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One of the major goals of genetic studies is to understand the organization and function of genes. To achieve this, alterations in DNA are created, either spontaneously or by the experimenter, fished out in some fashion, and the effects of these changes are observed. It is analogous to systematically breaking the pieces of a radio, seeing what happens, and from this information deducing the function of each component.

Bacterial genetics has helped to unravel the mysteries of DNA replication, the production of proteins from DNA, the genetic code, gene regulation and more! Genetic analysis will impact your life in many ways. The human genome project (which will sequence all 23 of the human chromosomes) involves the use of sophisticated bacterial genetic techniques. A growing list of mammalian and other proteins that are naturally found in extremely low amounts, are being expressed in microorganisms and purified including 1) human insulin-produced in bacteria for diabetics, 2) human growth hormone for treating dwarfism, and 3) proteins for the production of vaccines against diseases. On a more familiar level, bacterial genetics is being employed to improve the strains used to make cheese, beer, yogurt, and other fermented food products.

Here are a few key terms to help you understand bacterial genetics. A mutant is a strain that has an altered growth property (termed the phenotype) when compared to a designated bench mark strain, which is referred to as the wild-type strain (wt). A mutation is an alteration in the DNA sequence of an organism. Some mutations will cause a detectable change in its phenotype, others will not and are called silent mutations. The genotype of a microorganism is its DNA sequence. For example, if the wild-type strain is mutated so that it is unable to synthesize its own tryptophan, it is referred to as a tryptophan minus mutant (its phenotype is termed Trp^-). Its genotype is the specific base pair change that has taken place and this can be determined by DNA sequencing. If this mutant is plated onto minimal medium lacking tryptophan, most of the organisms will be unable to grow. However, a small fraction of the plated bacteria will revert back to Trp⁺ and form colonies. These are referred to as revertants. They can arise from a conversion of the mutant genotype back to wild-type, or from another secondary mutation that counteracts the initial mutation. Do not confuse a revertant with the wild-type strain, since reversion can also be caused by secondary mutations. An auxotroph refers to a mutant strain that is unable to synthesize a needed nutrient that the wt stain is able to make. For example a Trp^- mutant is termed a tryptophan auxotroph. A prototroph is an organism not requiring the nutrient in question, a wild-type strain is prototrophic. For more information, a good microbiology textbook will have a chapter on Bacterial Genetics.

Two short exercises here will introduce you to the techniques and strategies used when performing genetics. In the first experiment we look at selection of mutants. Taking a wild-type strain and demonstrating the development of drug resistance. In the second experiment, the transfer of genetic traits, the transfer of DNA, from one strain to another by conjugation is demonstrated.

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