Chapter 18 Gene Mapping In Bacteria and Bacteriophages

Abstract

1. Bacteria transfer genetic material by three different processes. In all cases, transfer is unidirectional, and no complete diploid stage is formed. The processes are: a. Conjugation. b. Transformation. c. Transduction. 2. Much of the study of genetic transfer has been done in E. coli. Some important features of this organism are: a. It grows readily on defined medium that is either solid or liquid. b. It is easily manipulated using standard microbiology techniques. c. It can be titered by dilution and plating on solid medium. 3. Mutations in biosynthetic pathways are identified using minimal medium. The minimal medium for a particular species supports growth of wild-type cells (prototrophs), which are able to synthesize everything else needed from the precursors in the minimal medium. a. Auxotrophs are mutants that are unable to synthesize all needed nutrients. Auxotrophs thus do not grow on minimal medium. b. If the minimal medium is supplemented with the nutrient that the auxotroph is unable to synthesize, the auxotroph will be able to grow. c. For example, E. coli with the genotype trp ade thi+ will be unable to grow on minimal medium unless it is supplemented with tryptophan and adenine. It has the wild-type allele for thiamine and does not need supplementation of this vitamin. 4. Mutations also occur in utilization pathways, preventing the mutant from using various nutrients. a. For example, E. coli with the genotype lac+ will be able to use lactose as a carbon source, while a mutant strain (lac) cannot. b. Varying nutrients in media can detect such mutations. 5. To detect parental and progeny phenotypes in E. coli, colonies are tested for their growth requirements. Replica plating is useful, because otherwise it is difficult to recover bacteria whose phenotype is “no growth” on a particular medium.