Transposons are mobile genetic elements found in the hereditary material of humans and other organisms. They can replicate and the new copies can insert at novel sites in the genome. Because this threatens the whole organism, molecular mechanisms have evolved which can repress transposon activity. Professor Klaus Frstemann of the Gene Center of Ludwig-Maximilians-Universitt (LMU) in Munich and a team of researchers working with the fruitfly Drosophila melanogaster have now uncovered a new type of cellular defence that acts against DNA sequences present in high copy numbers inside the cell, even if they have not integrated into the genome. Small molecules of RNA (a class of nucleic acid closely related to the genetic material DNA) play the central role. "Transposons are genomic parasites, so to speak", says Frstemann. "If they are allowed to proliferate, the genome can become unstable or cancers can develop. We now want to find out whether mammalian cells possess this newly discovered defence mechanism and to elucidate precisely how it works." (EMBO Journal online, 30 July 2009.)
Transposons constitute a significant fraction of the genomes of most higher organisms. Indeed, it is estimated that these mobile elements, which include one or more genes, make up as much as half of the genetic material. "This demonstrates", says Frstemann, "that it is not always possible to tame these "selfish" genetic elements, although highly efficient mechanisms of defence have evolved. For instance, in the germ cells, which are required for reproduction, the system of so-called piRNAs ensures that transposon activity is inhibited but only if these RNAs are transmitted from the mother. Disruption of this system usually leads to a drastic reduction in the fertility of the progeny.
Germ cells are an ideal target for transposons, since these cells pass their genetic material together with integrated mobile elements on to all cells of the progeny. But normal
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