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.
(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
- Tnp – R
- Tnp – A
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.
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.
i. I elements,
ii. Mariner elements,
iii. Gypsy elements,
iv. Hobo elements, etc.
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