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Transposable Elements

Transposable Elements. DNA Sequences That Change Positions in the Genome. Types of Transposable Elements. Transposition: movement of a transposable element. Characteristics of Transposable Elements.

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Transposable Elements

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  1. Transposable Elements DNA Sequences That Change Positions in the Genome

  2. Types of Transposable Elements Transposition: movement of a transposable element

  3. Characteristics of Transposable Elements • All elements have direct repeats: short repeated sequences flanking the element, arise by transposition

  4. Characteristics of Transposable Elements • Some elements have terminal inverted repeats

  5. Characteristics of Transposable Elements • Carry gene for enzyme that catalyzes transposition • transposase for elements that use a DNA intermediate • reverse transcriptase for elements that use an RNA intermediate • May contain other genes

  6. Mechanisms of Transposition • Use of a DNA Intermediate • Replicative- new copy in new location, old copy retained at original site, element is used as template to produce the new copy

  7. Mechanisms of Transposition • Use of a DNA Intermediate • Non-replicative: moves to another site without replication of the element

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

  9. 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

  10. Retrovirus Characteristics

  11. 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

  12. Applying Your Knowledge • Retrotransposon • Transposon • Both retrotransposons and transposons • Neither retrotransposons nor transposons • Which type of transposable element • Uses a DNA intermediate for transposition? • 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?

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

  14. 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

  15. Examples of Transposable Elements • Bacterial Insertion Sequences and more Complex Transposons • Ac-Ds Elements in Corn • P elements in Fruit Flies

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

  17. 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

  18. Transposition of Ds Element Disrupts Gene Controlling Kernel Color

  19. Excision of Ds Element Leads to Variegated Kernels

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

  21. Autonomous and Non-autonomous Elements Ac Ds

  22. 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

  23. P element codes for transposase P element with gene of interest can insert into chromosomeswith help of plasmid containing only transposase. Use of the P Element As a Vector in Drosophila

  24. 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?

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