Bacterial Genetics Study Notes

Bacterial Genetics Study Notes

Lecture Overview

  • Objectives of the Lecture
      - By the end of the lecture, students should be able to:
        - Describe the bacterial genetic architecture.
        - Explain the basis of variation in bacteria.
        - Explain the various processes responsible for creating genetic variation in bacteria.

Bacterial Structure

  • Bacterial Structural Features
      - Bacteria are considered haploid, meaning they contain a single copy of their genetic material.

Genetic Mutation

  • Knowledge Check 1:
      - A scientist observes that a bacterium acquires resistance to an antibiotic after being infected by a phage. This process is an example of:
        - C) Transduction

  • Definition of Mutation
      - A mutation is defined as a change in the base sequence of DNA that can result in the insertion of a different amino acid or a stop codon into a protein and the appearance of an altered phenotype.

Types of Mutation

  1. Base Substitution
       - Occurs during DNA replication due to DNA polymerase errors or mutagen-induced alterations.
       - Examples:
         - Normal Sequence:
           - DNA: AAA ATG CII CIC
           - mRNA: UUU AUG CAA GAG
           - Amino Acids: Phe, Tyr, Glu, Glu
         - After Substitution:
           - DNA: AAA ATG TTT IS
           - mRNA: UUU
           - Amino Acids: Phe, Tyr, Lys, Glu
       - Types of Base Substitutions:
         1. Missense Mutation
         2. Nonsense Mutation

  2. Frame Shift Mutation
       - Occurs when one or more base pairs are added or deleted, leading to a misalignment of the ribosomal reading frame.
       - Results in the incorporation of incorrect amino acids downstream, often leading to inactive proteins.
       - Example:
         - Normal Sequence:
           - DNA: AAA ATG CTT STC
           - Amino Acids: Phe, Tyr, Glu, Glu
         - After Deletion:
           - DNA: A A A
           - Amino Acid Sequence Changes

  3. Transposon or Insertion Sequences Mutations
       - Result when transposons insert into the DNA, leading to significant changes in the genes affected.

Causes of Mutations

  • Mutations can arise from:
      - Chemicals
      - Radiation
      - Viruses

Mechanisms of Mutations

Types of Inducing Agents:

  1. Benzpyrene

  2. X-Rays

  3. 5-Bromo Uracil

  4. Nitrous Acid and Alkylating Agents

  5. UV Radiations

  • Each agent leads to different mutational mechanisms, e.g., base alterations, frame shifts, and dimers.

Virus-Induced Mutations

  • Bacterial viruses, particularly Mu, can cause a significantly high frequency of mutations by integrating viral DNA into the bacterial chromosome.

Conditional Mutations

  • Conditional Lethal Mutations
      - Useful in producing vaccines (e.g., influenza vaccine).
      - Expressed only under specific conditions, leading to variations such as temperature-sensitivity.

  • Mechanism (Temperature-Sensitive Mutations):
      - Function at lower permissive temperatures (e.g., 32°C) but fail at higher restrictive temperatures (e.g., 37°C) due to changes in protein conformation.
      - An example of this is an influenza virus strain that replicates at 32°C but is ineffective at 37°C, thus preventing pneumonia.

Transfer of DNA within Bacterial Cells

Transposons

  • Transposons can move DNA within the bacterial chromosome or to a plasmid. This activity contributes to antibiotic resistance spread.

Programmed Rearrangements

  • Consist of moving genes from inactive storage to active expression loci, affecting antigen presentation in bacteria such as Neisseria gonorrhoeae.

Consequences of DNA Transfer

  1. Antibiotic resistance is primarily spread by conjugation.

  2. Important exotoxins are transferred through transduction.

Conjugation

  • Defined as the mating process of two bacterial cells allowing DNA transfer from donor to recipient controlled by F plasmid, which harbors conjugation-related genes.

  • R plasmids, capable of encoding antibiotic-degrading enzymes and resistance mechanisms, also transfer via conjugation (e.g., β-lactamases).

Transduction

  • Defined as the transfer of bacterial DNA via bacteriophages.

Transformation

  • Transformation involves the direct transfer of DNA from one bacterial cell to another.

Recombination

  • Post-DNA transfer, the integrated DNA can undergo recombination, with types as follows:

  1. Homologous Recombination
       - Requires extensive homology for DNA pairing and exchange.

  2. Nonhomologous Recombination
       - Little to no homology required for the process.

Knowledge Checks

  1. Antibiotic Resistance Gene Discovery
       - Mechanism likely: A) Conjugation

  2. Borrelia recurrentis and Immune Evasion
       - Likely Mechanism: C) Programmed rearrangements

Conclusion

  • Understanding of bacterial genetics, mutations, and transfer mechanisms is critical for advancements in medical microbiology and therapies.