Ch 17t - Chapter 17: Recombination Molecular Genetics...

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Chapter 17: Recombination Molecular Genetics Lecture Outlines – C.M. Kearney Fig. 17.1: Recombination is responsible for a large number of genetic phenomena, including the generation of diversity, the insertion of viruses into the genome, and transposition. Fig. 17.2: The Holliday model for homologous recombination. Start with two dsDNAs of very similar sequence lined up next to each other so that each locus is next to its homologous locus in the other dsDNA. This, of course, occurs during meiosis. Next, a fortuitous set of nicks appears right across from each other. One end from each helix invades the other helix, and this is sealed to the free end remaining in the helix by DNA ligase. This forms the Holliday structure. Branch migration occurs next. Since the homologous sequence is so similar to the strand in the next helix, it can be swapped for it. Just move the junction down along the helices to do this. Now that we’ve done a little swapping, we’re ready for “resolution”, the cutting of the branch. First, let’s form the “chi form.” To do this, you must manipulate the 3-dimensional Holliday structure in your head. The big “X” (chi) in the figure provides a better 3-D picture of the helix to help you visualize this. You see, once you have the branch of the Holliday structure, you really just have to bend the “red” helix in a “V” and the blue helix in a “V” (the Holliday structure in the figure) to get four helices connected by the branch point (the chi form in the figure). Now, you can easily see that the chi branch point could be cut either “vertically” or “horizontally” and you’d get two different products (bottom drawings in figure). In one case, you just get an exchange of the small amount of the branch migration. In the other case, you get “reciprocal strand exchange”, or one half of one helix bound to the other half of the other helix, plus a staggered end at the junction. Fig. 17.3:
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Ch 17t - Chapter 17: Recombination Molecular Genetics...

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