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lecture12 - Lecture 12 Transposable elements Transposons...

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Lecture 12 Transposable elements Transposable elements Transposons are usually from 10 3 to 10 4 base pairs in length, depending on the transposon type. The key property of transposons is that a copy of the entire transposon sequence can at a low frequency become inserted at a new chromosomal site. The mechanism by which transposons insert into new sites differs from one kind of transposon to another, but the details are not important to understand how transposons can be used. It is worth contrasting the recombination events that occur during transposition to the homologous recombination events that we have considered in meiosis and in phage crosses. In homologous recombination, crossovers occur between like sequences. While this type of recombination can generate new combinations of alleles the arrangement of genes is left undisturbed. In contrast, transposition involves recombination between unrelated sequences, namely the ends of the transposon and a site in the target sequence. Transposition therefore results in a new arrangement of genes along the chromosome. The generic structure of a transposon looks like this: Host DNA Transposon Tn5 Host DNA Transposase Kanamycin Gene resistance Inverted repeat sequences Transposon Element Function Transposase An enzyme that cuts the target DNA more or less at random and splices the transposon ends to the target sequences, Other steps in transposition are performed by host enzymes. Inverted Repeats These sequences direct transposase to act at the ends of the transposon. Note that because the sequences are inverted, the two ends have identical sequence. Selectable Marker(s) Transposons are thought to have evolved by providing a selective advantage to the host cell. Many transposons carry genes that confer antibiotic resistance or some other benefit to the host.
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The study of transposition mechanism and the biology of transposons is an interesting subject in genetics but for our current purposes we are going to concentrate on how transposons can be used for bacterial genetic analysis. For this purpose we will focus on
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