Transposable Elements or Transposition (both in prokaryotes and eukaryotes).

1. SYED MUHAMMAD KHAN (BS HONS. ZOOLOGY)
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Transposable Elements/Transposition
Transposable elements, or transposons are sequences of DNA that move from one
location in the genome to another (this phenomenon is called transposition).
Transposition is when a transposon / transposable element changes its position in
the genome.
Barbara McClintock discovered transposons in the 1940s in her studies on the
genetics of maize. Transposons are also called jumping genes (although this term is
only applicable for non-replicative transposons). Transposons are present in both
prokaryotes and eukaryotes.
Some transposons carry genes that are valuable to their hosts, the most familiar
being genes for antibiotic resistance. Not only is this a clear benefit to the bacterial
host, but it is also valuable to molecular biologists because it makes the transposon
much easier to track.
Mechanisms of Transposition:
Transposition occurs in two ways:
Non-Replicative Transposition: In this case, DNA leaves one place and jumps to the
other, it is called non-replicative transposition (or “cut and paste”) because both
strands of the original DNA move together from one place to the other without
replicating.
Figure: Non-replicative / Cut and Paste Transposition.

2. SYED MUHAMMAD KHAN (BS HONS. ZOOLOGY)
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Replicative Transposition: In this case, DNA replicates, i.e. one copy of the
transposon remains at its original site as another copy inserts at the new site. This is
called replicative transposition (or “copy and paste”) because a transposon moving
by this route also replicates itself.
Figure: Replicative Transposition
Bacterial / Prokaryotic Transposons
Some major bacterial transposons are listed as follows:
1. Insertion Sequences:
Insertion sequences (IS) / Insertion elements are a type of bacterial transposons /
transposable elements, i.e. they are short DNA sequences that can move from one
location in the genome to another. They are the simplest form of bacterial
transposons.
They are shorter than other transposons and contain genes that only code for
proteins that allow transposition, i.e. no other genes are present. They consist of:
 Short inverted repeats at both ends of the insertion sequence
 Genes coding for an enzyme called transposase that carries out transposition
Transposition involves duplication of a short sequence in the target DNA; one copy
of this short sequence is present on each side of the insertion sequence after
transposition.

3. SYED MUHAMMAD KHAN (BS HONS. ZOOLOGY)
pg. 3
2. Tn3 Transposon:
Tn3 transposon uses a replicative transposition mechanism. Tn3 contains genes for
antibiotic resistance and two genes that are instrumental in transposition. Tn3
transposes by a two-step process, each step of which requires one of the Tn3 gene
products.
Fusion: In the first step, the two plasmids (donor plasmid – having the Tn3
transposon, and the target plasmid) fuse. Tn3 replicates and the two plasmids are
coupled through a pair of Tn3 copies. This step is catalyzed by the transposase
enzyme (product of the Tn3 transposase gene tnpA).
Resolution: The second step in Tn3 transposition is a resolution of the fused
plasmids which break down into two independent plasmids, each bearing one copy
of Tn3. This step is catalyzed by the resolvase enzyme (product of the resolvase
gene tnpR).
Figure: Replicative Transposition in Tn3 Transposon.
3. Tn10 Transposon:
Tn10 Transposon uses a non-replicative / cut and paste the transposition
mechanism. The steps are as follows:
 The strands of donor and target DNAs are nicked and joined.

4. SYED MUHAMMAD KHAN (BS HONS. ZOOLOGY)
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 Then new nicks appear in the donor DNA on either side of the transposon.
 The donor DNA is released but the transposon is left behind, i.e. the transposon
is still bound to the target DNA.
 The remaining nicks in the target DNA are sealed, yielding a recombinant DNA
with the transposon integrated into the target DNA.
 The donor DNA has a double-stranded gap which may be repaired.
Figure: Non-Replicative Transposition in Tn10 Transposon.
Eukaryotic Transposons
The first transposons were discovered in maize (which is a eukaryote). Some
eukaryotic transposons are as follows:
1. Ac & Ds Transposons in Maize:
Ac (activator) and Ds (dissociation) are two transposable elements (jumping genes –
that change their location in DNA) found in maize. They affect the expression of the

5. SYED MUHAMMAD KHAN (BS HONS. ZOOLOGY)
pg. 5
color gene (C gene), hence altering the color of maize kernels. They were the first
transposable elements to be discovered (discovered by Barbara McClintock). The
maize kernel in its wild-type is purple, if it is mutated it becomes colorless. The action
of Ds and Ac transposons cause purple spots on the kernels (they mutate it and then
cause it to revert to wild-type).
Ds (dissociation) element transposes into the C gene (color gene) and mutates it
(making the kernel colorless), and then transposes out again, causing it to revert
(convert back) to wild-type (purple colored kernel). Ds cannot transpose on its own (it
is not autonomous), i.e. it needs the help of the Ac (activator) element.
Ac (activator element) is an autonomous transposon / transposable element (does
not require the assistance of other transposons), it helps Ds to transpose out of the
C gene. Ac transposon makes the transposase enzyme that is needed by Ds
transposon to transpose out of the C gene.
Figure: Ac and Ds transposition in maize: (a) A wild-type maize kernel has an active
C gene that causes synthesis of purple pigment (wild-type). (b) A Ds element inserts
into C, inactivating it and preventing pigment synthesis. The kernel is therefore
colorless (mutated). Ds cannot transpose out of the C gene by itself, i.e. it is stuck.
(c) Both Ac and Ds are present. This allows Ds to transpose out of C in many cells,
giving rise to groups of cells that make pigment. Such groups of pigmented cells
cause the formation of purple spots on the kernel (reverting to wild-type).

6. SYED MUHAMMAD KHAN (BS HONS. ZOOLOGY)
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2. Retrotransposons:
Retrotransposons are retroviruses like transposons, i.e. they replicate via an RNA
intermediate. Retroviruses cause tumors in vertebrates, and some of them (the
human immunodeficiency viruses, or HIVs) cause AIDS.
Retroviruses replicate through an RNA intermediate. When a retrovirus infects a cell,
it makes a DNA copy of itself, using a virus-encoded reverse transcriptase to carry
out the RNA→DNA reaction, and an RNase H to degrade the RNA parts of RNA–
DNA hybrids created during the replication process. A host tRNA serves as the
primer for the reverse transcriptase. The finished double-stranded DNA copy of the
viral RNA is then inserted into the host genome, where it can be transcribed by host
polymerase II.
Retrotransposons fall into two groups with different modes of replication:
 LTR containing Retrotransposons: These replicate in a manner very similar to
retroviruses, except that they do not pass from cell to cell in virus particles. They
contain LTRs (long terminal repeats).
 Non-LTR retrotransposons: These include the retrotransposons that lack LTRs.
Several eukaryotic transposons, i.e. in yeast and Drosophila, transpose by a
mechanism similar to that of retrovirus replication. They start with DNA in the host
genome, make an RNA copy, then reverse transcribe it to DNA that can insert in a
new location.