Transposable

elements in

eukaryotes

Submitted by:-

Class:- M.sc (P) Botany

Roll no. 02

Introduction

First experiment

Discovery of Ac Ds elements

P elements in Drosophila

Retrotransposons

Viral retrotransposons

Non-viral retrotransposons

Significance of Transposable elements

Conclusion

References

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.

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.

Discovery of Ac Ds elements Genetic marker that McClintock followed is C’ i.e. a dominant inhibitor of

aleurone coloration.

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

Structure of Ac Ds elements

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.

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

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

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.

Mechanism of Ty transpositionGag pol env

These are retrotransposons in Drosophila

When inserted in chromosome, creates a target

site duplication of 5 np, one copy on each side of

transposon.

gypsy element

Retrovirus like element found in Drosophila.

Creates 4 np target site duplication.

Target site duplications are oriented in same

direction.

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.

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.

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.

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.

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.)

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