The process of movement and integration of a piece of DNA into different sites in the chromosomes called transposition. DNA segments that carry the genes required for transposition are transposable elements or transposons, or jumping genes. It is present in procaryotes, viruses, and eucaryotic chromosomes. It generates new gene combinations. It does not require extensive areas of homology between the transposon and its destination/target site.

2. Transposons
• The process of movement and integration of a piece of DNA into
different sites in the chromosomes called transposition.
• DNA segments that carry the genes required for transposition are
transposable elements or transposons, or jumping genes.
• It is present in procaryotes, viruses, and eucaryotic chromosomes.
• It generates new gene combinations.
• It does not require extensive areas of homology between the
transposon and its destination/target site.

3. Discovery
• Transposons were first discovered in the
1940s by Barbara McClintock during
her studies on maize genetics.
• She was awarded the Nobel prize in
1983.
• They have been most intensely studied
in Bacteria.

4. Simplest transposable elements - Insertion
sequences (IS)
• An IS element is a short sequence of DNA.
• Around 750 to 1,600 base pairs [bp] in length.
• It contains only the genes that code for an enzyme
required for transposition.

5. Structure of IS
• Both ends of the gene possess identical or very similar sequences of nucleotides in
reversed orientation known as inverted repeats.
• Inverted repeats are usually about 15 to 25 base pairs long and vary among IS elements
so that each type of IS has its own characteristic inverted repeats.
• Between the inverted repeats is a gene that codes for an enzyme called transposase. This
enzyme is required for transposition and accurately recognizes the ends of the IS.
• Direct repeats (DRs) in host DNA, flank a transposable element.
• DNA, flank a transposable element.

6. Naming
• Each type of element is named by giving it the prefix IS followed by
a number.
• In E. coli several copies of different IS elements have been
observed.

7. Composite transposons
• Transposable elements also can contain genes other than those required for transposition.
• Example, antibiotic resistance or toxin genes.
• These elements often are called composite transposons.
• It consist of a central region containing the extra genes, flanked on both sides by IS elements that are
identical or very similar in sequence.
• Many composite transposons are simpler in organization.
• They are bounded by short inverted repeats, and the coding region contains both transposition genes and
the extra genes.

8. Naming
• It is believed that composite transposons are formed when two IS elements
associate with a central segment containing one or more genes. This
association could arise if an IS element replicates and moves only a gene or two
down the chromosome.
• Composite transposon names begin with the prefix Tn.

9. Replicative transposons

10. Transposition Process
• In procaryotes, transposition can occur by two basic
mechanisms.
Simple transposition
/ cut-and paste
transposition.
Replicative
transposition.

11. a) Simple transposition
• It involves transposase-catalyzed excision of the transposon.
• It cleaves the new target site and ligate the transposon into
this site.
• Target sites are specific sequences – 5-9 bp long.
• A short direct-sequence repeats flanked at ends of the
inverted repeats.

12. Mechanism of Simple Transposition

13. b) Replicative transposition
• The original transposon remains at the parental site on the chromosome and a
replicate is inserted at the target DNA site.
• Eg: Tn3 transposon
• Replicative transposition occurs at two stages,
• First stage : Tn3 transposase enzyme (tnpA gene), it fuse the DNA
containing Tn3 with the target DNA to form a cointegrate molecule. cointegrate
has two copies of the Tn3 transposon.
• Second stage: the cointegrate is resolved by resolvase enzyme (tnpR gene)
to yield two DNA molecules, each with a copy of the transposon. Resolution
involves a crossover.

14. Mechanism of Replicative Transposition

15. Role of transposons
• They cause a mutation or stimulate DNA rearrangement, leading to deletions of
genetic material.
• They carry stop codons or termination sequences, thereby block translation or
transcription, respectively.
• They carry promoters and can activate genes near the point of insertion. So
they can turn genes on or off.
• It is also located in plasmids and participate in such processes as plasmid
fusion, insertion of plasmids into chromosomes, and plasmid evolution.
• Eg: antibiotic resistance