2. CONTENT
Introduction
General characteristics of TE
Types of Transposable Elements
Mechanism of Transposition
Transposition
Retrotransposons
Mechanism of Retrotransposition
Uses of Transposons
3. INTRODUCTION
Segment of DNA that movefrom one genomic location to another.
Thesimplest transposable elements areinsertion sequences(IS), first TE
discovered in bacteria.
Comprises about 45% in humans.
4. A transposable element (TE or transposon) is a DNA
sequence that can change its position within a genome,
sometimes creating or reversing mutations and altering
the cell's Genome size. Transposition often results in
duplication of the TE. Barbara McClintock's discovery of
these jumping genes earned her a Nobel Prize in
1983.
Discovered largely from
cytogenetic studies in maize,
but since found in most
organisms.
BARBARA
McCLINTOCK’S
5. GENERAL CHARACTERISTICS OF TE
Theywere found to beDNA sequences that codefor enzymes, which bring
about the insertion of an identical copy of themselves into a new DNA site.
Transposition events involve both recombination and replication processes
which frequently generate two daughter copies of the transposable
elements. Onecopy remains at the parent site and another appears at the
target site.
A transposable elementis nota replicon. Thus, it can not replicate apart from
the host chromosome.
6. Types of transposable elements:-
Different type of transposable elements are
present in both prokaryotes and eukaryotes.
1. Insertion sequences
2. Transposons
3. Bacteriophage mu
7. INSERTION SEQUENCE
IS were first transposable elements identified as spontaneous
in some bacterial operon.
The IS are shorter (800 to 1500 base pairs) and do not code
for proteins.
In fact, IS carry the genetic information necessary for their
transposition (the gene for the enzyme transposase).
There are different IS such as IS1, IS2, IS3 and IS4 and so
on in E.coli.
8. Transposons
• Transposons are similar to IS elements but carry additional gene.
• Transposons are several thousand base pair long and have genes coding for
one or more protein.
• On either side of transposon is a short direct repeat. The sequence into which
the transposable element insert is called target sequence.
• Two types of transposons :-
1. COMPOSITE TRANSPOSONS
2. NON-COMPOSITE TRANSPOSONS
9. COMPOSITE TRANSPOSON
A composite transposon is similar in function to simple transposons and
insertional sequence(IS) in that it has protein coding sequence flanked by
inverted, repeated sequences that can be recognized by transposase
enzyme. A composite transposon, however e.g. carry gene for antibiotic
resistance and is flanked by two separate IS elements which may or may
not be exact replicas. Instead of each IS element moving separately, the
entire length of DNA spanning from one IS element to other is transposed as
complete unit. Composite transposon will often carry one or more genes
conferring antibiotic resistance.
10. • Composite transposons may be thousands of base pairs long. The IS
elements are both of the same types and are called IS-L (for “left”) and
IS-R (for “right”). Depending upon the transposon, IS-L and IS-R may be
in the same or inverted orientation relative to each other. Because the
ISs themselves have terminal inverted repeats, the composite
transposons also have terminal inverted repeats.
• Figure shows the structure of the composite transposon Tn 10 to
illustrate the general features of such transposons. The Tn 10
transposon is 9,300 bp long and consists of 6,500 bp of central,
nonrepeating DNA containing the tetracycline resistance gene flanked at
each end with a 1,400-bp IS element. These IS elements are designated
IS10L and IS10R and are arranged in an inverted orientation. Cells
containing Tn 10 are resistant to tetracycline resistance gene contained
within the central DNA sequence.
• Transposition of composite transposon occurs because of the function of
the IS elements they contain. One or both IS element supplies the
transposase. The inverted repeats of the IS elements at the two ends of
the transposon are recognized by transposase to initiate transposition
(as with transposition of IS elements).
11. Transposition of Tn 10 is rare, occurring once in 10 cell generations. This is
the case because less than one transposase molecule per cell generation
is made by Tn 10. Like IS elements, composite transposons produce target
site duplications after transposition.
