Transposition
A transposition is a physical movement of genetic material (i.e., DNA) within a genome or the movement of DNA across genomes (i.e., from one genome to another). Because these segments of genetic material contain genes, transpositions resulting in changes of the loci (location) or arrangements of genes are mutations. Transposition mutations occur in a wide range of organisms. Transposons occur in bacteria, and transposable elements have been demonstrated to operate in higher eukaryotic organisms, including mammalian systems.
Transposition mutations may only occur if the DNA being moved, termed the transposon, contains intact inverted repeats at its ends (terminus). In addition, functional tranposase enzymes must be present.
There are two types or mechanisms of transposition. Replicative transpositions involve the copying of the segment of section DNA to be moved (transposable element) before the segment is actually moved. Accordingly, with replicative transposition, the original section of DNA remains at its original location and only the copy is moved and inserted into its new position. In contrast, with conservative transpositions, the segment of DNA to be moved is physically cut from its original location and then inserted into a new location. The DNA from which the tranposon is removed is termed the donor DNA, and the DNA to which the transposon is added is termed the receptor DNA.
Transposons are not passive participants in transposition. Transposons carry the genes that code for the enzymes needed for transposition. In essence, they carry the mechanisms of transposition with them as they move or jump (hence Barbara McClintok's original designation of "jumping genes") throughout or across genomes. Transposons carry special insertion sequences (IS elements) that carry the genetic information to code for the enzyme transposase that is required to accomplish transposition mutations. One of the most important mechanisms of transposase is that they are the enzymes responsible for cutting the receptor DNA to allow the insertion of the transposon.
Transitions are a radical mutational mechanism. The physical removal of both DNA and genes can severely damage or impair the function of genes located in the transposons (especially those near either terminus). Correspondingly, the donor molecules suffer a deletion of material that may also render the remaining genes inoperative or highly impaired with regard to function.
McClintok's discovery of transposons, also termed "jumping genes," in the late 1940s (before the formation of the Waston-Crick model of DNA) resulted in her subsequent award of a Nobel Prize for Medicine or Physiology.
Transposition segments termed retrotransposons may also utilize an RNA intermediate complimentary copy to accomplish their transposition.
Transposition can radically and seriously affect phenotypic characteristics including transfer of antibiotic resistance in bacterium. Following insertion, transposed genetic elements usually generate multiple copies of the genes transferred, further increasing their disruption to both the genotype and phenotypic expression.
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