Mobile DNA Element-Transposable !

COMPILED BY
PRITHIVI RAJ GURUNG
Central Department of Botany, TU, Nepal

OUTLINE
Origin of Junk DNA Hypothesis
Types of Junk DNA
Mobile DNA Element: Overview
Rate of Transposition, Induction and Defence
Classification of Transposons
Transposable Elements in Bacteria
Mobile Genetic Elements in Eukaryotes
Drosophila Transposons
Human Retrotranspons
Transposons as Mutagens
Genetic Transformation using Transposons
Transposons and Genome Organization
Transposable Elements and Evolution
Transposons and Diseases

But First
About the mystery………

AMAZING DISCOVERY OF Barbara McClintock
 She discovered movable genes, pieces of DNA that could integrate
themselves into new loci in DNA.
 The idea that genes could jump around the genome was so
dramatic and arcane – must either be a one- off…. Or not even real.
 Barbara McClintock’s intuitions about gene regulation and
epigenetic inheritance as well as her discovery of transposons all
came decades before molecular technologies made it possible to
prove, and even give names to these phenomena.
Only later did the discovery of transposons in bacteria and
eukaryotes make McClintock ‘s mobile genetic elements real.

To understand how she found these mobile genetic elements,
we need to know something about reproduction of maize......

The Aleurone Tissue Layer
 The phenotypes studied by McClintock were in the
aleurone tissue of the seeds.
 Aleurone cells, the outer layer of corn kernels (the seeds)
are derived from the triploid endosperm tissue.
 The tissue provides a protective tissue layer for the seed.
 In addition when aleurone cells express anthocyanin
pigments, kernels become darker in color, or show dark
spots.

The problem McClintock was addressing in the 1940s and
early 1950s
Coloration(purple) , colorless (white/yellow) or variegated
(mosaicism, or spotted/ streaked) of maize kernels (seeds) is
inherited.
Inheritance of kernel color phenotype must be studied against a
triploid genetic background.
Variegated color was proposed to result when an unstable mutation
that produced colorless kernels ‘reverted’ in some cells but not
others to create spotted or streaked phenotype.
McClintock’s studies revealed that kernel coloration is based on at
least three genes.

The 3 genes initially studied by McClintock
Two genes controlling presence or absence of Kernel color
1. C’(dominant); C(recessive)
C’(inhibitor allele)- colorless (yellow) kernels, regardless
of the rest of the genetic background
2. Bz(dominant- purple color); bz(recessive- dark brown
kernels)
Third gene required to get variegated kernel color
Ds (Dissociator gene): Without Ds, kernels were either
colored or colorless depending upon the possible
genotypes above. Because the Ds gene effect was
random and only affected some aleurone layer cells, it
was suspected to be the region of chomosomal damage
or breakage.

What McClintock did?
Having earlier demonstrated crossing over in maize, McClintock
mapped the 3 genes to chromosome 9. She then selectively bred and
mated corn with the following genotypes:
Eggs (from female producing
colored kernels) homozygous
for recessive alleles of the 3
genes:
CCbzbz_ _ (no Ds gene)
Pollen (sperms from males
producing colorless kernels)
homozygous for dominant
alleles of the 3 genes:
C’C’BzBzDsDs
C’CBzbz_Ds
Diploid zygote
C’CCBzbzbz_ _Ds
Triploid aleurone cells
Expected phenotype = all colourless kernels
(Because of dominant C’ allele)

What McClintock found?
There were many colorless kernels on the
hybrid cob.
There were also many mosaic kernels with
dark spots/streaks against a colorless
background.

What McClintock reasoned?
If some aleurone layer cells in some kernels suffered
chromosome breakage (//) at the Ds (Dissociator) locus,
the allele, the C’ allele would become inactive, causing
these cells to revert to making pigment as directed by the
bz allele.
The effective (operative) genotype in the affected cells is
CCbzbz. These cells will produce brown pigments against
the colorless background of the kernels.

