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Genetics Lectures 27-29.pdf

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Biology (Sci)
BIOL 202
Tamara Western

Lecture 27Genetics of Transposable Elements Barbara McClintockunstable alleles of maizeLoss of a piece of a choromsome Called Ds locus for disocciator between centromere and DsRequired presence of unlinked Activator Ac locus Ds locus only broke when Activator was present as well on the chromosome at a different locus AcactivatedDs were jumping to the colour locus to give the color spots and then leave again Mobile elements but only in the presence of Ac also a jumping geneHybrid Dysgenesis of DrosophilaCrossed wild isolate males x lab strain femalesF1 progeny were sterilechromosome breakage made unstable mutations in germ line Unstable mutations of white eyes with red spots from dysgenic crossEnlarged mutant alleles in EcoliUnstable lacgal mutants of E coli found to contain extra DNADid a density gradient and found 2 bands Normaland large mutant allelesFirst evidence was that these unstable alleles were caused by something moving into the genes and coming out again Could find the same sequence in the large gal allele at multiple times in the chromosome In Ecoli there were several mobile elements making it unstable This was the evidence of the mobile genetic elements What is happeningLoss of protein function due to insertion of large piece of DNA into coding region o TE inserting into the regulatory region losing colour because losing transcription factor that regulation colour productionInsertions also can result in changes in gene expression if lands in a regulatory region or if transcription of gene now driven by transposable element regulatory sequences o Can also overactivate the gene or activate it at the wrong time if the promoter region is affected Getting a base white flower because the TE currently in the pigmentation gene is knocking out the pigment gene Can have restoration of the colour when the TE removed itself early so you have large coloured spots These are called revertant sectorsCells where the TE transposed out of the gene later in development multiply into small revertant coloured sectorsStable mutations caused by TE when they are not able to cleave themselves out cleanlyCould have mutation or deletion of portions of the TE required for recognition by transposase such that the TE is immobilized Or knock out the function of the transposase gene only if one transposase to start withError in transposition leading to imprecise excision leaving a couple base pairs behind This is more likely to happen than one transposase Transposon footprints that could knock out the function of something Fate of progeny depends on whether somatic or germline event TE insertions or movements that occur in somatic cells are nonheritable TE insertions or movements that occur in germline cells will be passed on to progeny TE insertions or movements that occur in germline cells will be passed on to stable mutant progeny This most likely could be due to imprecise transposition of a transposable elementMOST OF THE PROGENY WILL BE UNSTABLE MUTATIONS LIKE THE PARENTS Bacterial insertion sequences StructureEncodes transposase protein performs transpositionTerminal inverted repeats required for transpositionTarget site duplication created during insertionA bacterial transposon is more complicated and carry host gene and seen in two types composite hitch hikers and simple transposons Bacterial transposons Composite eg Tn9 and Tn10Bacterial host genes flanked by 2 Insertion SequencesFrequently an antibiotic resistance it will move as a unitSimple eg Tn3No insertion sequences ISL and ISRFlanked by inverted repeatsEncodes transposaseAlso carries beneficial host gene antibiotic resistanceMultiresistant plasmids We have 5 different transposons in this plasmid coding for 7 antibiotic resistance genes Eukaryotic transposable elements 2 types Class IRetrotransposons Reverse Transcriptase ex Yeast Ty1 Drosophila copiaLTR Retrotransposons long terminal direct repeats POL is the RTase here GAG can be involved in making a protein coat Found in many species NonLTR Retrotranspons LINEs and SINEs Alu Element is the most abundant in the human genome
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