1-20-11 Mutation _ Mutagenesis II

1-20-11 Mutation _ Mutagenesis II - Mutation and...

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1 Mutation and Mutagenesis Part II January 20, 2011 Mutations can occur spontaneously or be induced. Spontaneous mutations Result from abnormalities in cellular/biological processes Errors in DNA replication Oxidative damage from metabolism Induced mutations Caused by environmental agents Agents that are known to alter DNA structure are termed mutagens These can be chemical or physical agents (more on this later) OCCURRENCE AND CAUSES OF MUTATION Are spontaneous mutations random occurrences? Or do they arise from environmental influences? This is a question that biologists have asked since the 1800's: Jean Baptiste Lamarck Proposed that physiological events and life experience (e.g. use and disuse) determine whether traits are passed along to offspring. Charles Darwin Proposed that genetic variation occurs by chance, and… Natural selection determines what variations survive. Spontaneous Mutations Are Random Events These two opposing theories of the 19th century were tested in bacteria in the 1940s and 1950s Salvadore Luria and Max Delbruck studied the resistance of E. coli to bacteriophage T1 ton r ( T on e r esistance) They wondered if ton r is due to spontaneous mutations, or to a physiological adaptation that occurs at a low rate? The physiological adaptation theory predicts that the number of ton r bacteria will be essentially constant in different bacterial populations growing under the same conditions. The spontaneous mutation theory predicts that the number of ton r bacteria will fluctuate in different bacterial populations. Their test therefore became known as the fluctuation test .
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2 The Luria-Delbruck fluctuation test Figure 16.6 E.. coli is grown in the absence of T1 phages 20 million cells each 20 million cells each Great fluctuation in the number of ton r colonies Relatively even distribution of ton r colonies Many ton r bacteria Mutation occurred at an early stage of population growth, before T1 exposure No ton r bacteria Spontaneous mutation did not occur Several independent ton r mutations occurred during different stages of population growth. These were mixed together in the big flask, producing an average value of ton r cells Spread onto agar plates coated with T1 phages Joshua and Ester Lederberg were also interested in the relationship between mutations and the environment. At that time (1950s), there were two theories: Directed mutation theory Selective conditions could promote the formation of specific mutations allowing the organism to survive This was consistent with Lamarck’s viewpoint Random mutation theory Environmental factors simply select for the survival of those individuals that happen to possess beneficial mutations This was consistent with Darwin’s viewpoint Random mutations can give an organism a survival advantage The Lederbergs devised a technique to distinguish between these two theories - "replica plating
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1-20-11 Mutation _ Mutagenesis II - Mutation and...

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