19_an_introduction_to_genetic_analysis - An Introduction to...

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An Introduction to Genetic Analysis Chapter 19 Mechanisms of Recombination Chapter 19 Mechanisms of Recombination Key Concepts Recombination occurs at regions of homology between chromosomes through the breakage and reunion of DNA molecules. Models for recombination, such as the Holliday model, involve the creation of a heteroduplex branch, or cross bridge, that can migrate and the subsequent splicing of the intermediate structure to yield different types of recombinant DNA molecules. Recombination models can be applied to explain genetic crosses. Many of the enzymes participating in recombination in bacteria have been identified. Introduction Throughout our analysis of linkage, we studied the recombination of genes by crossing-over. In this chapter, we consider molecular mechanisms for generating recombination by crossing- over. Figure 19-1 depicts a basic crossover event, in which two homologous molecules are aligned and subsequently undergo recombination. When Benzer's work and that of others revealed that recombination occurs within genes, it became evident that recombination had to be very precise, because even single-base-pair errors could disrupt the integrity of the gene. How can recognition of homologous chromosomes and recombination events be so precise? The answer lies in the power of base-pair complementarity. We shall see how base-pair complementarity and the formation of heteroduplex regions between complementary regions of homologous chromosomes lead to the recombination events that we have been studying. Breakage and reunion of DNA molecules The experiments discussed in Chapter 5 provide good indirect evidence in favor of breakage and reunion. One of the first direct proofs that chromosomes (in this case, viral chromosomes) can break and rejoin came from experiments on λ phage done in 1961 by Matthew Meselson and Jean Weigle. They simultaneously infected E. coli with two strains of λ. One strain, which had the genetic markers c and mi at one end of the chromosome, was “ heavy ” because the phages were produced from cells grown in heavy isotopes of carbon ( 13 C) and nitrogen ( 15 N). The other strain was c+ 14mi + for the markers and had “light” DNA because it was harvested from cells grown on the normal light isotopes 12 C and 14 N. The two DNAs (chromosomes) can be represented as shown in Figure 19-2a . The multiply infected cells were then incubated in a light medium until they lysed. 勇勇勇勇勇勇勇勇,勇勇勇勇勇勇勇勇勇勇勇勇勇勇勇. 1
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An Introduction to Genetic Analysis Chapter 19 Mechanisms of Recombination The progeny phages released from the cells were spun in a cesium chloride density gradient. A wide band was obtained, indicating that the viral DNAs ranged in density from the heavy parental value to the light parental value, with a great many intermediate densities (Figure 19- 2b ). Interestingly, some recombinant phages were recovered with density values very close to the heavy parental value. They were of genotype c 14 mi + , and they must have arisen
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