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Transcript
TRANSPONSONS or TRANSPOSABLE ELEMENTS A talk by Dr Paul Kalitsis on 23/8/07 These are some notes taken whilst view the PowerPoint presentation and some may be of assistance in filling the gaps. Barbara McLintock (1940s) was the founder of “jumping genes” which led to the discovery of transposable elements (TE). She suggested that genes could change loci and produce phenotypic changes, eg. kernel colour in maize. This is before genes were known about. Genome size – C-value paradox (C-value is the amount of DNA per haploid genome). This is probably no longer a paradox since the discovery of transposable elements. Maize 2,500 Mb Arabidopsis 150 Mb Lilium 35,000 Mb What causes such a discrepancy? It has been suggested that it is due to TE. Retrotransposons These are sections of DNA that are transcribed to mRNA. Once in the cytoplasm cDNA is formed, then dsDNA which is then incorporated into the genome. This then leads to multiple copies in the genome. Interspersed repeats are equivalent to TEs. LINEs – long interspersed nuclear elements. SINEs – short interspersed nuclear elements. Function of TEs Development – differentially expressed Imprinting Tissue differentiation – affects processes in tissue formation Evolution – a very dynamic process In a lily, David Smyth found many repeats of a particular TE. RNAi is a primitive immune system. dsRNA from a TE produced after RNA polymerase acts on mRNA. The RNAi machinery cuts up the dsRNA of the TE Fragile X syndrome is characterized by a triplet CCG repeat that is present in the 5’ untranslated region that expands in affected individuals. When this triplet repeat expands beyond a certain length the gene is silenced due to cytosine methylation of the repeat. This is a form of epigenetic silencing. It’s been shown that yeast being stressed by nutrition or chemicals leads to increased TE activity. In mice the IAP element upstream of the Agouti gene can be switched to either yellow or brown epigenetically. This switching mechanism can be driven by changing the diet of the mice. Human diseases Up to 1% of diseases are due to TEs. Applications Mutagenesis Gene delivery and therapy eg viruses. Could place specific gene in specific locus. Cooption/domestication of TEs TEs adopt a new function in the host genome.
