This laboratory investigates one form of genetic recombination in bacteria. This process, called conjugation, occurs when one bacteria transfers DNA to another bacteria. Two different strains of Escherichia coli are used: an Hfr strain with the F factor integrated into the bacterial chromosome acting as the donor, and an F-strain lacking the fertility factor acting as a recipient. The F-strain is auxotrophic for certain genetic markers and the ordered transfer of markers from the Hfr strain to the F-strain is used to map gene locations on the bacterial chromosome.
This protocol represents a cost-effective modification of the Ames Test that allows students to investigate the mutagenic potential of various common substances. Potential mutagens are tested using well-characterized auxotrophic strains of Salmonella typhimurium. By analyzing the results, students determine if any of their compounds may be mutagenic. Follow-up experiments are designed to determine the dose response of these potential mutagens. Using this protocol, we have achieved reproducible results with several known mutagens, including sodium azide and ultraviolet irradiation. This approach enables faculty to control costs and results to improve student understanding of mutagenesis, biochemical pathways, experimental design, and data analysis.
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