09 - Bacterial genetics What are the genes What is the...

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1 MCB 140 09-19-08 1 “Bacterial” genetics MCB 140 09-19-08 2 “What are the genes? What is the nature of the elements of heredity that Mendel postulated as purely theoretical units? … Frankly, these are questions with which the working geneticist has not much concern himself… If the gene is a material unit, it is a piece of a chromosome; if it is a fictitious unit, it must be referred to a definite location in a chromosome. … Therefore, it makes no difference in the actual work in genetics which point of view is taken.” T.H. Morgan The Relation of Genetics to Physiology and Medicine Nobel Lecture, June 4, 1934 3 MCB 140, 2/25/05 DNA Æ RNA Æ protein central dogma of molecular biology MCB 140 09-19-08 4 7.20 MCB 140 09-19-08 5 Max Delbrück (1906 -1981) & Salvador Luria (1912 - 1991), Cold Spring Harbor Laboratory, Long Island NY, Summer 1941 MCB 140 09-19-08 6 Today even the layman thinks of resistant bacteria as originating from mutation … but when Luria and Delbrück first got together, conventional bacteriologists were by no means clear that microorganisms could be tought about genetically… Many believed that resistance was some kind of adaptation induced, in a few of the bacteria in a culture, by the exposure to the antibacterial agent. Judson p. 55
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2 MCB 140 09-19-08 7 The idea smacks of the pre- Mendelian, pre-Darwinian notion of the inheritance of acquired characteristics; Luria damned bacteriology as the “last stronghold of Lamarckism.” Judson p. 55 MCB 140 09-19-08 8 Let’s all do science in Nevada One Saturday evening … Luria went to a faculty dance… There, watching the fluctuating returns obtained by colleagues gambling on a slot machine, he thought of the experiment that would distinguish between resistance induced in bacteria and resistance resulting from previous spontaneous mutation upon which selection acts. Judson p. 55 MCB 140 09-19-08 9 What Luria perceived was that previous spontaneous mutation would pay out jackpots of resistant bacteria that would fluctuate much more widely in size than those paid out by induction. He tried the first experiment on the following morning and wrote off to Delbrueck; Delbrueck promptly replied that Luria really ought to go to church … Judson p. 55 MCB 140 09-19-08 10 7.4 MCB 140 09-19-08 11 What Luria actually did Sample set A: 1. Inoculate bacteria into individual cultures (1 bacterium per culture). 2. Let it grow up to a large number. Sample set B: 1. Take an aliquot of bacteria, and start a culture (which will therefore not be clonal). 2. Let
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09 - Bacterial genetics What are the genes What is the...

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