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DNA transposons (also called Class II elements[1]) are a group of transposable elements (TEs) that can move in the DNA of an organism via a single- or double-stranded DNA intermediate.[2] 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.[3]

Movement of Transposons

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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.[4] 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.[5] This systems creates duplicates of the gene sequence in the genome each time the TE moves.

Examples of DNA Transposons

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Families of DNA Transposons

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There are nine superfamilies of DNA Transposons defined.[6] The following is a list of these superfamilies, with some group members:

Examples in Maize

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Barbara McClintock first discovered and described DNA transposons in Zea mays,[9] 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

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The Mariner transposon, found in many animals but studied in Drosophila was first described by Jacobson and Hartl.[10] Mariner is well known for being able to excise and insert horizontally in to a new organism.[11] 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.[12] 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

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Bacterial transposons are especially good at facilitating horizontal gene transfer between microbes. The first example of the Mariner transposon was discovered in Trichomonas vaginalis.[13] 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.[14]

Class II TE Activity in Humans

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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".

References

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  1. ^ Wicker, Thomas; Sabot, François; Hua-Van, Aurélie; Bennetzen, Jeffrey L.; Capy, Pierre; Chalhoub, Boulos; Flavell, Andrew; Leroy, Philippe; Morgante, Michele. "A unified classification system for eukaryotic transposable elements". Nature Reviews Genetics. 8 (12): 973–982. doi:10.1038/nrg2165.
  2. ^ Feschotte, Cédric; Pritham, Ellen J. (December 2007). "DNA Transposons and the Evolution of Eukaryotic Genomes". Annual Review of Genetics. 41 (1): 331–368. doi:10.1146/annurev.genet.40.110405.090448.
  3. ^ International Human Genome Sequencing Consortium (Feb 2001). "Initial sequencing and analysis of the human genome". Nature. 409 (6822): 860–921. doi:10.1038/35057062. PMID 11237011.
  4. ^ Madigan M, Martinko J, eds. (2006). Brock Biolog of Microorganisms (11th ed.). Prentice Hall. ISBN 0-13-144329-1.
  5. ^ Kapitonov VV, Jurka J. Trends Genet. 2007 Oct;23(10):521-9. Epub 2007 Sep 11. Review. PMID 17850916
  6. ^ Sinzelle, L; Izsvák, Z; Ivics, Z (March 2009). "Molecular domestication of transposable elements: from detrimental parasites to useful host genes". Cellular and molecular life sciences : CMLS. 66 (6): 1073–93. doi:10.1007/s00018-009-8376-3. PMID 19132291.
  7. ^ Oosumi, T; Belknap, WR; Garlick, B (14 December 1995). "Mariner transposons in humans". Nature. 378 (6558): 672. doi:10.1038/378672a0. PMID 7501013.
  8. ^ Fraser, MJ; Smith, GE; Summers, MD (August 1983). "Acquisition of Host Cell DNA Sequences by Baculoviruses: Relationship Between Host DNA Insertions and FP Mutants of Autographa californica and Galleria mellonella Nuclear Polyhedrosis Viruses". Journal of Virology. 47 (2): 287–300. PMC 255260. PMID 16789244.
  9. ^ McClintock, Barbara (June 1950). "The origin and behavior of mutable loci in maize"Proc Natl Acad Sci U S A36 (6): 344–55. Bibcode:1950PNAS...36..344Mdoi:10.1073/pnas.36.6.344PMC 1063197 . PMID 15430309.
  10. ^ Jacobson JW, Medhora MM, Hartl DL. Proc Natl Acad Sci U S A. 1986 Nov;83(22):8684-8. PMID 3022302
  11. ^ Lohe AR, Moriyama EN, Lidholm DA, Hartl DL (January 1995). "Horizontal transmission, vertical inactivation, and stochastic loss of mariner-like transposable elements". Mol. Biol. Evol12 (1): 62–72. doi:10.1093/oxfordjournals.molbev.a040191PMID 7877497.
  12. ^ Deprá M, Valente VL, Margis R, Loreto EL. Gene. 2009 Dec 1;448(1):57-63. doi: 10.1016/j.gene.2009.08.012. Epub 2009 Aug 29. PMID 19720121
  13. ^ Draft genome sequence of the sexually transmitted pathogen Trichomonas vaginalis. Carlton JM, Hirt RP, Silva JC, Delcher AL, Schatz M, Zhao Q, Wortman JR, Bidwell SL, Alsmark UC, Besteiro S, Sicheritz-Ponten T, Noel CJ, Dacks JB, Foster PG, Simillion C, Van de Peer Y, Miranda-Saavedra D, Barton GJ, Westrop GD, Müller S, Dessi D, Fiori PL, Ren Q, Paulsen I, Zhang H, Bastida-Corcuera FD, Simoes-Barbosa A, Brown MT, Hayes RD, Mukherjee M, Okumura CY, Schneider R, Smith AJ, Vanacova S, Villalvazo M, Haas BJ, Pertea M, Feldblyum TV, Utterback TR, Shu CL, Osoegawa K, de Jong PJ, Hrdy I, Horvathova L, Zubacova Z, Dolezal P, Malik SB, Logsdon JM Jr, Henze K, Gupta A, Wang CC, Dunne RL, Upcroft JA, Upcroft P, White O, Salzberg SL, Tang P, Chiu CH, Lee YS, Embley TM, Coombs GH, Mottram JC, Tachezy J, Fraser-Liggett CM, Johnson PJ. Science. 2007 Jan 12;315(5809):207-12.
  14. ^ Devaud M, Kayser FH, Bächi B. Antimicrob Agents Chemother. 1982 Aug;22(2):323-9. PMID 6100428