LS4_13_rII_phage

LS4_13_rII_phage - Benzer and the rII locus What is a gene?...

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Benzer and the rII locus
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What is a gene? Approaches to a fne scale analysis oF gene structure. A a b B X Old model of gene organization - “Bead Theory” The gene is the fundamental unit of structure . Recombination takes place between but not within genes. The gene is the fundamental unit of change . One allele is converted to another at the whole gene level. Alleles are completely different from one another. The gene is the fundamental unit of function . Genes only function as whole units, no partial function.
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Seymore Benzer and the rII genes of T4 phage • A physicist in the 1940’s -His experiments with semiconductors helped lead to the invention of the transistor. In the 1950’s he decided to dabble into the field of biology -His studies and discovery of the numerous mutations in the rII genes of the T4 phage lead to the understanding of the relationship between genes and proteins.
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Benzer’s Experiments Addressed Three Basic Questions: • What is the basic element of structure? • What is the basic element of change? • What is the basic element of function?
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Why did Benzer choose to work on rII locus in T4 bacteriophage? 2. Easy to generate a lot of mutants with the bacteriophage system -can screen large number of phages (> 10 9 ) in short period of time (15 hours, overnight culture). 3. Easy assays -use selective agents to detect the presence of very small portion of recombinants within a large proportion of parentals ( conditional mutant ) 1. Mutants breed true What is a conditional mutant?
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Conditional mutations • Mutations cause certain phenotype only in certain environment. Restrictive condition mutant phenotype (non-permissive) Permissive condition wild type a. auxotrophs b. temperature sensitive c. host range d. amber-amber suppressor hosts • Examples:
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rII (Rapid Lysis) Mutant Plaques rII (conditional lethal) affects lysis and host range of T4 phage: rII + produces small plaques in both strains B and K rII only grows on E. coli strain B (permissive host). cannot grow in E. coli strain K (non-permissive host). produces large plaques in strain B • There are three rapid lysis mutations in T4 phage : rI, rII , rIII rII - rII + E.coli B E.coli B or K
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• rII mutants grow on E. coli B but not on E. coli K -Note: rII can infect E. coli K but cannot lyse it. Large plaques Wild type plaques r + r No growth Wild type plaques r + Bacteria Strain Phage rII E. coli B E. coli K rII + rII (Rapid Lysis) Mutant Plaques • 2 insights…
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