Bacterial Genetics

How is bacterial DNA organized compared to eukaryotic DNA?

Bacteria: single circular chromosome, no nuclear membrane, compact nucleoid, ~4,300 genes, 87% coding DNA. Eukaryotes: multiple linear chromosomes, histones, introns, ~1.5% coding DNA.

What are exceptions to the single chromosome rule in bacteria?

Vibrio cholerae has >1 chromosome; Myxococcus is multicellular with complex genome organization.

What are plasmids and why are they clinically important?

Extrachromosomal, circular dsDNA; autonomously replicate; can carry antibiotic resistance, toxin genes, virulence factors, novel metabolic functions.

What are transposons?

Mobile genetic elements; can move within genome by cut-and-paste (conservative) or copy-and-paste (replicative); often carry resistance genes (e.g., Tn3 with β-lactamase, Tn10 with tetracycline resistance).

What is an operon?

Cluster of genes under one promoter/operator; allows coordinated regulation. Example: lac operon (induced by lactose, enhanced by cAMP-CAP when glucose low).

What is the trp operon?

Genes for tryptophan synthesis; repressed when tryptophan high; attenuation mechanism fine-tunes expression.

What is the van operon?

Encodes vancomycin resistance (vanHAX); alters peptidoglycan D-Ala-D-Ala → D-Ala-D-Lac, preventing vancomycin binding.

How does bacterial replication occur?

Bidirectional from a single origin; replication forks proceed until termination; tightly coupled to cell division.

What are the two main sources of genetic diversity in bacteria?

Spontaneous mutations (vertical transmission, slow) and horizontal gene transfer (rapid).

What is gene amplification and why is it important?

Duplication of genes increases dosage; e.g., Vibrio cholerae ctx gene amplification → more cholera toxin; Staph aureus efflux pump duplication → multidrug resistance.

What is vertical transmission?

Genetic information passed from parent to daughter cells during binary fission.

What is horizontal transmission?

Gene transfer between unrelated bacteria; rapid acquisition of new traits (e.g., resistance, virulence).

What is transformation?

Uptake of naked DNA from environment; requires competence. Example: Streptococcus pneumoniae acquiring capsule genes → virulence. (dies and is absorbed)

What is transduction?

Phage-mediated gene transfer. Generalized: random bacterial DNA packaged. Specialized: specific genes near phage integration site transferred.

What is conjugation?

Plasmid-mediated transfer via sex pilus (Gram-negative) or adhesins (Gram-positive). F+ donor transfers F plasmid to recipient → both become F+.

What is transposition?

Movement of transposable elements within genome or plasmids; can spread resistance genes (e.g., Tn10 tetracycline resistance).

What is lysogenic conversion?

Integration of prophage DNA into bacterial genome → new properties (e.g., toxin production).

What is the difference between lytic and lysogenic phages?

Lytic: immediate replication, host cell lysis. Lysogenic: phage integrates as prophage, replicates with host, can later induce lytic cycle.

What is the clinical relevance of horizontal gene transfer overall?

Drives emergence of multidrug-resistant organisms (MRSA, VRE, NDM-1 Enterobacteriaceae) and new virulent strains (capsulated pathogens, toxin-producing bacteria).