TRANSPOSABLE ELEMENTS-TRANSPOSONS

AND RETROTRANSPOS

ONSBYP.DIVYA,M.TECH BIOTECHNOLOGY

WHAT ARE TRANSPOSABLE ELEMENTS?

Segment of DNA that can move from one position in a genome to another.

Also called as mobile gene, jumping gene, or transposons.

Retrotransposons – transposable element that moves by being first transcribed into RNA copy that is then reconverted to DNA by RT and inserted elsewhere in the chromosome.

Short ,inverted repeats of 20 to 40 bp. Contributes to spontaneous mutation,

genetic rearrangement.

DISCOVERY OF TRANSPOSONS Discovered by Barbara McClintock in

1950s Ac and Ds elements ( controlling

elements) in maize that influences kernel colour in maize.

Ac is activator element( autonomous) and Ds is dissociation element ( non-autonomous)

Ds elements does not remain constant but moves to a new position causing chromosomal breakage at new site.

Ac element is needed by Ds elements for transposition.

TYPES OF TRANSPOSONS DNA only transposons- the mobile elements

exists as DNA throughout the life cycle. The translocationg DNA segment is directly cut out of donor DNA and joined to target site by transposase enzyme. Follows replicative pathway.

Retroviral -like retrotransposons – An RNA polymerase enzyme first transcribes the DNA sequence to RNA . Then RT enzyme transcribes this RNA to DNA . Then this DNA is inserted finally into new site by integrase enzyme. Moves via an RNA intermediate produced by

promotor in LTR(long terminal repeats)

Non retroviral retrotransposons- each of these transposons moves by making a DNA copy of an RNA molecule that is transcribed from it . It moves via an intermediate that is often

produced from neighboring promotor.

TYPES OF TRANSPOSONS

MODES OF TRANSPOSITION Replicative transposition- direct

interaction between the donor transposon and the target site ,resulting in copying of donor element.

Conservative transposition – involves excision of the element and re-integration at a new site.

MODES OF TRANSPOSITION

Retrotransposition-The element makes an RNA copy of itself which is reversed-transcribed into a DNA copy which is then inserted (cDNA)

MODES OF TRANSPOSITION

EXAMPLES OF TRANSPOSONS Transposable elements are widespread

in both prokaryotes and eukaryotes.Bacterial transposons-

Insertion sequences- simplest forms of transposons which are found in bacterial chromosomes and plasmids.

Size – 768 bp-5 kb Ends of IS elements consists of inverted terminal

repeats. IS elements can transfer between bacteria

during conjugation

STRUCTURE OF INSERTION SEQUENCE

INSERTION OF IS ELEMENTS IN CHROMOSOMAL DNA

Composite transposons – consists of two insertion sequences flanking an internal region which usually includes at least one gene which codes for resistance to antobiotic.

EXAMPLES OF TRANSPOSONS

Tetracycline resistance gene is coded . One IS element is autonomous (IS10R) Another IS element is non-

autonomous(IS10L) Inactivation of one element by mutation

would not harm ability to transpose and would assure continued transposition of the entire transposon. Few more examples-

Tn 5- kanamycin resistance Tn 10 – chloramphenicol resistance

SIGNIFICANCE OF BACTERIAL TRANSPOSABLE ELEMENTS These transposons are called as “

SELFISH DNA” . They exist without any benefits to the

host’s phenotype and they exist only for propagation, contributing nothing to the host cell.

Bacteria containing large number of transposons have advantage as they increase mutation rate and bacteria can adapt to changing environmental conditions.

Eukaryotic transposons- Transposons in Drosophila-

The F1 generation produced by crossing certain strains show a type of sterility called hybrid dysgenesis that happens as a result of mutation induced by transposons.

The transposition is favoured by an enzyme transposase that is coded by an element of gene called the “p-element”.The p-element also encodes a repressor protein that inhibits the transposition.

EXAMPLES OF TRANSPOSONS

HYBRID DYSGENESISP strain – contains p-element in their genomeM strain-lacks p-element in their genome P female x M male P male x M female

No hybrid dysgenesis Hybrid dysgnesis (fertile) occurs (Sterile)

When p female mates with m male , p female’s egg which contains a repressor protein inhibits the transcription of the transposase gene of the p element . Since there is no production of transposase , the p element cannot be transposed into a gene .As a result , mutation does not take place and the progeny produced is fertile

When p male mates with m female , p male contains p elements but m female has no repressor protein that can suppress the transposition of the p element.Thus , transposition is carried out and this results in sterile progeny (hybrid dysgenesis)

MAMMALIAN RETROTRANSPOSONS LINEs ( Long Interspersed Nuclear

Elements)-Contain a reverse transcriptase

like gene that is involved in retrotransposition. For eg: human mobile element LINE 1 which is 6.1kb.

SINEs (short Interspersed Nuclear Elements)

Has no reverse transcriptase gene but transposes by “borrowing” RT enzyme from other retroelements. For eg: alu in human genome.

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