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Chapter 9 - Transposition and Non-Homologous Recombination

Chapter 9 - Transposition and Non-Homologous Recombination...

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Nonhomologous recombination includes: 1.   Transposition : the hopping of DNA elements by the action of  transposases . 2.  Site-specific excision  of intervening DNA between two genes or  sites. 3.  Integration  of DNA into specific sites by  integrases . 4.  Inversion  of the intervening DNA between  specific sites by  invertases . 5.  The  separation of entwined circular DNAs  by  resolvases . 6.  The  rearrangement  of DNAs by  topoisomerases . Transposition and  Nonhomologous Recombination   HOUR
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Transposons, retrotransposons and  homing introns True transposons  are DNA elements that carry with them their own  specific  transposase  every time they move.  They were originally  discovered by  Barbara McClintock  in  maize  in the 1950s, and  rediscovered 20 years later in bacteria. Homing DNAs  can  insert only  at  very specific sites  into DNA, and  usually do so through an RNA intermediate.  Most of the homing  elements are  introns  that encode a  reverse transcriptase  that also  functions as a  site-specific DNA endonuclease . Retrotransposons  are transposons that are  transcribed  first into an  RNA, which then is  reversely transcribed  into DNA by a  reverse  transcriptase  when the element integrates at a different site.  Retrotransposons are related to retroviruses . Only few exist in  bacteria, but they are abundant in yeasts, fungi, plants and  animals.
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Fig 9.1 Donor DNA Target  ( Recipient DNA Donor DNA Target DNA Transpositions are rare events Frequencies range from  10  to  10   per generation Target site Duplicated target site Transposon Transposon An overview of transposition
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A.    Insertion sequences  ( IS ) are  simple transposons , varying in       length from 750 - 2000 bp. B.   The names of insertion sequences   usually consist of the capital  letters  “IS” followed by a number written in italics.   EXAMPLES:  IS 1 , IS 2 , IS 10 , IS 50 . There are exceptions, such as   (also  γδ called Tn 1000 ) C.   Characteristically, the  components of insertion sequences  are: 1.   Terminal inverted repeats   ( TIR ) of variable lengths. 2.   One or two genes encoding  a transposase .   3.   Flanking short direct repeats of target DNA  (3 to 9 bp). D.   Each bacterial species has its own group of different IS elements but different species may share one or more of these because  many  transposons are  promiscuous . E.  Why are transosons promiscuous?  They catch a ride by inserting  into conjugative plasmids and viruses.
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