Transcript Brooker Chapter 17

RECOMBINATION AND
TRANSPOSITION
AT THE MOLECULAR LEVEL
CHAPTER 17
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Transposable Elements And Transposition
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Transposition
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transposable elements (TEs)
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integration of small DNA segments into
chromosomes
Can occur at many locations within genome
“jumping genes”
DNA segments that move
1st identified by Barbara McClintock in corn
McClintock Discovers Moving Loci in Corn
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Babara McClintock identified many unusual features
of corn chromosomes
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This observation initiated a six-year study that
culminated in 1951 with the following proposal
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She noticed that in one strain of corn, chromosome 9
tended to break at a high rate at the same site
termed this a mutable site or mutable locus
Mutable sites are actually locations where transposable
elements have been inserted into the chromosomes
She received the Nobel Prize in 1983 for this work
Transposition Pathways
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3 general types of transposition
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Simple transposition
Replicative transposition
Retrotransposition
Simple Transposition
Figure 17.12
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Known as Insertion Sequences - IS
bacterial
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Tn10
eukaryotic
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Ac/Ds
Replicative Transposition
Figure 17.12
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Involves replication of the TE and insertion of the copy into
another chromosomal location
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Only found in bacteria
Retrotranposons & Retrotransposition
Figure 17.12
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Very common but only occurs in eukaryotes
These types of elements are termed retroelements or
retrotransposons
Similar organization to retroviruses
Simple & Replicative Transposons
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Both contain a gene encoding a
transposase enzyme
Transposase function
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recognizes direct and indirect repeats
cuts DNA for both excision and insertion
Regulatory Sequences of Transposable Elements
transposon
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Direct repeats – DNA sequences that are identical and run
in the same direction (5’3’)
5’ ATGACTGAC 3’
3’ TACTGACTG 5’
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and
5’ ATGACTGAC 3’
3’ TACTGACTG 5’
Inverted repeats - DNA sequences that are identical (or very
similar) but run in opposite directions
5’ CTGACTCTT 3’
3’ GACTGAGAA 5’
Figure 17.13
and
5’ AAGAGTCAG 3’
3’ TTCTCAGTC 5’
Composite Transposons
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Contain additional genes that are not necessary for
transposition per se
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Only the two inverted repeats at the ends of the transposon
are involved in the transposition event
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Only these are adjacent to direct repeats
Figure 17.13
Elements of Replicative Transposons
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Organization is similar to insertion sequences
Resolvase gene is found between the inverted repeats
Both enzymes are needed to catalyze the transposition of
these types of elements
Figure 17.13
Retrotransposons – Retroviral-Like Elements
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Evolutionarily related to known retroviruses
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LTR – long terminal repeat
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uses RNA as a template to synthesize a cDNA (complementary DNA)
Int – integrase
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act as promoters to transcribe viral genes – in this case RT and Int genes
RT – reverse transcriptase
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Retroviruses - RNA viruses that make a DNA copy that integrates into the host’s
genome
recognizes DR sequences, cuts host DNA and insert retroelement sequences
There is no excision of retroelements or retroviruses!
There are ~100,000 copies of the L1 retroelement in humans
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Virtually all have lost function of RT and/or Int genes
Non-viral Retroelements
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Share little sequence similarity with retroviruses
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derived from normal eukaryotic genes
some have RT or RT-like gene, many such genes are not functional
Alu family of non-viral retroelements
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derived from a single ancestral gene known as the 7SL RNA gene
has been copied by retroposition to > 500,000 copies
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~ 6% of the human genome
An example of a SINE – short interspersed element
Figure 17.13
Describing Function of Transposable Elements
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Autonomous
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contain all the information necessary for transposition to occur
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Nonautonomous
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lack a gene or sequence element necessary for transposition
If element is missing – transposon will not transpose
if Transposase is mutated, element can still transpose if enzyme from another
transposon “helps” it
McClintock’s Ds element is nonautonomous
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functional transposase, RT, Int etc…
DNA elements – DRs, IRs, LTR, etc…
lacks transposase gene
McClintock’s Ac locus (Activator) is autonomous
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has functional transposase enzyme
If Ds and Ac are both present in genome, transposition of Ds can occur
Transposase Catalyzes Excision & Insertion
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The enzyme transposase catalyzes the removal of
a TE and the its reinsertion at another location
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Transposase recognize the inverted repeats at the
ends of a TE and bring them closer together
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The remainder of the general scheme for simple
transposition is shown in Figure 17.14
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Transposase Catalyzes Excision & Insertion
Figure 17.14
They are in the same direction
and are repeated at both ends
of the element
Figure 17.14
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Transposable Elements Influence Mutation & Evolution
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Over the past few decades, researchers have found that
transposable elements occur in the genomes of all species
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In some cases, repetitive sequences in eukaryotic
genomes are due to the proliferation of TEs
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In mammals, for example
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LINEs
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Long interspersed elements
Usually 1,000 to 5,000 bp long
Found in 20,000 to 100,000 copies per genome
SINEs
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Short interspersed elements
Less than 500 bp in length
Example: Alu sequence
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Present in 500,000 to 1,000,000 copies in the human genome
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The biological significance of transposons in
evolution remains a matter of debate
There are two schools of thought
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1. TEs exist because they simply can!
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In other words they are like parasites
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They can proliferate within the host as long as they do not harm the
host to the extent that they significantly disrupt survival
This has been termed the selfish DNA theory
2. TEs exist because they offer some advantage
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Bacterial TEs carry antibiotic-resistance genes
TEs may cause greater genetic variability through recombination
TEs may cause the insertion of exons into the coding sequences
of structural genes
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This phenomenon, called exon shuffling, may lead to the evolution
of genes with more diverse functions
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Transposable elements can rapidly enter the
genome of an organism and proliferate quickly
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Drosophila melanogaster
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A TE known as the P element was introduced into the species in
the 1950s
Remarkably, in the last 50 years, the P element has expanded
throughout D. melanogaster populations worldwide
The only strains without the P element are lab stocks collected
prior to 1950
Transposable elements have a variety of effects on
chromosome structure and gene expression
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Transposons Have Become Important
Tools in Biology
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The features of transposons have made them an
important experimental tool in molecular biology
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1. The introduction of transposons into a cell is a
convenient way to abolish the expression of a gene
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2. It can be used to clone a particular gene in an
approach known as transposon tagging
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An early example of transposon tagging involved an
X-linked gene in Drosophila that affects eye color
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Wild-type = red ; Mutant = white
In 1981, Paul Bingham, Robert Levis and Gerald
Rubin use transposon tagging to clone this gene
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They started with a wild-type population of Drosophila that
carried a transposon called copia
From this red-eyed strain, a white-eyed strain was
obtained
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The copia element transposed into the X-linked eye color gene,
thereby inactivating it
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Figure 17.17
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Figure 17.17
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