Transposable Elements DNA Sequences That Change Positions in the Genome

Types of Transposable Elements Transposition Examples 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 transposition

Characteristics of Transposable Elements Some elements have terminal inverted repeats

Characteristics of Transposable Elements Carry 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 genes

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

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

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

Retrovirus Characteristics

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

Applying Your Knowledge Retrotransposon Transposon Both retrotransposons and transposons Neither retrotransposons nor transposons Which type of transposable element Transposes as DNA without an RNA intermediate? Contains long terminal repeats on its ends? Generates direct repeats as a result of transposition? Carries a gene for reverse transcriptase? Can insert a copy in a new location while leaving the old copy at the original site?

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

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

Examples of Transposable Elements Bacterial Insertion Sequences and more Complex Transposons Ac-Ds Elements in Corn Gret1 Retrotransposon in Grapes P 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 element Ds 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 element Animation available at http://www.dnalc.org

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 Type Properties Example Autonomous Can transpose without the presence of another element Non-autonomous Requires the presence of another functional element to move Autonomous element provides transposase or reverse transcriptase Ac Ds

Gret1 Retrotransposon in Grapes

P element inserts in multiple locations Transposition is repressed The P Element in Drosophila Codes for a Transposase and a Repressor of Transposition No repressor Repressor produced P element inserts in multiple locations 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.

Applying Your Knowledge Ac-Ds Elements Alu Element Insertion Sequence P element Which type of transposable element Contains only the sequences needed for transposition in bacteria? Represents a SINE found in humans? Is used to insert genes into fruit fly chromosomes? Causes reversible alterations for kernel color in corn?