Genetic Diversity and Transformation in Prokaryotes

Learning Objectives

• Define genetic diversity and its significance for survival and adaptation.
• Distinguish vertical gene transfer from horizontal gene transfer.
• Describe the spontaneous emergence of mutations.
• Explain the role of DNA polymerase error rates in genetic diversity.
• Discuss environmental factors contributing to genetic diversity.
• Outline the three principal mechanisms of horizontal gene transfer.
• Summarize the transformation mechanism.

What is Genetic Diversity?

• Definition: Genetic diversity refers to the variation in DNA sequences within and among populations.
• Importance:
  • Essential for species survival and adaptation to changing environments.
  • Newly acquired genes may provide a selective advantage, increasing chances of survival and reproduction.
  • Example: A genetic bottleneck (a drastic reduction in population) can reduce diversity, affecting resilience against threats (e.g., antibiotics).

Genetic Diversity: Eukaryotes vs. Prokaryotes

• Prokaryotes:
  • Reproduce via binary fission, resulting in clonal offspring (identical genetic information).
• Eukaryotes:
  • Utilize meiosis, sexual recombination, and mobile genetic elements, fostering greater genetic diversity.

Mechanisms of Genetic Diversity in Prokaryotes

1. Mutations:
   • Spontaneous changes in DNA sequence due to replication errors.
   • Passed on to daughter cells via vertical gene transfer.
2. Horizontal Gene Transfer (HGT):
   • Transfers genetic material between organisms, enhancing genetic diversity.
     • Transformation: Uptake and integration of foreign DNA.
     • Transduction: DNA transfer via bacteriophages.
     • Conjugation: Plasmid exchange between cells.

Mutations as a Source of Genetic Diversity

• Spontaneous Rate: E. coli has approximately 4.6 million bases and an error rate of one mistake per billion bases, leading to the emergence of mutations, especially given rapid replication (every ~20 minutes).
• Environmental Factors:
  • Reactive Oxygen Species can damage DNA (e.g., Thymidine and Guanosine).
  • Electromagnetic Radiation (X-rays, UV) may cause DNA breaks or mutations (e.g., pyrimidine dimers).
  • Chemical Mutagens may alter nitrogenous bases, affecting replication and function.

Horizontal Gene Transfer Mechanisms

1. Transformation:
   • Import of free "naked" DNA from the environment into bacterial cells.
   • Occurs in both Gram-positive and Gram-negative bacteria but not universally.
2. Transduction:
   • Introduction of foreign DNA via bacteriophages.
3. Conjugation:
   • Direct transfer of DNA/plasmids from a donor to a recipient, requiring cell contact and specialized machinery.

Transformation Mechanism

• Definition: The process by which bacteria take up free DNA from their surroundings.
• General Mechanism:
  • Results in the donor's DNA replacing part of the recipient's DNA.

Competence in Transformation

• Natural Competence: Some bacteria are naturally competent, meaning transformation is a part of their biology (requires a transformasome).
• Laboratory Induced Competence:
  • Techniques include:
  • Heat-shock with CaCl2
  • Electroporation for membrane permeabilization.

Transformation in Gram-positive Bacteria

• Process:
  • Requires a competence factor (CF) released at high cell densities.
  • Involves a series of steps leading to the assembly of a transformasome, allowing the uptake of DNA.
• Components of Transformasome:
  • Binding proteins, transmembrane pore-formers, nucleases (for degradation), and transporters (for DNA acquisition).

Transformation in Gram-negative Bacteria

• Key Differences:
  • Additional outer membrane barrier that needs to be crossed.
  • Some Gram-negative bacteria are always competent; others become competent during nutrient starvation.
  • Specificity of DNA uptake can be sequence-specific, constrained by recognition processes involving pili.

Antibiotic Resistance through Transformation

• Mechanism Outline:
  1. Binding: A DNA fragment with a resistance gene binds to surface receptors.
  2. Degradation: Bacterial enzymes cut DNA into smaller fragments.
  3. Uptake: One strand is imported, while the other is degraded.
  4. Incorporation: The imported DNA is integrated into the bacterial chromosome, potentially conferring antibiotic resistance.
• Example: Transformation can convert a tetracycline-sensitive bacterium to one that can grow in the presence of tetracycline.