McClintock soon found that Ds cannot act alone….
Her first experiments were of course reproducible, except
for some crosses of true- breeding dominant and
recessive genotypes using different breeding stocks of
maize.
For example, when the original males were crossed with a
female from a different lineage but the same homozygous
recessive CCbzbz_ _ genotype, the progeny had all
colorless kernels.
It seemed that the Ds gene contributed by the male was
unable to function (i.e., ‘break’) in females of the new
breeding stock.
McClintock realized that the female in the original cross
must have contributed a factor that activated the Ds gene
to break.

Interpreting the results of the second cross…
It seemed that the Ds gene contributed by the male was
unable to function (i.e., ‘break’) in females of the new
breeding stock.
McClintock realized that the female in the original cross
must have contributed a factor at an independent genetic
locus that activated the Ds gene to break! In this scenario
the male lacked the gene.
She called the new gene the activator, or Ac gene. Based
on the dependence on Ds on the Ac locus, the loci are
recognized as a 2- element, Ac/Ds system influencing
mosaicism in male kernels.

With further study, she conclude “Ds is a mobile element”
McClintock later showed that Ac- dependent ‘breakage’
was in some cases associated with inactivation of a
normal Bz gene, leading to a loss of purple color kernels.
The simplest interpretation for the ability of a single gene
to disrupt the behaviour of two different, independent
genes was that, far from simply breaking the chromosome
at a fragile Ds locus, the Ds gene could actually jump into
either the Bz gene or C’ loci, disrupting one or the other
gene’s functions.
McClintock had discovered a mobile element long before
transposons were found in bacteria and eukaryotes. For
this and the rest of her remarkable career, she earned a
Nobel Prize.

Ac is a transposon!
With the advent of recombinant DNA technologies, we now
know that:
The Ds element lacks a key gene for a transposase enzyme
necessary for transposition. The Ac element has this gene
and is capable of independent transposition. The Ac
element can provide the transposase needed to mobilize
the Ds element.
Ds and Ac are related: Ac and Ds sequences are similar; Ds
is a truncated version of Ac, but other than the
transposase, has structural features needed for mobility.

Ds and Ac transposition share features with other
transposons.
 11 bp inverted repeats at either end of the Ac and Ds
element
 8bp direct repeats (Not inverted repeats) of ‘target DNA’
at the site of insertion of either transposon.

MOBILE DNA ELEMENT Part II
By PRITHIVI RAJ GURUNG

Origin of Junk DNA Hypothesis
The idea that a large portion of the genomes of eukaryotes is made
up of useless evolutionary remnants comes from the problem known
as the ‘c- value paradox’, ‘c’ meaning the haploid chromosomal DNA
content.
There is an extraordinary degree of variation in genome size between
different eukaryotes, which does not correlate with organismal
complexity or the numbers of genes that code for proteins.
Early DNA-RNA hybridisation studies and recent genome sequencing
results have confirmed that >90 % of the DNA of vertebrates does
not code for a product.
Much of this variation is due to non-coding (i.e. not producing an
RNA or protein product), often very simple, repeated sequences.

With the discovery that many of these sequences seemed to have
arisen from mobile DNAs which are able to reproduce themselves,
the selfish or parasitic DNA hypothesis was born.
However, recent research has begun to show that many of these
useless-looking sequences do have a function, and that they may
have played a role in ‘with in kind’ diversification.
Types of Junk DNA:
1. introns, internal segments in genes that are removed at
the RNA level;
2. pseudogenes, genes inactivated by an insertion or deletion;
3. satellite sequences, tandem arrays of short repeats; and
4. interspersed repeats, which are longer repetitive sequences
mostly derived from mobile DNA elements.

Mobile DNA Element: Overview
Mobile genetic elements include transposons, which move within a
single cell (and its descendants), plus those viruses whose genomes
can integrate into the genomes of their host cells.
In bacteria, transposable elements can move to new positions on
the same chromosome (because there is only one chromosome) or
onto plasmids or phage chromosomes; in eukaryotes, transposable
elements may move to new positions within the same chromosome
or to a different chromosome.
In both bacteria and eukaryotes, transposable elements insert into
new chromosome locations with which they have no sequence
homology; therefore, transposition is a process different from
homologous recombination (recombination between matching DNA
sequences)and is called nonhomologous recombination.

