LECTURE7 - Lecture 7: Transposable elements: Types...

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Unformatted text preview: Lecture 7: Transposable elements: Types & mechanisms Discovery of transposable elements Prokaryotic transposable elements Insertion sequences vs transposons Conservative vs replicative transposition Eukaryotic transposable elements Class I Eukaryotic TEs: retrotransposons Class II Eukaryotic TEs: DNA elements Transposable elements & host genome evolution Numbers of TEs Maintenance of TEs Roles of TEs in host genome evolution Readings : Ch 14 pp 487-50 Genetics & the detection of the unexpected Fig Ch 14 intro Unstable alleles!!! Unstable alleles!!! Chromosome breakage at Dissociator (Ds) locus affects several loci distal from breakage point Required presence of unlinked Activator (Ac) locus Genetics & the detection of the unexpected: Barbara McClintock & unstable alleles of maize Fig 14-3a Rare unstable alleles Affect only a single gene distal to usual breakage point Suggested that Ds is mobile, but only in the presence of Ac Couldnt map Ac so seemed to be mobile itself Genetics & the detection of the unexpected: Barbara McClintock & unstable alleles of maize Fig 14-3b Crosses of wild isolate males (P) x lab strain females (M) F1 progeny sterile chromosome breakage, unstable mutations in germ line Unstable mutations of white from dysgenic cross reminiscent of what seen before Genetics & the detection of the unexpected: Hybrid dysgenesis in Drosophila Fig 14-15 Unstable lac- & gal- mutants of E. coli Found to contain extra DNA Hybridization experiments for gal- insertion mutants found that same sequence over & over again Genetics & the detection of the unexpected: Enlarged mutant alleles in E. coli Bacterial insertion sequences Hybridization experiments for gal- insertion mutants found that same sequence over & over again In various gal- alleles In other parts of the bacterial chromosome Found other repeated Insertion Sequences through similar hybridization experiments Bacterial insertion sequences Structure: Encodes transposase protein (performs transposition) Terminal inverted repeats (required for transposition) Target site duplication (created during insertion) EG 21.1 Bacterial transposons Composite Bacterial genes flanked by 2 Insertion Sequences Depending on transposon, both ISs could move on own OR one IS may be disabled & transposition depends on the intact IS EG 21-2,3 See also Fig 14-6a Bacterial transposons Simple No insertion sequences Flanked by inverted repeats Encodes transposase Tn3 Fig 14-6b Mechanisms of transposition Replicative adds new copy Conservative cut & paste into new location Fig 14-9 Transposase- mediated Cuts at target site Cuts out from old site Inserts in new site Host repairs cut sites to generate target site duplication Conservative transposition Fig 14-8...
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LECTURE7 - Lecture 7: Transposable elements: Types...

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