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475 Lecture 30

Transposons are DNA elements that can move from one location in the genome to another. They were discovered by Barbara McClintock in corn and contain inverted repeat sequences at each end. There are three main types of transposons: insertion sequences, composite transposons consisting of two insertion sequences bracketing other genes, and noncomposite transposons containing several genes including a transposase between inverted repeats. Transposition usually causes a short duplication of target DNA and mechanisms exist to prevent frequent transposition, such as programmed frameshifting. Composite transposons can transpose DNA between the insertion sequences either as a unit or individually, which can result in deletions or inversions. Assays using suicide vectors or conjugal transfer can

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63 views6 pages

475 Lecture 30

Transposons are DNA elements that can move from one location in the genome to another. They were discovered by Barbara McClintock in corn and contain inverted repeat sequences at each end. There are three main types of transposons: insertion sequences, composite transposons consisting of two insertion sequences bracketing other genes, and noncomposite transposons containing several genes including a transposase between inverted repeats. Transposition usually causes a short duplication of target DNA and mechanisms exist to prevent frequent transposition, such as programmed frameshifting. Composite transposons can transpose DNA between the insertion sequences either as a unit or individually, which can result in deletions or inversions. Assays using suicide vectors or conjugal transfer can

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Transposons

• Transposons are DNA elements that can “hop”, or transpose from one place to another (below).
• Discovered by Barbara McClintock in corn.
• Transposase is the enzyme that that promotes transposition.
• Transposons have inverted repeat (IRs) sequences at each end (below right).
• There are three types of transposons: 1. Insertion Sequences
(IS elements), which are the smallest and have IRs and a
transposase gene; 2. Composite transposons, which are two
IS elements bracketing other genes; 3. Noncomposite
transposons, which have several genes including a
transposase between two IRs.
Transposons
• For most transposons, transposition causes a short direct repeat in the target DNA.
• Trasposons employ mechanisms that prevent frequent transposition, such as programmed frameshifting
in IS3.
Composite transposons
• Composite transposons consist of two IS elements
of the same type bracketing some intervening DNA.
• Both inside- and outside-end transposition may occu.r
• Outside-end transposition makes use of the two
IR sequences furthest from each other, in which case
DNA between the two IS elements is transposed as
well (right).
• Inside-end transposition makes use of the two IR
sequences closest to each other but from different IS
elements, which can result in the formation of a new
composite transposon (below).
Consequences of composite
transposon hopping
• R-factors (plasmids containing may resistance genes)
have been assembled by IS elements (below).
• Inside-end transposition can cause deletion or inversion
of DNA between the original site of the transposon and
its destination site.
- Whether a deletion or inversion occurs depends on
how the inside ends attach to the target DNA.
- Crossing of the inside ends causes inversion.
- If they do not cross, a deletion results.
Noncomposite transposons
• Noncomposite transposons combine the qualities of both IS elements and composite transposons.
• Like IS elements, They have single IR sequences at each end.
• Like composite transposons, they carry selectable marker genes.
Assays of transposition
• Phage and plasmid suicide vectors can be used to introduce transposons into a recipient cell
and antibiotic resistance conferred by the transposon can be used to select for cells where the
transposon has hopped into the recipient’s DNA.
• Transposition from a nontransferrable plasmid to one that can be transferred can be detected
by conjugal transfer of a transposon-encoded resistance marker gene (below).

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