Number of Bacteria Resistant to T1 Virus Are Mutations Random or Resistant Resistant Directed by the Environment? colonies Culture Culture colonies found Once biologists appreciated that Mendelian traits were, in fact, alternative versions of DNA sequences, which resulted from mutations, a very important question arose and needed to be answered: Are mutations random events number found number 107 11 1- 1 0- 12 0 2 13 that might happen anywhere on the DNA in a chromo- some, or are they directed to some degree by the environ- ment? For example, do the mutagens in cigarettes damage DNA at random locations, or do they preferentially seek out and alter specific sites such as those regulating the cell cycle? This key question was addressed and answered in an elegant, deceptively simple experiment carried out in 1943 by two of the pioneers of molecular genetics, Salvadore Luria and Max Delbruck. They chose to examine a particu- lar mutation that occurs in laboratory strains of the bacte- rium E. coli. These bacterial cells are susceptible to T1 viruses, tiny chemical parasites that infect, multiply within, and kill the bacteria. If 105 bacterial cells are exposed to 1010 T1 viruses, and the mixture is spread on a culture dish, not one cell grows-every single E. coli cell is infected and killed. However, if you repeat the experiment using 10 bacterial cells, lots of cells survive! When tested, these surviving cells prove to be mutants, resistant to T1 infec- tion. The question is, did the T1 virus cause the mutations, or were they present all along, too rare to be present in a sample of only 105 cells but common enough to be present in 109 cells? 3 0 14 0 4 1 15 0- 5 16 5- 6 17 0 - 7 64 18 19 0 35 - 20 10 3- 5 -0 1- 2 O- 1 3-1 5 2 107-1 35-1 4-1 Analysis 1. Applying Concepts Is there a dependent variable in this experiment? Explain. 2. Interpreting Data T1-resistant colonies found in the 20 individual What is the mean number of cultures? 3. Making Inferences Comparing the 20 individual cultures, do the cultures exhibit similar numbers of T1-resistant а. bacterial cells? b. Which of the two alternative outcomes illustrated, (a) or (b), is more similar to the outcome obtained by Luria and Delbruck in this experiment? Drewing Conclusions Are these data consistent with he hypothesis that the mutation for T1 resistance among E. coli bacteria is caused by exposure to TI vius? Explain. To answer this question, Luria and Delbruck devised a simple experiment they called a "fluctuation test," illustrated here. Five cell generations are shown for each of foor independent bacterial cultures, all tested for resistance in th fifth generation. If the T1 virus causes the mutations (top row), then each culture will have more or less the same Cufure 2 Culture 1 Culture 3 Culture 4 number of resistant cells, with only a little fluctuation (that is, variation among the four). If, on the other hand, mutations are spontaneous equally likely to occur in any generation, cultures and therefore 0 0 0 0 0 0 Q 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 Q 0 0 0 AA 0000 00000 then bacterial 00000006000600000 Ó66006000000 00000 00000000000 0000 000000 in which T1-resistance mutation occurs the а. in earlier generations will possess far more resistant cells by the fifth generation than cultures in which the mutation Culture 1 Culture 2 Culture 3 Culture 4 occurs in later generations, resulting in wide fluctuation among the four cultures. The table presents the data obtained for 20 0 0 O 0 0 0 0 0 0 0 OO 0 0 0 O 0 0 0 0 0 O 0000000000000 6660000000000000 66000000000006600 they individual 000000000 0000000 000 cultures. b. Inquiry & Analysis O O9

Number of Bacteria Resistant to T1 Virus Are Mutations Random or Resistant Resistant Directed by the Environment? colonies Culture Culture colonies found Once biologists appreciated that Mendelian traits were, in fact, alternative versions of DNA sequences, which resulted from mutations, a very important question arose and needed to be answered: Are mutations random events number found number 107 11 1- 1 0- 12 0 2 13 that might happen anywhere on the DNA in a chromo- some, or are they directed to some degree by the environ- ment? For example, do the mutagens in cigarettes damage DNA at random locations, or do they preferentially seek out and alter specific sites such as those regulating the cell cycle? This key question was addressed and answered in an elegant, deceptively simple experiment carried out in 1943 by two of the pioneers of molecular genetics, Salvadore Luria and Max Delbruck. They chose to examine a particu- lar mutation that occurs in laboratory strains of the bacte- rium E. coli. These bacterial cells are susceptible to T1 viruses, tiny chemical parasites that infect, multiply within, and kill the bacteria. If 105 bacterial cells are exposed to 1010 T1 viruses, and the mixture is spread on a culture dish, not one cell grows-every single E. coli cell is infected and killed. However, if you repeat the experiment using 10 bacterial cells, lots of cells survive! When tested, these surviving cells prove to be mutants, resistant to T1 infec- tion. The question is, did the T1 virus cause the mutations, or were they present all along, too rare to be present in a sample of only 105 cells but common enough to be present in 109 cells? 3 0 14 0 4 1 15 0- 5 16 5- 6 17 0 - 7 64 18 19 0 35 - 20 10 3- 5 -0 1- 2 O- 1 3-1 5 2 107-1 35-1 4-1 Analysis 1. Applying Concepts Is there a dependent variable in this experiment? Explain. 2. Interpreting Data T1-resistant colonies found in the 20 individual What is the mean number of cultures? 3. Making Inferences Comparing the 20 individual cultures, do the cultures exhibit similar numbers of T1-resistant а. bacterial cells? b. Which of the two alternative outcomes illustrated, (a) or (b), is more similar to the outcome obtained by Luria and Delbruck in this experiment? Drewing Conclusions Are these data consistent with he hypothesis that the mutation for T1 resistance among E. coli bacteria is caused by exposure to TI vius? Explain. To answer this question, Luria and Delbruck devised a simple experiment they called a "fluctuation test," illustrated here. Five cell generations are shown for each of foor independent bacterial cultures, all tested for resistance in th fifth generation. If the T1 virus causes the mutations (top row), then each culture will have more or less the same Cufure 2 Culture 1 Culture 3 Culture 4 number of resistant cells, with only a little fluctuation (that is, variation among the four). If, on the other hand, mutations are spontaneous equally likely to occur in any generation, cultures and therefore 0 0 0 0 0 0 Q 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 Q 0 0 0 AA 0000 00000 then bacterial 00000006000600000 Ó66006000000 00000 00000000000 0000 000000 in which T1-resistance mutation occurs the а. in earlier generations will possess far more resistant cells by the fifth generation than cultures in which the mutation Culture 1 Culture 2 Culture 3 Culture 4 occurs in later generations, resulting in wide fluctuation among the four cultures. The table presents the data obtained for 20 0 0 O 0 0 0 0 0 0 0 OO 0 0 0 O 0 0 0 0 0 O 0000000000000 6660000000000000 66000000000006600 they individual 000000000 0000000 000 cultures. b. Inquiry & Analysis O O9

Biochemistry

6th Edition

ISBN:9781305577206

Author:Reginald H. Garrett, Charles M. Grisham

Publisher:Reginald H. Garrett, Charles M. Grisham

Chapter17: Metabolism: An Overview

Section: Chapter Questions

Problem 13P: What Art the Features of the Series of -omes? Define the following terms: a. Genome b. Transcriptome...

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