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The Genetic Material
Typology: Essays (university)
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[Presented by: Steve Carr (scarr@mun.ca), 14 January 2014]
Max Delbruck (1906 -1981) & Salvador Luria (1912 -1991) Shared 1969 Nobel Prize in Physiology or Medicine
Bacteriology in 1940s not heavily influenced by genetic thinking Bacteria have no nuclei: do they have " genes "? Bacterial "phenotypes" are the manifestations of 10^6 s of bacteria simultaneously Bacteria don't have sex: crosses not possible [Discovery of bacterial sex led to 1958 Nobel Prize]
bacteriophages ("phages") - "subcellular parasites that infect, multiply within, & kill bacteria." T1 phages are active on E. coli
Ton r^ phenotype is stable all descendant bacteria are Ton r phenotype persists in the absence of T
Two Hypotheses (d'Herelle 1926 vs Brunet 1929)
Hypotheses make different predictions as to numerical distribution of Ton r^ phenotypes among bacterial cultures.
where n = number of Ton r^ bacteria observed out of N = number of Ton s^ bacteria plated, and a = probability of conversion from Ton s^ to Ton r Then, n should be a constant fraction of N
where a = mutation rate (# mutations / cell / generation) g = # generations to go from 1 N bacteria, so that N = 2 g^ doublings occur, of which n = ga2g^ produce mutant Ton r^ bacteria
i
g-i
g mutants] Then, n should increase wrt N , as g increases
Suppose c cultures are started from a single Ton s^ mutant each after g generations there are N = 2 g^ bacteria in each culture
Statistical foundations of Luria - Delbruck experiment
Consider four cultures each started from a single bacterium after g = 4 generations, expect 16 cells from 15 divisions @, total 64 cells from 60 divisions plate each culture separately w/ T1, count total # Ton r Suppose 10 Ton r^ colonies observed: what distribution (" fluctuation ") expected?
Induction Hypothesis:
Ton r^ induction occurred only in fourth generation upon exposure to T probability of induction (a) is uniform / bacterium a = 10 inductions / 64 cells = 15% observe = 3, 1, 5, & 1 Ton r^ colonies mean = 10 / 4 = 2.5 Ton r^ per culture variance = 2. Follows a Poisson Distribution: variance = mean
Mutation Hypothesis
Ton r^ mutation has occurred spontaneously, prior to exposure to T mutation rate (a) = 2 events / 60 cell divisions = 0.033 mutations / cell / generation mean = (2 + 0 + 8 + 0) / 4 = 2.5 Ton r^ as before After 4 generation, earlier Ton r^ mutations leave more offspring (as in Culture 3) variance = 10. after 5 generations, when the number of Ton r^ cells has doubled in each culture: variance = 48.
Mutation Hypothesis predicts variance >> mean, as g increases
"The first experiment was done on the following Sunday morning. (In a letter dated January 21 [1943], Delbruck exhorted me to go to church"
Twenty x 200 ul "individual cultures" One x 10 ml "bulk culture" Inoculate with ~ 10^3 bacteria @ Grow for g = 17 generations ~10^8 bacteria / ml Plate entire "individual cultures" & 200 ul aliquots of "bulk culture" on petri dish w/ T