Barbara McClintock discovered transposons in the 1940s while studying maize. Transposons are segments of DNA that can move within genomes. There are two classes - Class 1 retrotransposons copy and paste RNA into DNA, while Class 2 DNA transposons cut and paste DNA segments. Transposons make up a large portion of many genomes and can cause mutations, but may also create genetic diversity that benefits hosts over time. McClintock received the Nobel Prize for discovering transposons and proposing they play a regulatory role in gene expression.

1. PRATHYUSHA
Msc.BIOTECHNOLOGY
SCHOOL OF BIOSCIENCES
MAHATMA GANDHI UNIVERSITY

2. ‘’Jumping Genes’’
 Special segments of DNA that can move around to
different positions in the genome of a single cell.
 Found in almost all organisms.
 Mainly known as “transposable elements”.
 Can move from one site to another in the same or
different DNA by the process called transposition.
 Any gene into which a transposable element inserts
itself can no longer function.
 First recognised as mutagens.

3. HISTORY!!!!!
These elements were first
identified more than 50 years
ago by geneticist Barbara
McClintock of Cold Spring
Harbour Laboratory in New
York. McClintock, however, was
among the first researchers to
suggest that these mysterious
mobile elements of the genome
might play some kind of
regulatory role, determining
which genes are turned on and
when this activation takes place.
In 1983 the Nobel Prize in
Physiology or Medicine was
awarded.

4. MAIZE
Barbara McClintock
studied about
transposons in maize.
They are also found in
almost all
organisms(prokaryotes &
eukaryotes) and typical
in large numbers. For
example , TE’s make up~
50% of human genome &
70% of maize genome.

5. Class 1 transposons or Retrotransposons
Class 2 transposons or DNA transposons

6. CLASS 2- DNA TRANSPOSONS
 “Cut and Paste” mechanism.
 Cut out of its location & inserted to new location.
 Requires enzyme transposase.
 Transposase are encoded within some of these
transposons.
 Transposase binds to:
 Both ends of transposons which consist of inverted
repeats (identical sequences reading in opposite
directions).
 A sequence of DNA that makes up the target site. Some
transposons require target site : others can insert
anywhere.

7.  The DNA is cut in an offset manner(like “sticky ends” produced by
some restriction enzymes).
 The transposon is ligated into host DNA.
 The gaps are filled by “Watson & Crick” base pairing.
 This creates identical direct repeats at each ends of the transposon.
 Often transposon lose their gene for enzyme. But somewhere in the
cell there is a transposon that can synthesise the enzyme , their
inverted repeats are recognized and they , too , can be moved to a
new location.

9. MAIZE Drosophila
TRANSPOSONS IN MAIZE & Drosophila
 1st transposon discovered by
Barbara McClintock.
 Worked with maize(Zea mays).
 Responsible for variety of gene
mutations, especially :
 Insertions & deletions
 Translocations.
 In developing somatic tissues
like corn kernels ,a mutation(ex
; c) that alters colours will be
passed onto all the descendant
cells.
 Awarded Nobel Prize in 1983.
 P elements.
 Do little harm : gene expression is
usually repressed.
 When male flies containing P
elements mate with female flies
lacking them, the transposase
become active in germ line
producing so many mutations
that their off springs are sterile.
 Transgenic flies with any desired
gene can be produced by
injecting the early embryo with
an engineered P element
containing that gene.

11. TRANSPOSONS IN BACTERIA
 Requires additional enzyme “resolvase”.
 Carry genes for one or more imparting resistance
to antibiotics.
 When such a transposon is incorporated in
a plasmid, it can leave the host cell and move to
another. This is the way that the alarming
phenomenon of multidrug antibiotic
resistance spreads so rapidly.

12. INSERTION SEQUENCE
 Segments of bacterial DNA.
 When IS elements appear in the middle of genes, they
interrupt the coding sequence and inactivate the expression
of that gene.
 Owing to their size and in some cases the presence of
transcription and translation termination signals, IS
elements can also block the expression of other genes in the
same operon.
 IS elements were first found in E. coli in the gal operon—a
set of three genes taking part in the metabolism of the sugar
galactose.

13. These are relatively short, not exceeding
2000 bp (2kb).
1st IS in E.coli : IS 1 (800 bp long).
Types :
 IS 2
 IS 3
 IS 4
 IS 5

14. TRANSPOSON (tn) ELEMENTS
 Like IS units, Tn elements are mobile in both bacterial
and viral chromosomes and in plasmids.
 Provide a mechanism for the movement of genetic
information from place to place both within and
between organisms.
 Susumu Mitusuhashi first suggested that the genes
responsible for resistance to several antibiotics were
mobile and could move between bacterial plasmids and
chromosomes.

