Chapter 8: Genetic Mapping in Bacteria & Phages

Genetic Mapping in Bacteria & Phages shifts focus from eukaryotes to examine genetic recombination and mapping in bacteria and their viruses, known as bacteriophages, which serve as invaluable model systems due to their short reproductive cycles and ability to be studied in pure culture. Bacteria typically possess a single circular chromosome and are haploid, meaning all spontaneous mutations are immediately expressed. Genetic variation arises from the distinction between prototrophs (wild-type cells able to synthesize all necessary compounds on minimal medium) and auxotrophs (mutants requiring specific organic supplements). Bacterial genetic exchange, often facilitating horizontal gene transfer (e.g., antibiotic resistance spread via R plasmids), occurs through three mechanisms: Conjugation requires physical cell-to-cell contact, a finding confirmed by the Davis U-tube experiment, and is mediated by the F factor (fertility). The F factor is a circular DNA plasmid and episome. Integration of the F factor into the chromosome creates a High-frequency recombination (Hfr) strain. The sequential, linear transfer of genes during Hfr × F − matings is used for time mapping (interrupted mating technique), which confirmed the circularity of the bacterial chromosome. When an integrated F factor excises imperfectly and carries adjacent chromosomal genes, it forms an F' cell, which transfers this mixed genetic material to F − recipients, creating partially diploid cells called merozygotes. Transformation involves the uptake of small, naked extracellular DNA by a competent recipient cell, where one strand aligns homologously with the host chromosome, resulting in recombination and forming a heteroduplex. Closely linked genes can be transferred simultaneously via cotransformation, a phenomenon utilized for linkage mapping. Transduction is bacterial genetic exchange mediated by bacteriophages. Phages can follow a lytic cycle (reproduction and lysis) or, if they are temperate phages, enter lysogeny by integrating their DNA as a prophage into the host chromosome. Transduction occurs when the phage accidentally packages host DNA, transferring it to a new bacterium (e.g., generalized transduction discovered by Zinder and Lederberg), and simultaneous transfer of linked genes is termed cotransduction. Bacteriophages themselves undergo intergenic recombination (between genes), which can be analyzed using the plaque assay technique. Pioneering fine-structure mapping by Seymour Benzer focused on the rII locus in Phage T4, detecting extremely rare intragenic recombination events (exchange within a single gene). Benzer’s use of specialized selection systems and complementation analysis established that the rII locus was composed of two functional units, known as cistrons.