1. Transposons
Image courtesy of Lauren Solomon, Broad Communications The reddish streaks on these corn grains are caused by transposons.
Submitted to: Zaahir Salam
Dr. S.Kannan

2. Transposable Elements (Transposons)
“JUMPING GENES”
 Discovered by Barbara McClintock, largely from cytogenetic
studies in maize, but since found in most organisms.
1. Nobelprize.org
(1983 Nobel Prize in Physiology and
Medicine)
2. profiles.nlm.nih.gov/LL/
Corn (maize) varieties Barbara McClintock 1902-1992

3. We now know that only 1.1% to 1.4% of our DNA actually encodes proteins.
Snapshot of the human genome. The chart shows the proportions of our genome made up of various types of sequences.
More than 50% of our genome consists of short, repeated sequences, the vast
majority of which—about 45% of our genome in all—come from transposons.

4. What is a Transposon
• Segments of DNA that move from one genomic location to other.
• The simplest transposable elements are Insertion Sequences(IS).
• IS is a short sequence of DNA carrying only the genes needed for
transposition and bounded at both ends by sequences of nucleotides in
reverse orientation called Inverted repeats.

5. Between the IR’s there is the gene for transposase, which recognizes the end of
IS because the Enzyme is not specific for a particular sequence of genome
transposable elements appear to move to random destinations
Some also contain Antibiotic Genes or Toxin Genes. These are called Composite
Transposon.

6.  When a transposon inserts at a target site, the target sequence is
duplicated so that short, direct-sequence repeats flank the transposon’s
terminal inverted repeats.

7. Transposable Genetic Elements Move from
One Location to Another
• It is the third general type of recombination system: recombination
that allows the movement of transposable elements, or
transposons.
• These segments of DNA, found in virtually all cells, move, or“jump,”
from one place on a chromosome (the donor site) to another on
the same or a different chromosome (the target site).
• DNA sequence homology is not usually required for this movement,
called transposition; the new location is determined more or less
randomly.
• Transposons are perhaps the simplest of molecular parasites,
adapted to replicate passively within the chromosomes of host
cells.

8. • There are two distinct types:
– Class II transposons. These consist of DNA that moves
directly from place to place.
– Class I transposons. These are retrotransposons that first
transcribe the DNA into RNA and then use reverse
transcriptase to make a DNA copy of the RNA to insert in a
new location.

9. Bacterial Transposition
• Most have short repeated
sequences at each end that serve
as binding sites for the
transposase.
• When transposition occurs, a short
sequence at the target site (5 to 10
bp) is duplicated to form an
additional short repeated
sequence that flanks each end of
the inserted transposon.
• These duplicated segments result
from the cutting mechanism used
to insert a transposon into the
DNA at a new location.
Duplication of the DNA sequence at a target site when a transposon is inserted. The sequences that are duplicated following
transposon insertion are shown in red. These sequences are generally only a few base pairs long, so their size relative to that of a
typical transposon is greatly exaggerated in this drawing.

10. • There are two general pathways for
transposition in bacteria.
– Direct transposition
– 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 the 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 replicative 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 transposon are
present in the cointegrate, both having the
same relative orientation in the DNA.

11. Eukaryotic Transposition
• The difference is however, the mechanism of transposition seems to
involve an RNA intermediate.
• Eukaryotic DNA transposons from sources as diverse as yeast and fruit
flies have a structure very similar to that of retroviruses; these are
sometimes called retrotransposons. Retrotransposons encode an enzyme
homologous to the retroviral reverse transcriptase, and their coding
regions are flanked by LTR sequences.
• They transpose from one position to another in the cellular genome by
means of an RNA intermediate, using reverse transcriptase to make a DNA
copy of the RNA, followed by integration of the DNA at a new site.
Eukaryotic transposons. The Ty element of the yeast Saccharomyces and the copia element of the fruit fly Drosophila serve
as examples of eukaryotic transposons, which often have a structure similar to retroviruses but lack the env gene. The Delta
sequences of the Ty element are functionally equivalent to retroviral LTRs. In the copia element, INT and RT are homologous
to the integrase and reverse transcriptase segments, respectively, of the pol gene.

12. So Are Transposons Good or Bad?
• In the process of inserting into the genome, transposons can
– interrupt the normal coding of DNA,
– creating gene mutations with a variety of effects.
– They may turn nearby genes off, preventing their ability to create protein, or they may
turn them on, increasing the amount of protein made.
• There is evidence that transposons aren’t just “selfish genes” intent on
replicating themselves or genomic “junk” that provides no benefit to the
host. They may play a creative role in building new functional parts of the
genome .
• Recent research has shown that transposons may help plants respond and
adapt to environmental stress by regulating other genes.
• In bacteria, transposons often carry genes that impart resistance to
antibiotic substances, helping the bacteria survive.

13. Thank You