Bacterial genome structure

Bacterial Chromosome - Naked & Circular

• Structure: Single, circular, double-stranded DNA molecule located in the cytoplasm.
• Location: Contained within the nucleoid, a non-membrane-bound region. This contrasts with the eukaryotic nucleus.
• Composition:
  • Naked DNA: Lacks histones and a nuclear membrane, unlike eukaryotes.
  • Associated with histone-like proteins for packaging and condensation.
• Ploidy: Haploid (one copy of the chromosome), meaning mutations have an immediate phenotypic effect.
• Supercoiling:
  • The chromosome is highly folded and compacted into a supercoiled domain.
  • Maintained by topoisomerase enzymes, such as DNA gyrase.

⭐ Exam Favorite: DNA gyrase (a type II topoisomerase) is the primary target for fluoroquinolone antibiotics (e.g., ciprofloxacin), which inhibit DNA replication by trapping the enzyme-DNA complex.

Plasmids - Resistance Rings

• Extrachromosomal, circular, double-stranded DNA molecules; replicate independently of the bacterial chromosome.
• Carry non-essential genes that often confer a survival advantage.
  • Antibiotic Resistance: R-factors (resistance plasmids) are a primary source of multi-drug resistance (MDR).
  • Toxin Production: e.g., Shiga-like toxin, heat-labile (LT) and heat-stable (ST) enterotoxins in E. coli.
  • Pilus Formation: F-plasmids (fertility factors) enable genetic transfer via conjugation.

⭐ Plasmids are key vectors in genetic engineering (e.g., insulin production) and are the main drivers for the rapid horizontal spread of antibiotic resistance among bacterial populations.

Transposons - Mobile DNA Mayhem

• "Jumping genes": Segments of DNA that can move from one location in a genome to another (intramolecular) or between different DNA molecules (intermolecular), like from a plasmid to a chromosome.
• All transposons encode the enzyme transposase, which mediates their movement.
• Types:
  • Insertion sequences (IS): Simplest form, containing only the transposase gene flanked by inverted repeats.
  • Complex transposons: Carry additional genes, such as those for antibiotic resistance, flanked by two insertion sequences.

⭐ Vancomycin resistance in S. aureus (VRSA) is often acquired from Enterococcus faecalis (VRE) via conjugation involving transposon Tn1546, which carries the vanA gene cluster.

High‑Yield Points - ⚡ Biggest Takeaways

• Bacterial genomes are typically a single, circular chromosome located in the nucleoid.
• They are haploid, meaning they have only one copy of each gene.
• Bacterial DNA lacks introns and histones; it is compacted by supercoiling via enzymes like DNA gyrase.
• Plasmids are small, circular, extrachromosomal DNA molecules that replicate independently.
• Plasmids often carry non-essential genes, such as those for antibiotic resistance or virulence factors.