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Transposable Elements
DNA Sequences That Change
Positions in the Genome
Types of Transposable Elements
Type
Transposition Examples
Transposon
(Class I)
Use a DNA
intermediate
Corn: Ds element
Drosophila: P
element
Retrotransposons Use an RNA Yeast: Ty
(Class II)
intermediate Drosophila: Copia
Human: Alu
Human: L1
Transposition: movement of a transposable element
Characteristics of Transposable Elements
• All elements have
direct repeats:
short repeated
sequences
flanking 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 use a DNA
intermediate
– reverse transcriptase for elements that
use an RNA intermediate
• May contain other genes
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
Mechanisms of Transposition
• Use of a DNA Intermediate
– 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 doublestranded 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
1.
2.
3.
4.
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?
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
• 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
Properties
Type
Autonomous •Can transpose without
the presence of
another element
•Requires the
Nonautonomous presence of another
functional element
to move
•Autonomous element
provides transposase
or reverse transriptase
Example
Ac
Ds
The P Element in Drosophila Codes for a
Transposase and a Repressor of Transposition
Repressor
produced
Transposition is
repressed
No
repressor
P element
inserts in
multiple
locations
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
1.
2.
3.
4.
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?