Host Range Method By Luria

Host Range Method By Luria

	Host Range Method By Luria The E. coli strain B is infected and lysed by wild type T2 phage (h+). The phage cannot, however, infect the E. coli B/2 strain. When wild type T2 phage is plated on a mixture of Band B/2 strains, the cells are lysed but not B /2. The entire plaque appears turbid because of the presence of BIZ. Certain mutant phages called host range (h) lyse both strains of bacterial cells and produce plaques with clear centres and turbid haloes.
Hershey and Chase (1949) observed 'nonreciprocal recombination' in phages differing at the host range (b) and rapid lysis (r) loci. The cross between hr + X h + r formed equal proportions of hr and h +r + recombinants for the experiment as a whole. In many cases, however, lysates from single bacteria ("single bursts") did not contain equal proportions. One recombinant was present in higher quantities than the other. Many single bursts produced20% or more hr, but less than half this amount of h + r +. Single breakage and reunion implies reciprocal exchange between two parental chromosomes, and an equal proportion of the two types of recombinants. The presence of reduced proportions of one recombinant indicated that a copy choice mechanism was involved. Nonreciprocal gene conversion has also been found in fungi (Mitchell and Lindgren).
2. The second exception from break and reunion was the discovery that viral heterozygotes occurred as intermediates in recombination. Heterozygote formation is difficult to explain on the basis of breakage and reunion. Levinthal (1959) suggested a possible copy choice mechanism for heterozygote formation (A la).
3. The Chase and Doermann experiments (1958) provide the third argument against breakage and reunion. In recombination by breakage and reunion the red-like bendings involved in breakage would cause positive interference or no interference. Positive interference prevents crossover in regions adjacent to where crossover has already taken place. The experiments of Chase and Doermann, however, show that in many phage crosses there is evidence of high 'negative' interference. Genetic exchange in one region is accompanied by increased genetic exchange in adjacent regions. Negative interference is difficult to explain 011 the basis of the breakage and recombination theory. In the copy-choice mechanism, however, a copying switch from one strand to another need not prevent further adjacent switches

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