Transposable elements are important due to the genetic changes
they cause. For example, they can produce mutations by inserting
into genes (a process called insertional mutagenesis),they can
increase or decrease gene expression by inserting into gene
regulatory sequences (such as by disrupting promoter function or
stimulating a gene’s expression through the activity of promoters on
the element), and they can produce various kinds of chromosomal
mutations through the mechanics of transposition.
In fact, transposable elements have made important contributions to
the evolution of the genomes of both bacteria and eukaryotes
through the chromosome rearrangements they have caused.

RATE OF TRANSPOSTION, INDUCTION AND DEFENSE
One study estimated the rate of transposition of a particular
retrotransposon, the Ty1 element in Saccharomyces cerevisiae came
out to be about once every few months to once every few years.
Cells defend against the proliferation of TEs in a number of ways.
These include piRNAs and siRNAs, which silence TEs after they have
been transcribed.
If organisms are mostly composed of TEs, one might assume that
disease caused by misplaced TEs is very common, but in most cases
TEs are silenced through epigenetic mechanisms like DNA
methylation, chromatin remodeling and piRNA, such that little to no
phenotypic effects nor movements of TEs occur as in some wildtype
plant TEs.

Although each kind of transposable element has its own special
characteristics, most can be classified into one of three categories
based on how they transpose. (2073 Q1 10 marks)

The cut-and-paste transposons are found in both prokaryotes
and eukaryotes. The replicative transposons are found only in
prokaryotes, and the retrotransposons are found only in eukaryotes.
 CUT AND PASTE TRANSPOSITION
A cut-and-paste transposon is excised from one genomic
position and inserted into another by an enzyme, the
transposase, which is usually encoded by the transposon
itself.
The element is physically cut out of one site in a
chromosome and pasted into a new site, which may even be
on a different chromosome.

Figure: Cut and Paste Transposition
Formation of DNA loop
when the transposase
brings two inverted repeats
together
The insertion site is
marked by a short
direct repeat of the
target DNA sequence

 REPLICATIVE TRANSPOSITION
Transposition is accomplished through a process that
involves replication of the transposable element’s DNA.
A transposase encoded by the element mediates an
interaction between the element and a potential insertion
site.
During this interaction, the element is replicated, and one
copy of it is inserted at the new site; one copy also remains
at the original site.
 RETROTRANSPOSITION
Involves the insertion of copies of an element that were
synthesized from the element’s RNA.
An enzyme called reverse transcriptase uses the element’s
RNA as a template to synthesize DNA molecules, which are
then inserted into new chromosomal sites.

TRANSPOSABLE ELEMENTS IN BACTERIA
Bacterial transposons move within and between chromosomes and
plasmids.
Three Main Types: 1. IS Elements or The Insertion Sequences
2. The Composite Transposons and
3. The Tn3 like Elements.
These three types of transposons differ in size and structure.
The IS elements are the simplest, containing only genes that encode
proteins involved in transposition.
The composite transposons and Tn3-like elements are more
complex, containing some genes that encode products unrelated to
the transposition process.

IS ELEMENTS
Figure: Structure of an inserted
IS50element showing its terminal
inverted repeats and target site
duplication. The terminal inverted
repeats are imperfect because
the fourth nucleotide pair
(highlighted) from each end is
different.
The IS elements are normal constituents of bacterial chromosomes
and plasmids. Among bacteria as a whole, the IS elements range in
size from 768 bp to more than 5,000 bp and are found in most cells.
All IS elements end with perfect or nearly perfect terminal inverted
repeats (IRs) of 9 to 41 bp. This means that essentially the same
sequence is found at each end of an IS, but in opposite orientations.

