Chapter 8 Part Two: DNA Mutations and Horizontal Gene Transfer

DNA Mutations and Repair Mechanisms

• Types of Radiation:

  • Ionizing Radiation: X-rays, gamma rays; break DNA backbone.
  • UV Radiation: Causes thymine dimers, incorrect binding of thymines.

• Repair Mechanisms:

  • Photolyases: Enzymes that separate thymine dimers.
  • Nucleotide Excision Repair: Involves endonucleases, DNA polymerase, and DNA ligase to correct mismatched bases and repair damage.

Identifying Mutants and Chemical Carcinogens

• Mutants: Cells with mutations detectable by:

  • Positive selection (mutants grow/look different).
  • Negative selection (mutants can't grow/function).

• Ames Test: Assesses mutagenicity of substances by exposing mutant bacteria to mutagens.

Genetic Recombination and Horizontal Gene Transfer

• Genetic Recombination: Exchange of genes between DNA molecules, increasing diversity.

  • Crossing Over: Exchange of chromosomal segments, introducing foreign DNA.

• Horizontal Gene Transfer:

  • Transfer of genes between the same generation of cells.

Types of Horizontal Gene Transfer

Plasmids and Transposons

• Plasmids:

  • Circular, self-replicating DNA, independent of chromosomal DNA.
  • Carry genes for antibiotic resistance, pathogenicity.
  • Can transfer between bacterial cells, spreading resistance.

• Transposons (Jumping Genes):

  • DNA segments that move within and between DNA molecules.
  • Contain insertion sequences coding for transposase enzyme.
  • Can carry antibiotic resistance genes.

Transformation

• Definition: Uptake of naked DNA from the environment by bacteria.

  • Leads to genetic recombination, increasing genetic diversity.

• Griffith Experiment: Demonstrated transformation in bacteria using encapsulated and non-encapsulated strains.

Conjugation

• Process: Transfer of plasmids between bacterial cells via sex pili.
  • F Factor/Plasmid: Determines F+ or F- status of cells.
  • Can integrate into chromosome (HFR cell).

Transduction

• Mechanism:
  • Involves bacteriophages transferring DNA between bacteria.
  • Generalized Transduction: Random bacterial DNA packaged by phages.
  • Specialized Transduction: Specific genes transferred.

Summary

• Horizontal gene transfer and genetic recombination contribute to microbial diversity and evolution.
• These processes enhance bacterial resistance, pathogenicity, and survival, affecting natural selection and evolution.