry genes for enzymes usually found in RNA virusesCarry genes for enzymes usually found in RNA viruses

Retrovirus Characteristics

Types of Retrotransposons Non-viral RetrotransposonsNon-viral Retrotransposons –do not resemble retroviruses –two types in humans LINES = long interspersed elementsLINES = long interspersed elements –6-7 kb long –example: L1 has 600,000 copies, represents 15% of human DNA SINES = short interspersed elementsSINES = short interspersed elements –300 bp long –example: Alu has 1 million copies, represents 10% of human DNA

Effects of Transposition Transposable elements can: Cause mutations in adjacent genesCause mutations in adjacent genes Cause chromosomal rearrangementsCause chromosomal rearrangements Relocate genesRelocate genes

Possible Advantages of Transposable Elements Transposable elements may: Create genetic diversityCreate genetic diversity Act as promotersAct as promoters Allow recombination between plasmid and genomic DNA when multiple copies of the element are presentAllow recombination between plasmid and genomic DNA when multiple copies of the element are present Carry antibiotic resistance genes, conferring an advantage on bacterial cellsCarry antibiotic resistance genes, conferring an advantage on bacterial cells Increase the number of copies of an exon or geneIncrease the number of copies of an exon or gene

Examples of Transposable Elements Bacterial Insertion Sequences and more Complex TransposonsBacterial Insertion Sequences and more Complex Transposons Ac-Ds Elements in CornAc-Ds Elements in Corn Gret1 Retrotransposon in GrapesGret1 Retrotransposon in Grapes P elements in Fruit FliesP elements in Fruit Flies

Transposable Elements in Bacteria Insertion Sequences contain only the elements needed for transposition Composite Transposons contain DNA that has insertion sequences on both sides Antibiotic resistance genes are often included

Ac and Ds Elements in Corn Ac stands for activator elementAc stands for activator element Ds stands for dissociative elementDs stands for dissociative element Barbara McClintock showed that -- transposition of the Ds element altered kernel coloration --movement of the Ds element required the activity of Ac elementBarbara McClintock showed that -- transposition of the Ds element altered kernel coloration --movement of the Ds element required the activity of Ac element Animation available at

Transposition of Ds Element Disrupts Gene Controlling Kernel Color

Excision of Ds Element Leads to Variegated Kernels

Relatedness of Ac and Ds Elements For transposition, Ds elements require the transposase produced by the Ac element.

Autonomous and Non-autonomous Elements TypePropertiesExample Autonomous Can transpose without the presence of another elementCan transpose without the presence of another element Non- autonomous Requires the presence of another functional element to moveRequires the presence of another functional element to move Autonomous element provides transposase or reverse transcriptaseAutonomous element provides transposase or reverse transcriptase

Gret1 Retrotransposon in Grapes

The P Element in Drosophila Codes for a Transposase 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 Drosophila P element codes for transposase P element with gene of interest can insert into chromosomes with help of plasmid containing only transposase.