IS elements usually encode a protein, the transposase, that is
needed for transposition. The transposase binds at or near the ends
of the element and then cuts both strands of the DNA.
This cleavage excises the element from the chromosome or plasmid,
so that it can be inserted at a new position in the same or a different
DNA molecule. IS elements are therefore cut-and-paste transposons.
When IS elements insert into chromosomes or plasmids, they create a
duplication of part of the DNA sequence at the site of the insertion.
One copy of the duplication is located on each side of the element.
These short (2 to 13 nucleotide pairs), directly repeated sequences,
called target site duplications, arise from staggered cleavage of the
double-stranded DNA molecule.
INTEGRATION OF ‘IS’ ELEMENT IN CHROMOSOMAL DNA

Figure: Process of integration of an IS element into chromosomal DNA.

When a particular IS element resides in two different DNA
molecules, it creates the opportunity for homologous recombination
between them. For instance, an IS element in the F plasmid may pair
and recombine with the same kind of IS element in the E. coli
chromosome.
When an IS element
mediates recombination
between these molecules,
the smaller plasmid is
integrated into the larger
chromosome, creating a
single circular molecule. Such
integration events produce
Hfr strains capable of
transferring their
chromosomes during
conjugation.
Figure: Formation of a
conjugative R plasmid by
recombination between IS
elements.

THE COMPOSITE TRANSPOSON
Composite transposons are created when two IS elements insert
near each other. The region between the two IS elements can then
be transposed when the elements act jointly. In effect, the two IS
elements “capture” a DNA sequence that is otherwise immobile and
endow it with the ability to move.
Composite transposons, like the IS elements that are part of them,
create target site duplications when they insert into DNA.
Flanking IS
elements
are either
same in
orientation
or are
inverted

The Tn3 Element
Bacteria contain other large transposons that do not have IS elements
at each of their ends. Instead, these transposons terminate in
simple inverted repeats 38 to 40 nucleotide pairs long; however, like
the cut-and-paste transposons, they create target site duplications
when they insert into DNA.
There are three genes, tnpA, tnpR, and bla, encoding, respectively, a transposase,
a resolvase/repressor, and an enzyme called beta lactamase. The beta lactamase
confers resistance to the antibiotic ampicillin, and the other two proteins play
important roles in transposition.

Tn3 is a replicative transposon that moves in a two-stage process.
Figure: Transposition of
Tn3 via the formation of
a cointegrate.

MOBILE GENETIC ELEMENTS IN EUKARYOTES
Mobile genetic elements have been identified in many eukaryotes.
They have been studied extensively, with most research being done
with yeast, Drosophila, corn, and Human.
On the basis of mechanism of transposition, mobile genetic elements
of eukaryotes can be divided into two classes:
 Class I elements (RNA transposable elements)
Three families: 1. Retroviruses
2. Long Terminal Repeat (LTR) retrotransposon
3. non- LTR retrotransposon
 Class II elements (DNA to DNA transposable elements)
Four superfamilies: 1. Ac family 2. CACTA family
3. Mutator- like elements (Mu)
4. Tc1- Mariner- like elements (MLEs)

Class I Elements
These transpose by reverse transcription of an RNA intermediate
(DNA-RNA-DNA). They are mobile because of their ability to make
DNA copies of their RNA transcripts, and these DNA copies integrate
at new sites in the genome.
Transposition via RNA leaves the original copy in the genome, and
therefore results in a replicative increase in the copy number of
retrotransposons, so called copy and paste mechanism.
Mobile elements that transpose via RNA intermediates through
reverse transcription induce stable mutations.
In most or all plant species, they comprise the greatest mass of
transposable elements. Retrotransposons make up over 70% of the
nuclear DNA in maize and are equally or even more numerous in
other plant species with large complex genome.

RETROVIRUSES
Retrovirus genomes are composed of single-stranded RNA comprising
at least three genes: gag (coding for structural proteins of the viral
particle), pol (coding for a reverse transcriptase/integrase protein),
and env (coding for a protein embedded in the virus’s lipid envelope).
Retroviruses are distinguished from other types of retroelements by
the presence of an env gene in their genome. The protein encoded by
this gene allows retroviruses to enter and leave their host cell.
Retroviruses are therefore the only infectious type of retroelement:
they spread from cell to cell, and also organism to organism.
pol gene also has a DNA polymerase activity, which enables it to
synthesize a duplex DNA from the single- stranded reverse trancript
of the RNA. The enzyme has an RNase H activity, which can degrade
RNA part of RNA-DNA hybrid.

