Transposon

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Transposons are sequences of DNA that can move around to different positions within the genome of a single cell. In the process, they can cause mutations, and change the amount of DNA in the genome. Transposons are also called "jumping genes" or "transposable genetic elements". Transposons can move directly from one position to another within the genome, while retroposons have first to be transcribed to RNA and then back to DNA by reverse transcriptase.

Mechanism

A transposon needs the enzyme transposase, which is often encoded by the transposon itself. The ends of the transposon sequence consist of inverted repeats (identical sequences reading in opposite directions). The transposase binds to both the inverted repeats of the transposon and the target site on the genome, where the transposon will move to. This target site is the cut open, leaving sticky ends. The transposon is then ligated into the target site, the gaps are filled up, resulting in direct repeats.

Examples

Transposons causing diseases

Transposons are mutagenes. They can damage the genome of their host cell in different ways :

  • A transposon/retroposon that inserts itself into a functional gene will most likely disable that gene.
  • After a transposon left a gene, the resulting gap can probably not be repaired correctly.
  • Multiple copies of the same sequence (e.g., Alu) can hinder precise chromosomal pairing during mitosis, resulting in unequal crossovers, one of the main reasons for chromosome duplication.

Diseases that are often caused by transposons include Hemophilia A and B, SCID, porphyria, predisposition to cancer, and Duchenne muscular dystrophy.

Some multicellular organisms, e.g., C. elegans, have found a way to keep retroposons in check. A gene is not translated if a double-stranded RNA copy of that gene is present, as it is for, e.g., integrase.

See also : retroposon -- genetics