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
and then back to DNA by reverse transcriptase
. Transposons are very useful to researchers as a means to alter DNA inside of a living organism.
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 cut, leaving sticky ends. The transposon is then ligated into the target site, the gaps are filled in, resulting in direct repeats.
- The first transposons were discovered in maize (zea mays, aka corn) 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 maize 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. Artificial 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 multiresistant strains.
Transposons causing diseases
Transposons are mutagens. They can damage the genome of their host cell in different ways :
Diseases that are often caused by transposons include Hemophilia A and B, SCID, porphyria, predisposition to cancer, and Duchenne muscular dystrophy.
- 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.
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