substances of commercial value.

of exchanging genetic material with other bacteria and viruses, and it seems that much of this acquired DNA is responsible for its virulence. The gene that encodes the cholera toxin, for example, is found imbedded in a viral genome that infected the bacterium and became a permanent part of its genome long ago. Vibrio cholerae illustrates the importance of gene exchange between bacteria and viruses, a major theme of this chapter.

Since the 1940s, the genetic systems of bacteria and viruses have contributed to the discovery of many important concepts in genetics. The study of molecular genetics initially focused almost entirely on their genes; today, bacteria and viruses are still essential tools for probing the nature of genes in more-complex organisms, in part because they possess a number of characteristics that make them suitable for genetic studies (Table 8.1).

The genetic systems of bacteria and viruses are also studied because these organisms play important roles in human society. They have been harnessed to produce a number of economically important substances, and they are of immense medical significance, causing many human diseases. In this chapter, we focus on several unique aspects of bacterial and viral genetic systems. Important processes of gene transfer and recombination, like those that contributed to the pathogenesis of the cholera bacterium, will be described, and we will see how these processes can be used to map bacterial and viral genes. Information about John Snow and his contributions to the study of cholera

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