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DNA transposons (also called Class II elements ) are a group of transposable elements (TEs) that can move in the DNA of an organism via a single- or double-stranded DNA intermediate. DNA transposons have been found in both prokaryotic and eukaryotic organisms. They can make up a significant portion of an organisms genome, particularly in eukaryotes. In prokaryotes, TE's can facilitate the horizontal transfer of antibiotic resistance or other genes associated with virulence.

There are autonomous, as well as nonautonomous DNA transposons. The latter use the enzymatic machinery of the former for their amplification in a genome. It is estimated, that there are around 300,000 copies of DNA transposon fossils in the human genome and they make up around 3% of it.

Movement of Transposons
DNA transposons can move around in the genome. The system requires a transposase enzyme that catalyzes the movement of the DNA from its current location in the genome and inserts it in a new location. In transposition, the transposase "cuts" the DNA segment out and "pastes" it in elsewhere. Occasionally, genetic material not originally in the transposable element gets copied and moved as well. The ability of these elements to excise and insert themselves creates a mechanism for lateral gene transfer from one organism to another via a transposon migrating from one cell to another.

Helitrons
Helitrons are a group of eukaryotic class II transposable elements. This group does not move via the "cut and paste" method. Instead, helitrons replicate and move around the genome using a "rolling circle" mechanism, where a single stranded piece of donor DNA rolls in to a circular intermediate and inserts itself in to a target elsewhere in the genome. This systems creates duplicates of the gene sequence in the genome each time the TE moves.

Families of DNA Transposons
There are nine superfamilies of DNA Transposons defined. The following is a list of these superfamilies, with some group members:
 * Tc1-mariner
 * Tigger
 * hAT (name is derived from hobo transposon of Drosophila melanogaster)
 * Charlie
 * Mutator
 * Merlin
 * Transib
 * P
 * piggyBac (first discovered in Trichoplusia ni )
 * PIF/Harbinger
 * CACTA

Examples in Maize
Barbara McClintock first discovered and described DNA transposons in Zea mays, during the 1940's; an achievement that would earn her the Nobel Prize in 1983. She described the Ac/Ds system where the Ac unit (activator) was autonomous but the Ds genomic unit required the presence of the activator in order to move.

Examples in Drosophila
The Mariner transposon, found in many animals but studied in Drosophila was first described by Jacobson and Hartl. Mariner is well known for being able to excise and insert horizontally in to a new organism. Thousands of copies of the TE have been found interspersed in the human genome as well as other animals.

The Hobo transposons in Drosophila have been extensively studied due to their ability to cause gonadal dysgenesis. The insertion and subsequent expression of hobo-like sequences results in the loss of germ cells in the gonads of developing flies.

Examples in Bacteria
Bacterial transposons are especially good at facilitating horizontal gene transfer between microbes. The first example of the Mariner transposon was discovered in Trichomonas vaginalis. Transposition facilitates the transfer and accumulation of antibiotic resistance genes. In bacteria, transposable elements can easily jump between the chromosomal genome and plasmids. In a study by Devaud et al. in 1982, a multi-drug resistant strain of Acinetobacter isolated and examined. Evidence pointed to the transfer of a plasmid in to the bacterium, where the resistance genes were transposed in to the chromosomal genome.

Class II TE Activity in Humans
Class II transposable elements make up about 3% of the human genome. Today, there are no active DNA Transposons in the human genome. Therefore, the elements found in the human genome are called "fossils".