Figure 5–62 The life cycle of a retrovirus. The retrovirus genome consists of an RNA
molecule (blue) that is typically between 7000 and 12,000 nucleotides in length. It is
packaged inside a protein capsid, which is surrounded by a lipid-based envelope that
contains virus-encoded envelope proteins (green).

DNA copy
synthesized
is longer
than the
RNA
template
Reverse
transcriptase
copies the
3’sequence
of RNA to 5’
sequence of
DNA….. This
generates
LTRs.
Integration
event
generates
direct target
repeats

LTR Retrotransposons
Both LTR retrotransposons and retroviruses are characterized by the
presence of about 300- to 500- bp long direct repeats at both ends of
the element.
These so called LTRs contain control sequences for the initiation and
termination of transcription as well as for polyadenylation.
DNA between the LTRs is normally between 3 and 5 kb long. It
encodes a capsid protein, an RNase H, a reverse transcriptase, a
protease and an endonuclease which serves as an integrase.
LTR retrotransposons can be classified into two groups that are
distinguished by the arrangement of the integrase and reverse
transcriptase genes along the element.

These groups were named Ty 1- copia and Ty 3-gypsy, respectively
following the nomenclature given to the initial representatives of
both groups, which were first described for yeast (transposon yeast:
Ty 1 and Ty 3) and Drosophila (copia and gypsy).
Both types of retroelements are ubiquitous in many plant species.
gag prot int RT-Rnase H LTRLTR5’ 3’
gag prot RT-Rnase H int LTRLTR5’ 3’
Ty1- copia
Ty3- gypsy
Figure: LTR retrotransposons. Ty-1 copia and Ty-3 gypsy elements
differ from each other by the relative arrangements of int and RT-
Rnase H domains

Non- LTR Retrotransposons
Retrotransposons lacking the terminal repeats. They can be further
divided into LINEs and SINEs.
 LINEs
 Several kilobases long, have a chracteistic poly(A) tract at their 3’
end, and are by flanked by short direct repeats (3 to 16 bp) that
result from the repair of the staggered breaks generated by the
integration process
 Two open reading frames are usually present, one encoding a
capsid protein, and the other encoding an endonuclease and a
reverse transcriptase domain.
 Plant LINEs generally exhibit high levels of sequence
heterogeneity, which may be the consequence of the
accumulations of mutations in these mostly defective elements.

 SINEs
 With about 100 to 500 bp, SINEs are much smaller than LINEs.
 They are not able to transpose on their own, but require the
activity of a reverse transcriptase in trans.
 SINEs are derived from processed pseudogenes.
 Their intact ancestors are host genes encoding small
cytoplasmic RNAs such as tRNAs.
 Like LINEs, SINEs are flanked by short target site duplications
and harbor an A- rich tract at their 3’ end
 Like their tRNA progenitors, SINEs carry two internal promoters
recognised by host RNA polymerase II.

Gag EN RT (A)n
LINE
5’-UTR 3’-UTR
5’ 3’
tRNA-related LINE- related (A)n5’ 3’
SINE
Figure: Non- LTR retrotransponsons have no LTRs, but harbor an A(n)
tract at their 3’ end. They are subdivided into LINEs and SINEs. Only
LINEs carry genes involved in their own transposition (encoding a
capsid protein, an endonuclease and a reverse transcriptase)

CLASS II TRANSPOSONS
Class II transposons disperse via a DNA intermediate and are
characterized by short terminal inverted repeats (TIRs). The internal
region encode one or two genes responsible for transposition.
Transposition usually follows a non replicative cut- and- paste
mechanism. Hence, copy numbers are small to intermediate, and
class II transposons therefore comprise only a small part of the
genome.
They often integrate in gene rich regions, which make them useful
tools for gene isolation by transposon tagging.
In plants, class II transposons can be grouped into at least four super
families, three of which (Ac, CACTA,Mu) were first characterized in
maize.

