transposable elements

TRANSPOSABLE ELEMENTS

VARSHA YADAV ROLL NO.- 6674DEPARTMENT OF BIOTECHNOLOGY CENTRAL UNIVERSITY OF HARYANA

INTRODUCTION: Transposons are section of the genome that undergo

transposition with in the genome. Discovered by Barbara McClintock in corn in 1940. They are known as transposable elements, mobile

sequence, movable sequence, insertion elements, jumping elements or selfish DNA.

Later in 1960 bacteria and bacteriophages were shown to posses TE.

CLASSIFICATION ON THE BASIS OF MECHANISM OF TRANSPOSITION

CLASS 1: Insertional sequence were the first

transposable elements identified as spontaneous insertion in some bacteria operon.

They encode for transposase and are short about 800 – 1500 base pair.

IS carry the genetic information necessary for their transposition .

Cut and paste mechanism. There are different IS such as IS1, IS2, IS3, IS4.

CLASS 2 : - These are called retrotransposon. - Use the enzyme reverse transcriptase and then make a copy of DNA and then inserted into the genome in new position. - Behave like retrovirus such as HIV - Copy and paste mechanism - Inverted repeats are present on either side.

(A) COMPOSITE TRANSPOSONS Carry genes (ex. Gene for antibiotic resistance) flanked on both

sides by IS elements. Tn10 is 9.3 kb in size includes 6.5 kb of central DNA (includes

a gene for tetracycline resistance) and 1.4 kb inverted IS element.

IS element supply transposase and ITR recognition signals.

(B)NONCOMPOSITE TRANSPOSONS

Carry genes ( ex. Antibiotic resistance) but do not terminate with IS elements.

Ends are not IS element repeated sequence.

Tn3 is kb with 38 bp ITRs and includes 3 genes; β-lactamase, tnpA (transposase) and tnpB (resolvase).

EXAMPLES OF TRANSPOSABLE ELEMENTS

Ac-Ds system in MAIZE P Element in Drosophila LINEs and SINEs in Human

Ac-Ds element Ac stands for activator and Ds for dissociation

element. Both were located on chromosome 9. Transposition of Ds element altered kernel

coloration. Movement of Ds element requires activity of Ac

element that codes transposases. Ac element is 4563 bp long and its structure is

similar to that of bacterial transposons.

P Element Most significant transposable element found in

drosophila Range from 0.5 to 2.9 kb long , with 31 bp ITRs. They encode two protein ,one is transposase

enzyme that activate transposition and another is a repressor that inhibit transposition.

If P elements inserts into the coding region , it can terminate transcription ,in introns affect splicing.

If inserts into the promoter region, it affect the level of gene expression.

LONG INTERSPREAD OR SHORT INTERSPREAD ELEMENTS

LINEs accounts for 21% of human genome and about 6kb pairs in length.

SINEs accounts for 13% of human genome and are about 100-500bp long.

Most human transposons appear to be inactive but several mutation lead to their activation.

LINE inserted into the human dystrophin gene have result in Duchene muscular dystrophy.

LINE inserted into the APC and c-myc genes leading to mutation and cause development of some colon or breast cancer.

Conti….. A type of SINE called Alu elements integrated into

the BRCA2 gene and inactivate this tumor- suppresser gene and lead to breast cancer.

Potential effect of transposons on the genome-

EFFECT CAUSEGene mutation or inactivation Insertion into coding region or intron of a gene or inaccurate excision from the coding region of a genechange in gene regulation Alteration in splicing or alteration in transability of mRNAGene duplication Replication of a trasposonGene deletion Recombination between two transposonsChromosome breakage Excision of a transposon, followed by an inadequate DNA repairChromosome rearrengemet Recombination between transposon located at two different places

APPLICATION OF TE -Use as cloning vehicle.

-Provide raw material for evolution.

-Drug resistance transposones are useful in development of plasmid as cloning vehicles.

--Use of transposones to increase the rate of mutation due to insertional inactivation.