Transposons are segments of DNA that can move from one location in the genome to another. There are two main types: insertion sequences (IS), which are short DNA sequences that code for transposase enzymes; and transposons, which can carry additional genes and are flanked by IS elements or inverted repeats. Transposition occurs when the transposase enzyme cuts and inserts the transposon into a new target site, sometimes duplicating flanking DNA. This movement allows transposons to generate genetic diversity and influence genome evolution over generations.

1. Transposons
Dr. S. Sivasankara Narayani
Assistant professor
Department of Microbiology
Ayya Nadar Janaki Ammal College
sivakasi
06-02-2021
Dr.SS ., MRSB (UK)

2. UNIT - IV
Regulation of gene expression in bacterial system : Operon concept
– lac, trp, ara operon in E. coli - *DNA methylation and
heterochromatization. Transposons (brief account only).
06-02-2021
Dr.SS ., MRSB (UK)

3. Transposons
• Jumping Gene
06-02-2021
Dr.SS ., MRSB (UK)

4. INTRODUCTION
• Segment of DNA that move from one genomic location to another.
• The simplest transposable elements are
• insertion sequences(IS), first TE discovered in bacteria.
• Comprises about 45% in humans
06-02-2021
Dr.SS ., MRSB (UK)

5. Transposable element
• 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.
06-02-2021
Dr.SS ., MRSB (UK)

6. GENERAL CHARACTERISTICS OF TE
• They were found to be DNA sequences that code for 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.
• One copy remains at the parent site and another appears at the
target site.
• A transposable element is nota replicon.
• Thus, it can not replicate apart from the host chromosome.
06-02-2021
Dr.SS ., MRSB (UK)

7. Types of transposable elements
• Different type of transposable
elements are present in both
prokaryotes and eukaryotes.
• 1. Insertion sequences
• 2. Transposons
• 3. Bacteriophage mu
06-02-2021
Dr.SS ., MRSB (UK)

8. 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.
06-02-2021
Dr.SS ., MRSB (UK)

9. 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
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Dr.SS ., MRSB (UK)

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Dr.SS ., MRSB (UK)

11. COMPOSITE TRANSPOSONS
• 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.
06-02-2021
Dr.SS ., MRSB (UK)

12. • 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.
06-02-2021
Dr.SS ., MRSB (UK)

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Dr.SS ., MRSB (UK)

14. • 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.
06-02-2021
Dr.SS ., MRSB (UK)

15. • 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).
06-02-2021
Dr.SS ., MRSB (UK)

16. • 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.
06-02-2021
Dr.SS ., MRSB (UK)

17. 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.
06-02-2021
Dr.SS ., MRSB (UK)

18. • 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.
06-02-2021
Dr.SS ., MRSB (UK)

19. • 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.
06-02-2021
Dr.SS ., MRSB (UK)

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Dr.SS ., MRSB (UK)

21. 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
06-02-2021
Dr.SS ., MRSB (UK)

22. 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 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.
06-02-2021
Dr.SS ., MRSB (UK)

23. • 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
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Dr.SS ., MRSB (UK)

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Dr.SS ., MRSB (UK)

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Dr.SS ., MRSB (UK)

26. 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.
06-02-2021
Dr.SS ., MRSB (UK)

27. • 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.
06-02-2021
Dr.SS ., MRSB (UK)

28. • 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.
06-02-2021
Dr.SS ., MRSB (UK)

29. 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.
06-02-2021
Dr.SS ., MRSB (UK)

30. • 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.
06-02-2021
Dr.SS ., MRSB (UK)

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Dr.SS ., MRSB (UK)
Mechanism of retrotransposition

32. USES OF TRANSPOSONS
• As cloning vehicle.
• Transformation vectors for transferring genes between organisms.
• Also drug resistance genes encoded by many transposons are
useful in the development of plasmids as cloning vehicle.
• Transposon mutagenesis.
• 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).
• Used in genetic studies
06-02-2021
Dr.SS ., MRSB (UK)

33. sivasan91@gmail.com
06-02-2021
Dr.SS ., MRSB (UK)

34. QUESTIONS TO THINK
• Transposons
• Composite transposons
• Non composite transposons
• Transposition mechanism
• Retrotransposons
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Dr.SS ., MRSB (UK)

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Dr.SS ., MRSB (UK)