reeta yadav. roll no. 02. transposable element in eukaryotes
TRANSCRIPT
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Transposable
elements in
eukaryotes
Submitted by:-
Class:- M.sc (P) Botany
Roll no. 02
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Introduction
First experiment
Discovery of Ac Ds elements
P elements in Drosophila
Retrotransposons
Viral retrotransposons
Non-viral retrotransposons
Significance of Transposable elements
Conclusion
References
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INTRODUCTION
Transposable elements are also known as
“TRANSPOSONS” , “JUMPING GENES” ,
“MOBILE GENETIC ELEMENTS”.
Term was given by “Hedges and Jacob(1947).
These are DNA sequences able to transport
themselves to other location within the genome.
They can insert new copies of themselves
throughout the genome.
Each transposable element carries transposase gene
that encodes for enzyme activity required for its own
transposition.
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The 1st transposable element was discover- ed by
“ in Maize in late 1940s.
Her discovery of jumping genes, through an analysis of
genetic instability in Maize, earn her noble prize in 1983.
The instability involves chromosome breakage and was
found to occur at sites where transposable elements were
located i.e. at C locus of 9th chromosome.
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Discovery of Ac Ds elements Genetic marker that McClintock followed is C’ i.e. a dominant inhibitor of
aleurone coloration.
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Autonomous:- have transposase gene and can
transposase itself. e.g. Ac elements in maize
Non-autonomous:- lack transposase gene & can
transposase only in presence of an autonomous
element. e.g. Ds elements in maize
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Structure of Ac Ds elements
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flanked by 8 np target site duplications.
These may be complete or incomplete.
It possess a gene that codes for transposase protein which attaches to element & enables it to move to another position in genome.
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Drosophila have two strains:-
1) strains having P element c/d M –cytotype.
2) strains lacking P elements c/d P-cytotype.
CROSS 1 CROSS 2
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Their movement depends on the reverse transcription of RNA into DNA.
The enzyme reverse transcriptase uses RNA as template to synthesize DNA molecules, the process is called retrotransposition.
There are two main classes:-
1) Viral retrotransposons / Retro virus-like elements/ LTR retrotransposons
e.g Ty1 elements( in Yeast )
Copia elements (in Drosophila)
gypsy elements ( in Drosophila)
2) Non-viral retrotransposons/ Retroposons
e.g LINEs (L1) in Humans
SINEs (Alu) in Humans
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Their genetic organisation resembles that of eukaryotic
retroviruses.
35 copies in the haploid genome of yeast.
LTRs may be detached from Ty element forming “solo δ”.
These are flanked by 5 np target site duplications at each side
of Ty insertion.
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Mechanism of Ty transpositionGag pol env
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These are retrotransposons in Drosophila
When inserted in chromosome, creates a target
site duplication of 5 np, one copy on each side of
transposon.
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gypsy element
Retrovirus like element found in Drosophila.
Creates 4 np target site duplication.
Target site duplications are oriented in same
direction.
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NON VIRAL
RETROTRANSPOSONS(in
humans)
LINEs (Long Interspersed Nuclear
Elements) Most dominant & abundant T.E are L1, 6 kb long.
It has
-internal promoter(recognised by Polymerase II)
-2 open reading frames(ORF1 & ORF2)
L1 are authentic retroposons.
They may be complete or incomplete.
Only small number of them are transpositionally active.
Two more LINE sequences in Human genome are
L2 & L3.
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2nd most abundant class of T.E. in humans.
Less than 400bp long.
Reverse transcriptase needed for synthesis of
DNA from SINE’s RNA is furnished by LINE
type element.
SINEs depend on LINEs to multiply.
One of them is Alu elements.
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Significance of transposable elements
Genetic significance
Contributes to more than half the DNA in
Maize genome.
Johng Lim observed the role of T.E
-in evolution of chromosome structure
-in chromosomal rearrangements
Spontaneous mutations caused by T.E
insertions
such as P, retrovirus like elements &
retroposons
e.g mobilized P elements in Dysgenic
hybrids of Drosophila.
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Evolutionary issues
T.E are nature’s tool for genetic
engineering.
as –have ability to copy, transpose &
rearrange other sequences.
Can spread simply as
-replicate selfishly independent of normal
replication machinery.
-act as genomic parasites.
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Alan Kingsman & Susan Kingsman proposed
-retroviruses develop from retroposons by
addition of env gene(synthesize membrane
protein).
It makes the element capable of moving from
one cell to another(as T.E move from one
location to other in genome.)
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References Snustad, D.Peter & Simmons J.Michael.2003. Principles Of
Genetics(2nd edition) John Wiley & Sons. Newyork..page no. 446-459.
Weaver , F Robert & Hedrick W.Philip .1997. Genetics(3rd edition). Wm.C.Publishers page no. 365-370.
Web links:-
-http://www.nature.com/scitable/topicpage/transposons-the-jumping-genes-518
-http://www.google.co.in/url?sa=t&rct=j&q=&esrc=s&frm=1&source=web&cd=1&ved=0CCUQFjAA&url=http%3A%2F%2Fmercury2.iab.uaf.edu%2Fkevin_mccracken%2Fgenetics%2Flectures%2Fchapter_07b.ppt&ei=9Mz8Uq7eHISTrgf2xoGwBw&usg=AFQjCNGUP3bZdYOaPo9Vwt-0mO_rh-yRZw&bvm=bv.61379712,d.bmk
-http://www.google.co.in/url?sa=t&rct=j&q=&esrc=s&frm=1&source=web&cd=5&ved=0CD8QFjAE&url=http%3A%2F%2Fwww2.fiu.edu%2F~barbieri%2FCH17.ppt&ei=9Mz8Uq7eHISTrgf2xoGwBw&usg=AFQjCNE5FMptymqC5y1OXTaO_7Du6IlOgw
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