Lecture 10 - Lecture 10 Transposable Elements Kernels on an...

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Lecture 10 Transposable Elements
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Kernels on an ear of corn The maize kernel is not merely a seed but a one-seeded fruit comprised of many cells.
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Transposons, transposable elements, mobile elements Segments of DNA that can move around to different genomic positions in a single cell . New position may be on the same or a different chromosome In the process, they may •cause mutations •increase (or decrease) the amount of DNA in the genome
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The Ds transposable element helps cause chromosome breakage Figure 14-2 Ds (disassociation) located at the site of break
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Unusual phenotypes are caused by the Ds transposable element Figure 14-3a Ac (activator) : unlinked to Ds locus; required to activate breakage at Ds locus Break occurred in a single cell which divided mitotically to produce the affected sector. This could occur many times in a single kernel
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Today we know that Ac is a 4.5 kb transposon that encodes a single transposase protein. Ds elements are often, but not always, internally deleted, transposition-incompetent derivatives of Ac. Ds elements transpose when supplied with Ac -encoded transposase and chromosome breakage is not a general property of Ds transposons. It turns out that McClintock's chromosome-breaking Ds is a composite element, with one internally deleted 2 kb transposon inserted in inverted order precisely into the middle of a second copy. This composite transposon contains both termini in both orientations, resulting in the cleavage of both, rather than just one sister chromatid during transposition. This is often followed by joining of the broken chromatid ends, creating a dicentric and an acentric chromosome fragment.
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Unusual phenotypes are caused by the Ds transposable element Figure 14-3b “Strains” or genetic clones of the chromosome breaking line showed new phenotype – something changed… Colorless - unstable phenotype, reverts to wild type In some cells Ds leaves the C gene leading to expression of the pigment and purple spot…the earlier in development Ds leaves the larger the spot becomes as mitotic progeny accumulate
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What is the relationship between Ds and Ac? Ds
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This note was uploaded on 11/11/2009 for the course BIO 325 taught by Professor Saxena during the Spring '08 term at University of Texas at Austin.

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Lecture 10 - Lecture 10 Transposable Elements Kernels on an...

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