central dogma of molecular biology DNA RNA protein Bacterial genetics I MCB 140 2 25 05 MCB 140 2 16 05 3 1 7 17 MCB 140 2 16 05 MCB 140 2 16 05 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 4 2 1 Judson p 55 The idea smacks of the preMendelian pre Darwinian notion of the inheritance of acquired characteristics Luria damned bacteriology as the last stronghold of Lamarckism Max Delbr ck 1906 1981 Salvador Luria 1912 1991 Cold Spring Harbor Laboratory Long Island NY Summer 1941 MCB 140 2 16 05 MCB 140 2 16 05 7 5 MCB 140 2 16 05 Judson p 55 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 MCB 140 2 16 05 Let s all do science in Nevada Judson p 55 Today even the layman thinks of resistant bacteria as originating from mutation but when Luria and Delbr ck 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 8 6 2 MCB 140 2 16 05 MCB 140 2 16 05 Expose both to phage and count how many phageresistant colonies per culture are found Ask if there is a difference between these two sample sets Sample set B 1 Take an aliquot of bacteria and start a culture which will therefore not be clonal 2 Let them grow up to a large number 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 Judson p 55 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 11 9 MCB 140 2 16 05 Judson p 56 MCB 140 2 16 05 If the production of resistance began only at the moment of exposure to phage then it wouldn t matter whether the bacteria came from many individual cultures or one bulk culture When Luria performed the experiment though the twenty separate cultures showed much wider fluctuations from the average number of resistant colonies indicating that a few of the individual tubes contained resistant bacteria from near the beginning of the overnight growth period S Luria M Delbr ck 1943 Mutations of bacteria from virus sensitivity to virus resistance Genetics 28 491 511 7 4 12 10 3 Brock p 59 13 15 MCB 140 2 16 05 MCB 140 2 16 05 MCB 140 2 16 05 George Beadle left and Edward Tatum right receiving their Nobel Prizes MCB 140 2 16 05 16 14 4 MCB 140 2 16 05 Take strain of E coli that is auxotrophic for two distinct nutrients thiamine and leucine Take different strain of E coli that is also auxotrophic for two distinct nutrients but different ones biotine and cysteine Mix the two Ask if ANY NOVEL PHENOTYPES APPEAR Conjugation in bacteria MCB 140 2 16 05 19 17 14 11 MCB 140 2 16 05 MCB 140 2 16 05 J Lederberg E Tatum 1946 Novel genotypes in mixed cultures of biochemical mutants of bacteria Cold Spring Harbor Symp Quant Biol 11 113 114 Joshua Lederberg 20 18 5 Paramecium William Hayes MCB 140 2 16 05 MCB 140 2 16 05 23 21 Brock p 87 streptomycin Selman Waksman MCB 140 2 16 05 MCB 140 2 16 05 In the pre Hayes period mating in bacteria was envisioned as a conventional sex process perhaps modified by aspects of relative sexuality but nevertheless a standard haploid diploid meiosis mechanism After Hayes it was known that bacteria were not just small cells but constituted a completely different kind of cell The terms prokaryote and eukaryote were not introduced until 1962 B H and A H 24 22 6 MCB 140 2 16 05 Brock p 89 MCB 140 2 16 05 I discussed these results with Denny Mitchison and I think it was he who first suggested that one of the parents A might be acting as a gene donor and the other B as a recipient It was from this experiment that the concept of asymmetry in bacterial sexuality arose Parent B was the recipient or female the continued viability of which was essential for the whole process of recombination and segregation while the A donor or male cell was dispensable once genetic transfer had been effected W Hayes 1952 Recombination in Bact coli K 12 unidirectional transfer of genetic material Nature 169 118 Wait and see Mix the two on a minimalmedium plate containing streptomycin Take strain B which is streptomycin sensitive and auxotrophic for threonine and leucine Take strain A which is streptomycin resistant and auxotrophic for biotin and methionine Hayes expt 27 25 MCB 140 2 16 05 MCB 140 2 16 05 Cross 1 Strain A StrR B M Strain B StrS L T Result streptomycin completely inhibits prototroph formation i e appearance of B M L T bacteria if added before conjugation is complete Cross 2 Strain A StrS B M Strain B StrR L T Result streptomycin has no effect whatsoever You can add it all you want at any time and prototrophs will still form Rich husband poor wife is not the same as poor husband rich wife 28 26 7 14 13 14 12 MCB 140 2 16 05 MCB 140 2 16 05 31 29 14 14 14 12 MCB 140 2 16 05 MCB 140 2 16 05 32 30 8 33 35 MCB 140 2 16 05 MCB 140 2 16 05 E Wollman F Jacob 1955 Sur le mecanisme du transfert de materiel genetique au cours de la recombinaison chez E coli K12 CR Academie Sciences 240 2449 Interruption de la p n tration de l ADN m le dans le cytoplasme de la bact rie femelle l aide d un warring blender simple mixer m nager MCB 140 2 16 05 Cours de Physiologie et de g n tique bact riennes 1957 Illustration humoristique des cours par une s rie de dessins de …
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