Chapter 8: Genetics of Bacteria & Viruses

The field of molecular genetics fundamentally relies on bacteria and viruses as crucial model systems due to their rapid reproduction, small size, and ease of detecting heritable variations, which have enabled deep analysis into the molecular mechanisms governing genes and chromosomes. This chapter initiates the discussion by highlighting the serious public health threat posed by multi-drug-resistant (MDR) bacteria, such as Mycobacterium tuberculosis, whose rapid evolution is driven by genetic exchange mechanisms. The chapter differentiates between virulent bacteriophages like T4, which follow a destructive lytic pathway resulting in host cell lysis, and temperate phages such as lambda (λ), which can either engage in the lytic cycle or enter a non-destructive lysogenic pathway. In lysogeny, the λ chromosome integrates into the bacterial genome as a prophage via site-specific recombination. Bacterial genetics utilizes distinct mutant types, including auxotrophs (requiring supplementary nutrients) and prototrophs (wild-type growers), alongside drug-resistant strains. Genetic transfer in bacteria is unidirectional, typically resulting in partial diploids in the recipient cell, and occurs through three distinct parasexual processes. Transformation involves the uptake and integration of naked exogenous DNA by competent cells. Conjugation requires direct cell contact facilitated by F pili, often controlled by the F factor (a fertility episome or plasmid). Donor cells (F+ or Hfr, carrying an integrated F factor) transfer genetic material, enabling scientists to construct linkage maps of the circular E. coli chromosome calibrated in minutes through interrupted mating experiments. Additionally, F' factors, formed by the anomalous excision of the integrated F factor, can be transferred by sexduction to create useful partial diploids. The third process, transduction, uses a phage intermediary to move bacterial genes: generalized transduction transfers random genomic fragments, while specialized transduction transfers only specific genes adjacent to the prophage insertion site, exemplified by λ carrying gal or bio genes. These processes collectively accelerate the genetic diversification and adaptive capability of bacteria in response to environmental pressures.