Learning Objective

Describe the structure, replication, integration, and gene functions of plasmid DNA, including episomes and clinically important plasmid-encoded traits.

Plasmids

Plasmids are extrachromosomal genetic elements found primarily in bacteria (and some eukaryotes).

Key Characteristics

• Covalently closed, circular DNA molecules
• Small: ~1.5–400 kB
• Replicate autonomously (own origin of replication)
• Usually nonessential, but they provide selective advantages

Episomes

• A subclass of plasmids.
• Have insertion sequences homologous to bacterial chromosomal IS elements.
• It can integrate into the bacterial chromosome.
• Once integrated, → behaves like chromosomal DNA.

Activity

Genes Commonly Carried by Plasmids

Plasmids encode nonessential but beneficial traits, including:

• Fertility genes (tra operon) → forms F pilus for conjugation
• Antibiotic resistance genes
  • e.g., β-lactamases, aminoglycoside-modifying enzymes
• Most bacterial exotoxins
  • Tetani toxin (tetanus)
  • Anthrax toxin
  • Heat-labile and heat-stable enterotoxins
• Virulence factors
• Metabolic enzymes that improve survival in harsh environments

Activity