Mutations

Mutations - Mutations & Mutagenic Agents The Nature of...

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1 Mutations & Mutagenic Agents 1 The Nature of Spontaneous Mutations Luria and Delbruck 2 Bulk culture of T-1 nsitive 20 Individual cultures Luria and Delbruck’s Fluctuation Test Plate on 10 Petri Dishes containing T-1 phage Plate each on a Petri dish of T-1 phage sensitive E. coli 3 Count the number of bacterial colonies on each plate Griffiths 4
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2 Two Theories of Mutation Formation “Adaptation” Theory “Mutation” Theory 5 Fig 7.2 Jackpot Frequency of T1 Resistant Colonies Luria and Delbruck’s Data Follow a Poisson Distribution 4 6 8 10 12 requency Mean # of events 6 0 2 0 2 04 06 08 01 0 0 1 2 0 Colonies per Plate F (Graph of individual culture data from Luria and Delbruck Experiment) Characteristics a Poisson Distribution an be used to determine the probability of • Can be used to determine the probability of infrequent whole number events • Events must be independent • Poisson distributions are skewed to the right vents that follow a Poisson distribution are 7 • Events that follow a Poisson distribution are considered to be random The Poisson Distribution ) = - Binomial: f (i) = e m m i i ! f (i): frequency of class i : mean number of items ƒ b (x) = n!p x (1-p) n-x x!(n-x)! 8 m: mean number of items i! = i (i-1)(i-2)…(1) e.g. 3! = 3 X 2 X 1= 6 0! = 1 1! = 1
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3 Problem: 100 one dollar bills are tossed into a classroom containing 100 students. Assuming a Poisson distribution, how many students would ,y end up with no dollar bills? Solution: i = 0, m = 100/100 = 1 f (0) = e - 1 m 0 1 ! = 0.368 or 37 students would d up with no dollar bills 9 end up with no dollar bills. ) = - m i ind m where i= 0 Application of the Poisson Formula: Find the multiplicity of infection (MOI) of the Luria-Delbruck experiment f (i) e m i ! Find m where i= 0 f (0) = 11/20 = 0.55= e - m m 0 = e -m (1) 0! 1 10 if e -m = 0.55, then m = 0.60 (Griffiths, Table 6.1) e -m = 0.55 Griffiths 11 Mutation Rate The probability* that a cell will experience a mutation in one generation 12 *based on the Poisson distribution
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4 Determining Mutation Rates f (i) = e - μ n ( n) i i ! opulation mutation rate 13 μ : population mutation rate n: total cells in population Mutation Rate Example: alculate the mutation rate in the Luria Calculate the mutation rate in the Luria- Delbruck experiment where 0.2 ml samples of T1 sensitive bacteria were plated onto media containing T1 phage from a suspension culture containing 1 X 10 8 14 cells/ml f (i) = e - μ n ( n) i i ! n = 0.2 ml X 10 8 cells/ml = 2 X 10 7 cells f (i) = 0.55 where i = 0 () 0.55 = e ( n ) 0 1 ln 0.55 = ln ( e - 2 X 10^7 ) -0.6 = - μ ( 2 X 10 7 ) - 2 X 10^7 15 μ= 3 X 10 -8 mutations per cell per generation x 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 ln x -2.3 -1.6 -1.2 -0.9 -0.7 -0.5 -0.4 -0.2 -0.1 0 Mutation Frequency • The measured number of mutations per cell per generation • Less accurate than calculating the mutation rate due to the influence of jackpots • Determined by dividing the number of mutations by the number of cells in the population Example: In the Luria and Delbruck experiment, the 16
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Mutations - Mutations & Mutagenic Agents The Nature of...

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