15. BACTERIOPHAGES
 Behave in a similar fashion.
 Ability to insert their
genetic material into the
host organism.
 Bacteriophage Mu can
insert its DNA at various
places in the E.coli
organisms.
 Like IS units, if insertion
occurs within a gene,
mutant behaviour results at
that locus.
 Here it move by “copy &
paste” mechanism.

16. CLASS 1- RETROTRANSPOSONS
 “Copy & Paste” mechanism.
 Copy is made of RNA not DNA.
 RNA DNA.
 Long terminal repeats present(in many having 1000
bp’s).
 Generate direct repeats at their new sites of
insertion.
 Presence of these direct repeats indicate
occurrence of retro transposition.
 Some 50% of entire human genome contain RT.
REVERSE TRANSCRIPTASE

17. LINE......
 Human genome contains over 1 million lines.
 Most abundant of these belong to family : Line1.
 L1 elements are DNA sequences ranging from few 100 to
9000 bp’s.
 Only 50 L1 elements are functional ; that is they can be
transcribed and translated.
 Functional elements are 6500 bp long.
 Encode 3 proteins , including
 Endonuclease that cut DNA & a
 Reverse transcriptase that makes DNA copy of RNA
transcript.

18. L1 ACTIVITY
L1 DNA
L1 DNA
RNA PROTEINS
L1 element
R
N
A
P
O
L
2
TRANSLATION
Endonuclease cuts a strand
of “target DNA” , often in
the intron of a gene.
RNA &
PROTEINS RE-
ENTER THE
NUCLEUS
RT

19.  Occasionally , Li activity makes and inserts a copy of
cellular mRNA (thus a natural cDNA). Lacking introns
as well as necessary control elements like promoters ,
these genes are not expressed. They represent one
category of pseudo gene.
 Through this copy mechanism , the no: of Lines can
increase in the genome.
 The diversity of Lines between individual human
genomes makes them useful markers for DNA
“fingerprinting” .

20. HIV-1
 Cause of AIDS & other human retroviruses(e.g.,HTLV-
1,the human T-cell leukaemia or lymphoma virus)
 The RNA genome contains a gene for
 Reverse transcriptase and one for
 Integrase. Integrase ~ transposase. DNA copies can
be inserted anywhere in the genome.
 Molecules of both the enzymes are incorporated in the
virus particle.

22. TRANSPOSONS & MUTATIONS
 Transposons are mutagens.
 Can cause mutation in several ways..
1. Insertions into introns , exons , & even into the DNA
flanking the genes can destroy or alter genes activity.
2. Faulty repair at the old site(cut & paste transposition).
3. Commonest cause of duplications.
 Some cases of human genetic diseases include:
 Haemophilia A(f8) & Haemophilia B(f9).
 X-linked SCID.
 Porphyria.
 Predisposition to colon polyps & cancer.
 Duchenne muscular dystrophy.

23. PRACTICAL APPLICATIONS
As a result of the capacity of transposon mutagenesis to incorporate
genes into most areas of target chromosomes, there are a number of
functions associated with the process.
 Virulence genes in viruses and bacteria can be discovered by
disrupting genes and observing for a change in phenotype. This
has importance in antibiotic production and disease control.
 Non-essential genes can be discovered by inducing transposon
mutagenesis in an organism. The transformed genes can then be
identified by performing PCR on the organism's recovered
genome.
 Cancer-causing genes can be identified by genome-wide
mutagenesis and screening of mutants containing tumours.
Based on the mechanism and results of the mutation, cancer-
causing genes can be identified as oncogenes or tumour-
suppressor genes.

24. What good are transposons?
 Transposons have been called "junk" DNA and "selfish"
DNA.
 "selfish" because their only function seems to make
more copies of themselves
&
 "junk" because there is no obvious benefit to their host.
 Retrotransposons often carry some additional sequences
at their 3' end as they insert into a new location.
 Create new combinations of exons, promoters, and
enhancers that benefit the host.

25.  The longer the L1 element, the lower the level of gene
expression.
 L1 elements inserted into the introns of functional
genes reduce the transcription of those genes without
harming the gene product.
 Some 79% of our genes contain L1 elements, and
perhaps they are a mechanism for establishing the
baseline level of gene activity.
 Telomerase, the enzyme essential for maintaining
chromosome length, is closely related to the reverse
transcriptase of LINEs and may have evolved from it.

26.  RAG-1 and RAG-2.
 The proteins encoded by these genes are needed to
assemble the repertoire of antibodies and T-cells
receptors (TCRs) used by the adaptive immune system .
 The mechanism resembles that of the cut and paste
method of Class II transposons , and the RAG genes
may have evolved from them. If so, the event occurred
some 450 million years ago when the jawed vertebrates
evolved from jawless ancestors. Only jawed vertebrates
have the RAG-1 and RAG-2 genes.
 In Drosophila, the insertion of transposons into genes has
been linked to the development of resistance
to DDT and organophosphate insecticides.