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Trasposable elements

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  • Trasposable elements



    Trasposable elements


    Trasposable elements or Mobile genetic elements were discovered in bacteria in the late 1960s. They were also recognized
    because they caused a particular type of mutation which was unstable. The first type of element that was discovered (what
    we now know as an insertion element) will block the expression of any gene into which it inserts and it will also block the
    expression of any downstream genes that are expressed as part of the same operon. This kind of effect is called a polar effect

    At more or less the same time research into the structure, properties and origins of broad host-range plasmids and the
    spread of antibiotic resistance in bacteria revealed the presence of more complex, related, elements that were also potentially mobile

    their are four types of bacterial transposon


    Insertion Sequences

    Insertion Sequences or IS elements are the simplest mobile element. They consist of a fairly short
    (700 - 1500 bp) DNA segment flanked by a 10 - 40 bp inverted repeat sequence. The segment codes
    for the protein (transposase) that catalyses the transposition event:



    Transposons


    Simple transposons are similar to IS elements. They contain DNA segments flanked by short inverted repeat
    sequences. The DNA segments, however, usually code for a number of gene products. In addition to a transposase
    they may also code for a resolvase (this will depend on their mechanism of transposition) and they
    may contain for one or more antibiotic resistance genes:



    Composite Transposons

    Composite transposons are DNA segments that are flanked by an IS element at either end. In other words
    instead of each IS element moving independently, they now act in concert and move together along with the intervening DNA

    Each IS element is a typical IS element although only one of the two elements typically retains a functional transposase
    activity. The IS elements may be in the same or in the opposite orientation with respect to one another.

    The intervening segment often carries the genetic determinants for a number of antibiotic or other toxin resistances:



    Bacteriophage Elements


    The E. coli bacteriophage Mu is an unusual phage. It can infect E. coli as a normal phage. It is a temperate phage
    so it has both lytic and lysogenic growth cycles. The Mu prophage integrates into the host genome. However, it does
    not do so by a site-specific recombination - rather, it integrates randomly into the host genome by a transpositional
    mechanism. When Mu replicates, it does so by replicative transposition events in which each new copy of Mu is inserted
    somewhere else in the chromosome Mu is, in effect, a transposon that happens to have genes that package
    its DNA as well as genes that regulate its transposition

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