Transposon

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 repeat]?s (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 end]?s. The transposon is then ligated? into the target site, the gaps are filled up, resulting in [direct repeat]?s.

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 insertion?s, deletion?s and translocation?s, 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 element]?s. 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 stem]?s.

Retroposons

Retroposons, also called retrotransposons, copy themselves to RNA and then, via reverse transcriptase, back to DNA. Many retroposons have LTRs ([long terminal repeats]?) at their ends that can contain over 1000 base pairs each. Like transposons, they create direct repeats at their entry site, which can be used to detect them. About 40% of the human genome supposedly consists of retroposons.

• Several viruses, like HIV-1 or HTLV?-1 behave like retroposons, and contain both reverse transcriptase and integrase?, the retroposon equivalent of transposase.
• LINES ([Long interspersed elements]?) are long DNA sequences that represent reverse-transcribed RNA molecules originally transcribed by RNA polymerase II into mRNA (messenger RNA to be translated into protein by ribosomes). Also called pseudogene?s, they do not contain introns or promoters, but can code for reverse transcriptase or integrase, enabeling them to copy both themselves and other, non-coding LINES. As LINES move by copying themselves (instead of moving, like transposons do), they enlarge the genome. The human genome, for example, contains about 500.000 LINES, which roughly equals 16% of the genome. LINES are used to generate [genetic fingerprint]?s.
• SINES ([Short interspersed elements]?) are short DNA sequences that represent reverse-transcribed RNA molecules originally transcribed by RNA polymerase III into tRNA, rRNA and other small nuclear RNAs. The most common SINES are called [Alu element]?s. Alu elements are about 300 base pairs long, do not contain any coding sequences and can be recognized by the restriction enzyme AluI? (thus the name). With about 1 million copies, they make up about 11% of the human genome. Both LINES and SINES are also called "selfish DNA" or "junk DNA", as they do not serve any apparent purpose.

Transposons causing diseases

Transposons are mutagene?s. 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 crossover?s, 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.