Bio March 10

Overview of Chapter Six

• Chapter focuses on studying genetics in bacteria and viruses that infect immune cells.

• Emphasis on basic theory with no calculations required for this chapter.

• Objectives:

  • Working with microorganisms.

  • Understanding genetic exchange in bacterial cells.

Working with Microorganisms

• Importance of Microorganisms:

  • Practical applications in research.

  • Rapid generation of colonies from single bacterial cells, enabling quick experimental results.

  • Ability to survive in diverse environments; they have existed far longer than eukaryotes.

• Bacterial Cell Characteristics:

  • Bacterial cells lack compartmentalization; they operate as "bags of soup" without organelles.

  • Genome structure differs from eukaryotes:

    • Eukaryotes: Linear chromosomes.

    • Bacteria: Circular genome.

  • No meiosis; they exhibit simpler crossover mechanisms than eukaryotes.

  • Exhibiting phenotypes easily due to their rapid growth and ability to utilize various nutrients.

Nutritional Requirements and Growth Patterns

• Bacteria are autotrophs capable of synthesizing macromolecules from basic compounds.

• Auxotrophs: Bacterial strains unable to synthesize specific compounds; require external supply (e.g., biotin).

• Examples of carbon sources utilized by bacteria:

  • Lactose, sucrose, mannose, etc.

  • Some strains may be unable to utilize specific sugars but can grow on others (e.g., lactose).

Resistance and Mutation

• Rapid mutations in bacterial cells lead to antibiotic resistance.

• Growth mediums help differentiate between resistant and sensitive strains:

  • Selective Medium: Only resistant cells grow in the presence of antibiotics.

  • Differential Medium: All grow, but specific compounds indicate genotype presence (e.g., lactose fermentation).

Growth Medium Types

• Two main growth mediums:

  1. Selective Medium: Suppresses growth of unwanted cells (e.g., with antibiotics).

  2. Differential Medium: Grows all cells but allows differentiation of genotypes based on visual indicators (e.g., color change for lactose).

Techniques for Identifying Bacterial Phenotypes

• Start with liquid cultures to grow bacteria; then transfer to selective/differential plates.

• Stamping technique allows tracking specific colonies and ensures orientation consistency between replica plates.

Identifying Auxotrophs

• If a colony does not grow in minimal medium, it is identified as an auxotroph and cannot synthesize certain nutrients necessary for survival.

• Process also used to identify auxotrophs for various compounds like arginine and threonine.

Bacterial Genetic Exchange Mechanisms

• Three main methods of DNA transfer in bacteria:

  1. Transformation: Uptake of free DNA from the environment.

     • Bacterial cells can take up plasmids or DNA fragments through cell membranes.

  2. Conjugation: Direct DNA transfer between two bacterial cells via physical contact.

     • Requires formation of pili for connection.

  3. Transduction: DNA transfer mediated by viruses (bacteriophages) infecting a bacterium.

Experimentation and Gene Transfer Evidence

• Key Experiments:

  • E. coli strains demonstrated successful gene transfer through conjugation.

  • Donor and recipient strains were assessed for genetic recombination, confirming unidirectional transfer.

Conjugation Mechanism

• Bacterial conjugation involves a F (fertility) factor, enabling the formation of pili that facilitate DNA transfer between cells.

• Transfer process involves:

  • Nicking of one strand of the plasmid.

  • The single strand is transferred to the recipient cell,

  • Both donor and recipient cells replicate to complete the DNA strand.

Conclusion

• Understanding these mechanisms lays a foundation for studying genetic interactions in bacteria and their evolvement, especially resistance to antibiotics.