12. NON-COMPOSITE TRANSPOSON
• They like composite transposons, contain genes such as those for drug resistance.
Unlike composite transposons, they do not terminate with IS elements. However,
they do have the repeated sequences at their ends that are required for
transposition. Tn3 is a non-composite transposon.
• Tn3 has 38 bp inverted terminal repeats and contains three genes in its central
region. One of those genes, bla, encodes β-lactamase which breaks down
ampicillin and therefore makes cells containing Tn3 resistant to ampicillin. The other
two genes, tnpA and tnpB, encode the enzymes transposase and resolvase that are
needed for transposition of Tn3 (Fig. 12.4). Transposase catalyzes insertion of the
Tn into new sites, and resolvase is an enzyme involved in the particular re-
combinational events associated with transposition.
• Resolvase is not found in all transposons. The genes for transposition are in the
central region for non-composite transposons, while they are in the terminal IS
elements for composite transposons. Non composite transposons also cause target
site duplications when they move.
13. BACTERIOPHAGE mu
The longest transposon known so far.
Carries numerous gene for viral head and tail formation
The vegetative replication of mu produces about 100
viral chromosomes in a cell arises from the transposition
of mu to about 100 different target sites.
Therefore considered as giant mutator transposon.
14. MECHANISM OF TRANSPOSITION
• Movement of transposon occurs only when enzyme tranposase
recognizes and cleaves at either 5’ or 3’ of both ends of
transposon and catalysis at either at 5’ or 3’of both both the ends
of transposons and catalysis staggered cut at the target site.
• Depending on transposon, a duplication of 3 to 12 base of target
DNA occurs at the site where insertion is to be done. One copy
remains at each end of the tranposon sequence.
• After attachment of both ends of transposon to the target site, two
replication forks are immediately formed. At this stage there starts
two paths for carrying out onwards.
1. DIRECT or NON-REPLICATIVE
2. REPLICATIVE
15. TRANSPOSITION
Mechanism of movement of TE from one location
to another.
In this process staggered cuts are made in the
target DNA.
The TE is joined to single stranded ends of the
target DNA.
Finally DNA is replicated in the single stranded
gap.
16. There are two general pathway for
transposition in bacteria :-
1. Direct transposition
2. Replicative
transposition
In direct (or simple) transposition
cuts on each side of the
transposon excise it, and the
transposon moves to a new
location. This leaves a double
strand break in the donor DNA
that must be repaired.
At target site, a staggered cut is
made the transposon is inserted
into the break, and DNA
replication fills in the gaps to
duplicate the target site sequence.
In replicated, transposition the
entire transposon is replicated,
leaving a copy behind at the donor
location.
A cointegrate is an intermediate
in this process, consisting of the
donor region covalently linked to
DNA at the target site. Two
complete copies of the
transposons are present in the
17. Retrotransposons
Retrotransposons (also called transposons via RNA
intermediates) are genetic elements that
can amplify themselves in a genome and are ubiquitous
components of the DNA of many eukaryotic organisms.
These DNA sequences use a "copy-and-paste" mechanism,
whereby they are first transcribed into RNA, then converted
back into identical DNA sequences using reverse
transcription, and these sequences are then inserted into the
genome at target sites.
Retrotransposons are particularly abundant in plants, where
they are often a principal component of nuclear DNA.
In maize, 49–78% of the genome is made up of
retrotransposons.
In wheat, about 90% of the genome consists of repeated
sequences and 68% of transposable elements.
Around 42% of the human genome is made up of
retrotransposons.
20. USES OF TRANSPOSONS
1) As cloning vehicle.
2) Transformation vectors for transferring genes between organisms.
3) Also drug resistancegenes encoded by many transposons are
useful in the development of plasmids as cloning vehicle.
4) Transposon mutagenesis.
5) Use of transposon is to increase rate of mutation due to insertional
inactivation(e.g. production of different colour of grapes, corn and
other fruits).
6) Used in genetic studies
