Genetic - Transposable Element,Definition,Classification


Transposable elements (TEs), also known as “jumping genes” or transposons, are sequences of DNA that move (or jump) from one location in the genome to another. A transposon may be defined as: “a DNA sequence that is able to move or insert itself at a new location in the genome.” The phenomenon of movement of a transposon to a new site in the genome is referred to as transposition.
Barbara McClintock’s discovery of these jumping genes earned her a Nobel Prize in 1983.

Classification

On the basis of their transposition mechanism, transposons may be categorized into following types:

(i) Cut-and-Paste Transposons/Nonreplicative Transposons:

 

They transpose by excision (cutting) of the transposable sequence from one position in the genome and its insertion (pasting) to another position within the genome
Examples:– cut-and-paste type of transposons are IS-elements, P-elements in maize, hobo-elements in Drosophila etc.

(ii) Replicative Transposons:

 

In this type the transposable element creat a duplicate copy of itself and transfer it to the target site of recipient cell,thus the donor cell remain unchange.
 Example:- Tn3-elements found in bacteria are good examples of such type of transposons.

Transposable Elements in Prokaryotes:

The transposable first observation at molecular level was done in bacteria, which is a prokaryote.
Bacterial transposable elements are of the two types:

(1) Insertion Sequences or IS Elements:

  • They are the transposable sequences which can insert at different sites in the bacterial chromosomes and they are well organised
  • They are contain a short sigment of 9-40 bp sequence at the both end they are called “Inverted Terminal Repeats”
  • When they are inserted into a recipient cell they creat a copy of DNA at the end of their insertion,this 3-12bp containing sequences are called ‘target site duplication’
  • Is element mediate the insertion of episom into the bacterial chromosome by the homologous recombination.
Example: Tn1, Tn5, Tn9, Tn10 etc

(2) Composite Transposable Elements:

  • When two Is elements are joined each other they are formed composite transposons’
  • They are contain an antibiotic resistance gene in Composite element.

 

Example:

There are present sevarel type of composite transposons they are-

(1)Tn -5

  • The total length of this tn5 is 5700bp.
  • The length of the Is element is 1533bp.
  • The orientation of both Is element in Tn5 is in Opposite direction.
  • The Is50 in Tn5 contain about 9bp.
  • It contain Kanamycin resistant gen which flanking with two Is -50 in both side.

(2)Tn -9

  • It contain chloramphenicol resistant gen which flanking with two Is -1 in both side.
  • The total length of this tn5 is 2700bp.
  • The length of the Is element is 768bp.
  • The orientation of both Is element in Tn5 is in Same direction.
  • The Is – 1 in Tn9 contain about 23bp.

(3)Tn -10

  • It contain Tetracycline resistant gen which flanking with two Is -10 in both side.
  • The total length of this tn5 is 9300bp.
  • The length of the Is element is 1329bp.
  • The Is – 10 in Tn10 contain about 22bp.
  • The orientation of both Is element in Tn5 is in opposite direction

(4)Tn -3

  • It is a special composite transposon.
  • It contain a long coding region of 5000 nucleotide bp, and its pair with the 39-40 bp Inverted terminal repeats.
  • It’s coding region contain three special gene this are-
    • Tnp – A
      • Tnp – R
        • bla

Mechanism

The mechanism of composite transposons or Is element transfer is completed through this two step –
(i) Cut-and-Paste Transposons/Nonreplicative Transposons:

They transpose by excision (cutting) of the transposable sequence from one position in the genome and its insertion (pasting) to another position within the genome
Examples:- cut-and-paste type of transposons are IS-elements, P-elements in maize, hobo-elements in Drosophila etc.

(ii) Replicative Transposons:

 

In this type the transposable element creat a duplicat copy of itself and transfer it to the target site of recipient cell,thus the donar cell remain unchange.
Example:- Tn3-elements found in bacteria are good examples of such type of transposons.

Transposable Elements in Eukaryotes:

(a)Transposable element of Maize/Ac-Ds System:
  • The Ac and Ds system first analysed by Barbara Mc. Clintock in Maize.
  • Ac stand for Activator and Ds stand for Dissociation.
  • This Ac and Ds element can move freely  from one location to another location of chromosome, and change the phenotypic characters of Maize karnel.
  • Ac is required for activation of Ds element, after activation the Ds element help in the breakage of chromosome.
  • Ac can move autonomously  while Ds can move only in the presence of Ac.
Other transposable elements of maize are:
i. spm (suppressor mutator) system,
ii. dt (dotted) system,
iii. Mu (Mutator) system, etc.
(a)Transposable element Drosophila:
  • P – element;
  • It is discover during the study of ‘hybrid-dysgenesis’.
  • It is about 2.9kb in size.
  • It contain about 37bp in long Inverted Terminal Repeat.
  • It is encoded by transposase enzyme which help in transposition.
  • copia – element;
  • It’s transposition causes mutation in eye colour of drosophila.
  • It is about 8-9 kb in size.
  • It contain 276bp in Direct terminal Repeat.
Other important types of transposable elements found in Drosophila are:
i. I elements,
ii. Mariner elements,
iii. Gypsy elements,
iv. Hobo elements, etc.

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