Genetics / D.J. Cove.

Cove, D. J.

Date:: 1971

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Licence: Attribution-NonCommercial-NoDerivatives 4.0 International (CC BY-NC-ND 4.0)

Credit: Genetics / D.J. Cove. Source: Wellcome Collection.

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6. The genetics of bacteria The bacterial cell shows many structural differences from the eukary- otic cell. It is not surprising therefore that the genetic systems found in bacteria have no counterpart in eukaryotes. However sexual reproduction in both brings about the same end, the pooling of the genes from two different individuals, and their distribution in different combinations to their progeny. For a long time it was not thought that bacteria reproduced sexually. The process of transformation, des¬ cribed in the last chapter (see page 64) certainly caused recombinant progeny to be produced, but it was thought of as a laboratory curiosity rather than a natural method of bringing about recombination. The first detection in bacteria of a more conventional type of sexual process was by J. Lederberg and E. L. Tatum, who published their findings in 1946. These workers set about searching systematically for a process which could give rise to recombinant progeny. They used the bacterium, Escherichia coli, upon which an enormous amount of biochemical research had already been done, arguing that this would be an invaluable foundation for their genetic studies. Lederberg and Tatum thought that bacteria might undergo sexual reproduction in the same way as a unicellular haploid eukaryotic organism does, two cells fusing to form a zygote, which then underwent meiosis. However they reasoned that if such a process existed it must be extremely uncommon, otherwise it would have been detected previously. They therefore devised a selective technique to detect rare progeny with recombinant genotypes. This technique required bacterial stocks which carried several mutations. Since these could not be synthesised by breeding experiments, they utilised sequential mutagenic treat¬ ments. They first treated a wild type with a mutagen, and obtained from it a strain carrying a biochemical mutation leading to a require¬ ment for threonine. This threonine-requiring strain was then grown and treated again with mutagen, and a strain requiring both threonine and leucine was obtained from it. Finally this strain, after it had been 85