Chapter 15: Genetics of Bacteria and Bacteriophages

Genetics of Bacteria and Bacteriophages begins by establishing the importance of E. coli and its viruses in the history of molecular genetics, distinguishing between prototrophs (wild-type strains that can grow on minimal medium) and auxotrophs (mutant strains requiring nutritional supplements). The text details three primary methods of bacterial gene transfer: conjugation, transformation, and transduction. Conjugation is described as a unidirectional transfer of genetic material mediated by the F (fertility) factor, a plasmid that can exist independently or integrate into the bacterial chromosome to form High-frequency recombination (Hfr) strains. The summary explains how interrupted-mating experiments with Hfr strains allow geneticists to map genes based on their time of entry into a recipient cell, ultimately revealing the circular nature of the E. coli chromosome. The concept of F-duction (sexduction) involving F prime factors is also covered. The discussion then shifts to transformation, where competent bacteria take up extracellular DNA, and how cotransformation frequencies are utilized to determine linkage and gene order. The third mechanism, transduction, is explored through the life cycles of bacteriophages, specifically distinguishing between the lytic cycle (virulent phages) and the lysogenic cycle (temperate phages like lambda). The chapter differentiates between generalized transduction, where random bacterial DNA is packaged into phage heads, and specialized transduction, where specific genes near the prophage attachment site are transferred due to imprecise excision. Furthermore, the text delves into the genetic mapping of bacteriophages using recombination frequencies derived from plaque morphology and host range mutants. A significant portion of the chapter is dedicated to Seymour Benzer's fine-structure analysis of the rII region in bacteriophage T4. This section explains how deletion mapping and the cis-trans (complementation) test were used to define the cistron as the unit of function and to demonstrate that the unit of mutation and recombination is the individual DNA base pair, rather than the entire gene.