DNA transposon

DNA transposons (also called Class II transposable 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 organism's genome, particularly in eukaryotes. In prokaryotes, TE's can facilitate the horizontal transfer of antibiotic resistance or other genes associated with virulence. DNA transposons (also called Class II transposable 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 organism's 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. 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. Transposition requires three DNA sites on the transposon two at each end of the transposon called terminal inverted repeats and another at the target site. The transposase will bind to the terminal inverted repeats of the transposon and mediate synapsis of the transposon ends. The transposase then disconnects the element from the flanking DNA of the original donor site and mediates the joining reaction that link the transposon to the new insertion site. The ends of the mobile element attack the target DNA at staggered positions such that the newly inserted transposon is flanked by short gaps. Host systems repair these gaps resulting in the target sequence duplication that are characteristic of transposition. In many reactions, the transposon is completely excised from the donor site in what is called a 'cut and paste' transposition and inserted into the target DNA to form a simple insertion.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 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. Polintons are a group of eukaryotic class II transposable elements. They contain genes with homology to viral proteins and which are often found in eukaryotic genomes. They are the largest and most complex known DNA transposons. Polintons encode up to 10 individual proteins and derive their name from two key proteins, a DNA polymerase and a retroviral-like integrase, proteins necessary to replicate themselves. As of the most recent update in 2015, 23 superfamilies of DNA transposons were recognized and annotated inRepbase, a database of repetitive DNA elements maintained by the Genetic Information Research Institute: Barbara McClintock first discovered and described DNA transposons in Zea mays, during the 1940s; 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. 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.