Bacterial and Archaeal Genetics — Horizontal Gene Transfer & CRISPR Defense | Chapter 9 of Brock Biology of Microorganisms

Welcome to Last Minute Lecture. In this post, we summarize Chapter 9 of Brock Biology of Microorganisms, which explores how microbes generate genetic diversity and defend themselves against foreign DNA. You’ll learn about mutation types, DNA repair, and horizontal gene transfer via transformation, transduction, and conjugation. Plus, we cover mobile DNA elements and the powerful CRISPR–Cas defense system.

🎥 Watch the full podcast summary here:

This chapter is essential for understanding microbial evolution, genetic engineering, and biotechnology applications.

Mutations and Genetic Variation

• Mutation = heritable change in DNA sequence
• Types:
  • Point mutations: missense, nonsense, silent
  • Frameshift: insertions or deletions
  • Transitions (purine→purine) vs. transversions (purine↔pyrimidine)
  • Reversions and suppressor mutations restore phenotype
• Mutagens: base analogs, alkylating agents, UV, ionizing radiation
• Repair: error-prone SOS system and high-fidelity pathways

Genetic Recombination

• RecA protein mediates strand invasion and homologous recombination
• Recombination stabilizes foreign DNA and promotes diversity
• Complementation restores mutant function with wild-type gene
• Merodiploids carry duplicate copies of specific genes

Transformation: DNA Uptake from Environment

• Natural or artificial competence required
• Pili mediate DNA binding and uptake
• Incoming DNA becomes single-stranded and recombines with host chromosome via RecA

Transduction: Virus-Mediated Gene Transfer

• Generalized transduction: any gene accidentally transferred during lytic cycle
• Specialized transduction: specific genes transferred due to incorrect prophage excision
• Phage conversion: prophage alters host traits
• Gene Transfer Agents (GTAs): defective phage-like particles that transfer host DNA

Conjugation: Cell-to-Cell DNA Transfer

• F plasmid encodes conjugation machinery and pilus
• F+ donor transfers DNA to F– recipient via rolling circle replication
• Hfr strains = F plasmid integrated into chromosome → transfer chromosomal genes
• F′ plasmids carry chromosomal genes due to excision errors

Archaeal Gene Transfer

• Transformation and conjugation occur
• Mechanisms include vesicle fusion, pili, and nanotube bridges
• Marker selection must account for unique archaeal biology

Mobile DNA: Transposable Elements

• IS elements: small, encode transposase only
• Transposons: larger, may carry antibiotic resistance or other traits
• Mechanisms: conservative (cut-paste) or replicative (copy-paste)
• Widely used for mutagenesis and gene tagging in research

CRISPR–Cas: Adaptive Immunity in Prokaryotes

• CRISPR locus = repeats + spacers from past invaders
• Cas proteins use crRNAs to recognize and cleave incoming DNA
• Immunization adds new spacers from infections
• Phages counteract with anti-CRISPR proteins or PAM mutations
• CRISPR–Cas now used in gene editing and genome engineering

Glossary Highlights

• Mutation: heritable DNA sequence change
• Transformation: DNA uptake from environment
• Transduction: DNA transfer via bacteriophage
• Conjugation: DNA transfer through pilus contact
• RecA: recombination mediator protein
• F plasmid: encodes conjugation system
• Hfr cell: high-frequency recombination donor
• F′ plasmid: plasmid with chromosomal genes
• Transposon: mobile element with extra genes
• CRISPR: adaptive immune system in microbes
• Spacer: foreign DNA memory in CRISPR array
• Rolling Circle Replication: plasmid replication method during conjugation

Conclusion

Chapter 9 of Brock Biology of Microorganisms unveils the remarkable genetic flexibility of Bacteria and Archaea. From DNA repair and mutation to horizontal gene transfer and defense systems like CRISPR, these tools are central to microbial evolution and modern biotechnology.

🎬 Watch the video above for a complete podcast-style summary of this high-yield chapter.

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