C17 - C1. At the molecular level, sister chromatid exchange...

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C1. At the molecular level, sister chromatid exchange and homologous recombination are very similar. Identical (sister chromatid exchange) or similar (homologous recombination) segments of DNA line up and then cross over. Due to the molecular similarities of the two processes, one would expect that the same types of proteins would catalyze both events. At the genetic level, the events are different, however. Sister chromatid exchange does not result in the recombination of alleles because the chromatids are genetically identical. Homologous recombination usually results in a new combination of alleles after a crossover has taken place. C2. Branch migration will not create a heteroduplex during sister chromatid exchange because the sister chromatids are genetically identical. There should not be any mismatches between the complementary strands. Gene conversion cannot take place because the sister chromatids carry alleles that are already identical to each other. C3. Breakage would have to occur at the arrows labeled 2 and 4. This would connect the A allele with the B allele. The a allele in the other homologue would become connected with the b allele. In other words, one chromosome would be AB and the homologue would be ab. C4. The steps are described in Figure 17.6. A. The ends of the broken strands would not be recognized and degraded. B. RecA protein would not recognize the single-stranded ends, and strand invasion of the homologous double helix would not occur. C. Holliday junctions would not form. D. Branch migration would not occur without these proteins. And resolution of the intertwined helices would not occur. C5. The two molecular mechanisms that can explain the phenomenon of gene conversion are mismatch DNA repair and gap repair synthesis. Both mechanisms could occur in the double-stranded break model. C6. Usually, the overall net effect is not to create any new mutations in particular genes. However, homologous recombination does rearrange the combinations of alleles along particular chromosomes. This can be viewed as a mutation, since the sequence of a chromosome has been altered in a heritable fashion. C7. A recombinant chromosome is one that has been derived from a crossover and contains a combination of alleles that is different from the parental chromosomes. A recombinant chromosome is a hybrid of the parental chromosomes. C8. It depends on which way the breaks occur in the DNA strands during the resolution phase. If the two breaks occur in the crossed DNA strands, nonrecombinant chromosomes result. If the two breaks occur in the uncrossed strands, the result is a pair of recombinant chromosomes. C9. Gene conversion occurs when a pair of different alleles is converted to a pair of identical alleles. For example, a pair of Bb alleles could be converted to BB or bb. C10. Holliday model—proposes two breaks, one in each chromatid, and then both strands exchange a single strand of
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This note was uploaded on 04/07/2009 for the course BIO 325 taught by Professor Saxena during the Spring '08 term at University of Texas at Austin.

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C17 - C1. At the molecular level, sister chromatid exchange...

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