Chapter 9 - Transposition and Non-Homologous Recombination

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

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Unformatted text preview: 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 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. 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 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 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 ....
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Chapter 9 - Transposition and Non-Homologous Recombination...

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