Transposons of Ac family (e.g. Ac in maize, P- elements in Drosophila)
code for a single gene (a transposase). Transposons of the CACTA
family (e.g. En/Spm in maize, Tam1 from Antirrhinum majus) carry
two genes, encoding a transposase and a DNA- binding protein,
respectively.
Mutator- like elements also encode two genes and are characterized
by much longer TIRs than the other two families.
Tc1-Mariner- like elements (MLEs) is particularly widespread in
nature. Tc1 was initially detected in the nematode Caenorhabdites
elegans, Mariner in the fly Drosophila mauritiana, but related
elements were identified in other animals as well as in the nuclear
genome of Human, ciliates, fungi and plants.
Like members of the Ac family, MLEs contain a single gene coding for
a transposase that is flanked by TIRs.

Ac like
CACTA family
Mu like
tnp
tnp D tnp A
tnp D tnp A

Geneticists have identified several families of transposable
elements. Each family consists of a characteristic array of
transposable elements with respect to numbers, types, and
locations.
Each family has two forms of transposable elements: autonomous
elements, which can transpose by themselves, and non-autonomous
elements, which cannot transpose by themselves because they lack
the gene for transposition.
The nonautonomous elements require an autonomous element to
supply the missing functions. Often, the non-autonomous element is
a defective derivative of the autonomous element in the family.
A family consists of a single type of autonomous element
accompanied by many varieties of nonautonomous elements.

Drosophila Transposons
• ~15% of Drosophila genome thought to be mobile.
P elements
• Hybrid dysgenesis, defects arise from crossing of specific
Drosophila strains.
• Occurs when haploid genome of male (P strain) possesses ~40
P elements/genome.
• P elements vary in length from 500-2,900 bp.
• P elements code a repressor present in the cytoplasm, which
makes them stable in the P strain (but unstable when crossed
to the wild type female; female lacks repressor in cytoplasm).
• Used experimentally as transformation vectors.

Alu1 SINEs (short-interspersed sequences)
 ~300 bp long, repeated 300,000-500,000X. Flanked by 7-20 bp
direct repeats.
• Some are transcribed, thought to move by RNA intermediate.
• AluI SINEs detected in neurofibromatosis (OMIM1622200) intron;
results in loss of an exon and non-functional protein.
L-1 LINEs (long-interspersed sequences)
• 6.5 kb element, repeated 50,000-100,000X (~5% of genome).
• Contain ORFs with homology to reverse transcriptases; lacks LTRs.
• Some cases of hemophilia (OMIM-306700) known to result from
newly transposed L1 insertions.
HUMAN RETROTRANSPOSONS

Spontaneous mutations are often the result of transposable element
activity.
Geneticists have exploited the mutagenic potential of transposons to
disrupt genes. Transposon mutagenesis was pioneered in the 1970s
and 1980s using the P elements of Drosophila.
Other types of transposons have been used to induce mutations in
the genomes of nematodes, fish, mice, and various plants.
Mutagenesis with transposons has an advantage over traditional
methods of inducing mutations because a gene that has been
mutated by the insertion of a transposable element is “tagged” with
a known DNA sequence.
The transposon tag can subsequently be used to isolate the gene
from a large, heterogeneous mixture of DNA by using a probe
derived from a cloned version of the transposon.
TRANSPOSONS AS MUTAGENS

GENETIC TRANSFORMATION USING TRANSPOSONS
Rubin and Spradling developed a technique which is routinely used
to transform Drosophila with cloned DNA. An incomplete P element
serves as the transformation vector, and a complete P element
serves as the source of the transposase that is needed to insert the
vector into the chromosomes of an injected embryo.
Gene, called rosy (symbol ry), encodes the enzyme xanthine
dehydrogenase. Flies lacking this enzyme—that is, homozygous ry
mutants—have brown eyes, whereas flies homozygous for the
wildtype allele ry have red eyes.
Rubin and Spradling used recombinant DNA techniques to insert the
ry gene into an incomplete P element that had been cloned in a
bacterial plasmid denoted as P(ry+).
In another plasmid, they cloned a complete P element capable of
encoding the P element’s transposase. Rubin and Spradling then
injected a mixture of the two plasmids into Drosophila embryos that
were homozygous for a mutant ry allele.

