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

2. 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

3. Characteristics of Transposable Elements
All elements have flanking direct repeats: short repeated sequences on each side of 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 transpose as DNA
reverse transcriptase for elements that use an RNA intermediate
May contain other genes

6. 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

7. Mechanisms of Transposition
Transposition through DNA
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
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?

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
Gret1 Retrotransposon in Grapes
P elements in Fruit Flies

16. 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

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

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
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

22. Gret1 Retrotransposon in Grapes

23. 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

24. 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.

25. 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?