Transposable element

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

• The first transposons were discovered in corn (zea mays) by Barbara McClintock in 1940, for which she was awarded a Nobel Prize in 1983. She noticed insertions, deletions and translocations, caused by these transposons. These changes in the genome could, for example, lead to a change in color. About 50% of the total genome of corn consists of transposons.
• Transposons in Drosophila (the fruit fly) are called P elements. They seem to have first appeared in Drosophila melanogaster only about 50 years ago. Since then, they have spread through every population of the species. Artifical P elements can be used to insert genes into Drosophila by injecting the embryo.
• Transposons in bacteria usually carry an additional gene for a function other than transposase, often an antibiotic resistance. In bacteria, transposons can jump from the "regular" DNA to plasmids and back, allowing the transfer and permanent addition of, for example, antibiotic resistance, leading to multiresistent stems.

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