When the injected animals matured, Rubin and Spradling mated
them to ry mutant flies. Among the offspring, they found many that
had red eyes.
Subsequent molecular analysis demonstrated that these red-eyed
flies carried the P(ry) element.
In effect, Rubin and Spradling had corrected the mutant eye color
by inserting a copy of the wild-type rosy gene into the fly genome—
that is, they had genetically transformed mutant flies with DNA from
wild-type flies.
Geneticists have identified several transposons that can be used in
their place.
For example
 the piggyBac transposon from a moth can serve as a
transformation vector in many different species, and
the Sleeping Beauty transposon from salmon works well in
vertebrates, including humans, where it is being developed as a
possible agent for gene therapy.

TRANSPOSONS AND GENOME ORGANIZATION
Some genomic regions are especially rich in transposon sequences.
In Drosophila, for example, transposons are concentrated in the
centric heterochromatin and in the heterochromatin abutting the
euchromatin of each chromosome arm.
However, many of these transposons have mutated to the point
where they cannot be mobilized; genetically, they are the equivalent
of “dead.” Heterochromatin therefore seems to be a kind of
graveyard filled with degenerate transposable elements.
Several Drosophila transposons have been implicated in the
formation of chromosome rearrangements, and a few seem to
rearrange chromosomes at high frequencies.
One possible mechanism is crossing over between homologous
transposons located at different positions in a chromosome.
If two transposons in the same orientation pair and cross over, the
segment between them will be deleted

Crossing over can also occur between transposons located in
different chromosomes.
If we consider a case where the crossover involves two sister
chromatids. Each chromatid carries two neighboring transposons
oriented in the same direction. The transposon on the left in one
chromatid has paired with the transposon on the right in the other
chromatid.
A crossover between these paired transposons yields two
structurally altered chromatids, one lacking the segment between
the two transposons, the other with an extra copy of this segment.
Crossing over between neighboring transposons can therefore
duplicate or delete chromosome segments— that is, it can expand
or contract a region of the genome.

TRANSPOSABLE ELEMENTS AND EVOLUTION
TEs are found in most life forms, and the scientific community is still
exploring their evolution and their effect on genome evolution.
It is unclear whether TEs originated in the last universal common
ancestor, arose independently multiple times, or arose once and
then spread to other kingdoms by horizontal gene transfer.
Various viruses and TEs also share features in their genome
structures and biochemical abilities, leading to speculation that they
share a common ancestor.
Because excessive TE activity can damage exons, many organisms
have developed mechanisms to inhibit their activity.
Bacteria may undergo high rates of gene deletion as part of a
mechanism to remove TEs and viruses from their genomes, while
eukaryotic organisms typically use RNA interference to inhibit TE
activity.
Nevertheless, some TEs generate large families often associated
with speciation events. Evolution often deactivates DNA
Transposons.

Large quantities of TEs within genomes may still present
evolutionary advantages, however. Interspersed repeats within
genomes are created by transposition events accumulating over
evolutionary time.
Because interspersed repeats block gene conversion, they protect
novel gene sequences from being overwritten by similar gene
sequences and thereby facilitate the development of new genes.
TEs can contain many types of genes, including those conferring
antibiotic resistance and ability to transpose to conjugative plasmids.

TRANSPOSONS AND DISEASES
TEs are mutagens and their movements are often the causes of
genetic disease. They can damage the genome of their host cell in
different ways:
 a transposon or a retrotransposon that inserts itself into a
functional gene will most likely disable that gene;
 after a DNA transposon leaves a gene, the resulting gap will
probably not be repaired correctly;
 multiple copies of the same sequence, such as Alu sequences, can
hinder precise chromosomal pairing during mitosis and meiosis,
resulting in unequal crossovers, one of the main reasons for
chromosome duplication.
Diseases often caused by TEs include haemophilia A and B, severe
combined immunodeficiency, porphyria, predisposition to cancer,
and Duchenne muscular dystrophy.

BARBARA McCLINTOCK IN 1947
-THANK YOU-

Mobile DNA Element-Transposable !

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