Ch 9

Chapter 9: Bacterial and Viral Genetic Systems

9.1 Bacteria and Viruses Have Important Roles in Human Society and the World Ecosystem

Significance

Bacteria and viruses play pivotal roles in various aspects of human society and the ecosystem, including:

  • Bacterial Function in Oceans: Bacteria contribute to nutrient cycling and primary production in marine ecosystems.
  • Agriculture: Certain bacteria enhance soil fertility and plant growth, while others can cause plant diseases.
  • Natural Flora: Beneficial bacteria form part of the human microbiome, aiding digestion and providing immune support.
  • Disease: Pathogenic bacteria and viruses are responsible for numerous diseases in humans, animals, and plants.
  • Commercial Products: Bacteria are utilized in the production of antibiotics, enzymes, and fermented foods.

9.1 Bacterial and Viruses Have Important Roles in Human Society and the World Ecosystem (2 of 2)

Prokaryotes Overview

All prokaryotic organisms are unicellular and lack membrane-bound nuclei and organelles. They include:

  • Eubacteria: True bacteria with diverse characteristics and functions.
  • Archaea: Prokaryotes often found in extreme environments, distinct from eubacteria.
Characteristics of Bacteria
  • Exhibit diverse shapes and sizes.
  • Some possess photosynthetic abilities.
  • Replication occurs via binary fission after DNA replication.

9.2 Genetic Analysis of Bacteria Requires Special Approaches and Methods (1 of 2)

Bacterial Growth Techniques
  • Prototrophic: Refers to wild-type bacteria capable of synthesizing all necessary nutrients.
  • Auxotrophic: Refers to mutant bacteria that are unable to synthesize certain nutrients.
Media Types
  • Minimum Medium: Contains only substances required by prototrophic bacteria for growth.
  • Complete Medium: Contains all substances needed for growth, accommodating both prototrophic and auxotrophic bacteria.

9.2 Genetic Analysis of Bacteria Requires Special Approaches and Methods (2 of 2)

Bacterial Genome
  • Bacterial genomes are primarily a single, circular DNA molecule known as a chromosome (refer to Fig. 9.4).
  • Plasmids: Small, circular DNA molecules separate from the chromosomal DNA, often involved in gene transfer and various physiological functions.
    • Episomes: A type of plasmid that can exist independently or integrate into the bacterial chromosome.
    • F Factor: A specific type of plasmid that carries genes necessary for conjugation.
Concept Check 1
  • Correct Answer: They replicate independently of the bacterial chromosome.

9.3 Bacteria Exchange Genes Through Conjugation, Transformation, and Transduction (1 of 9)

Gene Transfer Mechanisms
  • Conjugation: A process where DNA is directly transferred from one bacterium to another.
  • Transformation: The uptake of free DNA from the environment by a bacterium.
  • Transduction: The transfer of DNA from one bacterium to another via bacteriophages.
Sub-processes of Gene Transfer
  • Conjugation Details:
    • The connection occurs through a cytoplasmic bridge between donor and recipient cells.
    • This process is characterized by one-way DNA transfer without reciprocal exchange of genetic material.
Concept Check 2
  • Correct Answer: Transduction.

9.3 Bacteria Exchange Genes Through Conjugation, Transformation, and Transduction (2 of 9)

Conjugation in Detail
  • F+ Cells: Donor cells that possess the F factor.
  • F- Cells: Recipient cells lacking the F factor.
  • Sex Pilus: A tubular structure through which DNA is transferred during conjugation.

9.3 Bacteria Exchange Genes Through Conjugation, Transformation, and Transduction (3 of 9)

Experimental Evidence
  • Lederberg and Tatum Experiment: Demonstrated genetic exchange between auxotrophic bacterial strains.
    • Auxotrophic Strains:
    • Strain Y10: Cannot synthesize Thr, Leu, or Thi.
    • Strain Y24: Cannot synthesize biotin, Phe, or Cys.
    • Mixing Y10 and Y24 resulted in some prototrophic colonies, showing recombination occurred and indicating that genetic exchange takes place.
Davis’s U-tube Experiment
  • Showed direct contact between bacterial cells is necessary for genetic exchange.
  • Filters prevented bacterial mixing, resulting in no prototrophic colonies, confirming that cell contact is essential.

9.3 Bacteria Exchange Genes Through Conjugation, Transformation, and Transduction (4 of 9)

Hfr Cells
  • Hfr Cells: Donor cells where the F factor is integrated into the bacterial chromosome, allowing for high-frequency recombination.
  • F’ Cells: Contain an F plasmid with some bacterial genes; usually resulting in partial diploid cells known as merozygotes.

9.3 Bacteria Exchange Genes through Conjugation, Transformation, and Transduction (5 of 9)

Interrupted Conjugation
  • Mapping genes through the time taken to transfer DNA helps determine relative positions of genes on the chromosome.
  • Genes are considered closer together if they take less time to transfer.

9.4 Bacterial Defense Mechanisms

Defense Systems
  • Restriction-Modification Systems: Utilize restriction enzymes to cut foreign DNA, protecting against intrusions like phage infections.
  • CRISPR-Cas Systems: A bacterial immune response that uses RNA-based memory to target foreign genetic elements.
    • Three Steps:
    1. Adaptation
    2. Expression
    3. Interference

9.5 Viruses Are Simple Replicating Systems Amenable to Genetic Analysis

General Characteristics of Viruses
  • A virus consists of a replicating structure (either DNA or RNA) surrounded by a protein coat.
  • Bacteriophage: Viruses that specifically infect bacteria, can be virulent (lytic cycle) or temperate (lysogenic cycle).
Lytic and Lysogenic Cycles
  • In the lytic cycle, the host cell is destroyed after reproduction of phages.
  • In the lysogenic cycle, phage DNA integrates into bacterial chromosomes and replicates with it.
Concept Check 6
  • Correct Answer: Lysogenic.

9.5 Viruses Are Simple Replicating Systems Amenable to Genetic Analysis (9 of 9)

Gene Mapping and Viral Genetics
  • Gene mapping in phages is done through counting recombinant plaques to infer gene positions and distances.
  • Retroviruses: RNA viruses that integrate into host genomes; reverse transcriptase synthesizes DNA from the RNA template.
Influenza Virus
  • Three types of influenza A, B, and C exist, with type A most commonly causing pandemics.
  • New strains arise due to antigenic shift.
Rhinoviruses
  • Small RNA viruses responsible for common colds, with significant genetic variation through recombination.