Lecture24S10n

Lecture24S10n - BIS101/Engebrecht Lecture24 We reviewed the...

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BIS101/Engebrecht Lecture24 5/27/10 We reviewed the consequence of chromosome missegregation in meiosis I and meiosis II. CQ1:In what parent and in what meiotic division did an XXY individual arise? This could have occurred in the mother at meiosis I or meiosis II, or in the father at meiosis I. The answer is E . Transposons Genes are genetically defined by the phenotype a mutation will cause and by their position in the genome. Transposons were discovered by McClintock in 1944 (Nobel Prize 1983) as mutations that cause phenotypes that could not be genetically mapped to a specific position. McClintock found that if she tried to map a gene, that defined a transposon, in a different, but related strain, it would map to a different location. Thus, these were termed jumping genes. Transposons are mobile genetic elements that insert themselves into new places of the genome. The genetic consequence of this is a mutation (if the transposon inserted into a ORF) or altered regulation of a gene (if the transposon jumped into the upstream regulatory region of a gene). Analysis of transposons also revealed that they could revert at a high frequency. When this happens, it is due to the precise excision of the transposon. Transposons make up 10-30% (or higher) of the genomes of higher eukaryotes. It is estimated that 1/500 human mutations are caused by transposons. Many of the transposons in the genome have lost the ability to jump (due to mutation); however, if transposase (the enzyme that allows them to jump) is provided in trans (produced by another transposon in the cell and therefore diffusible) they can jump. We saw previously that 50% of all spontaneous mutations in Drosophila are due to transposon insertions. We will primarily discuss the process of transposition in E. coli. Transposons were original discovered as an insertional mutation in a gal transducing lambda phage. (Think about the consequence of an insertional mutation.) It was found that the phage had more DNA due to the insertion of the transposon. The simplest transposon is referred to as an IS (insertional) element. These contain an inverted repeat at their ends and encode for transposase the enzyme that mediates the insertion of these elements into the target DNA. The steps in transposition are the following: 1) Transposase cleaves a target sequence and just outside of the inverted repeat ends of the IS element. This is a staggered cut and this sequence will be different for different IS elements. 2) The transposase inserts the IS into the cleaved target site 3) DNA polymerase fills the gap and DNA ligase glues the DNA back together. 4) This process results in a target site duplication In general, the insertion site is random. Transposons are larger than IS elements and usually are arranged as IS-genes-IS. In this situation, they are moving not only themselves but also the genes that are flanked by the IS elements. Unfortunately, transposons have picked up antibiotic resistance genes. This helps explain the ability of bacterial cells to rapidly acquire antibiotic
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Lecture24S10n - BIS101/Engebrecht Lecture24 We reviewed the...

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