Plasmids - Extrachromosomal Power-Ups
Plasmids are small, circular, double-stranded DNA molecules found in bacteria, separate from the bacterial chromosome. These extrachromosomal elements replicate independently and often carry non-essential genes that provide a selective advantage to the host.
- F-plasmids (Fertility): Contain tra genes, which code for the sex pilus, enabling conjugation.
- R-plasmids (Resistance): Carry genes conferring resistance to antibiotics or heavy metals; a major driver of antimicrobial resistance (AMR).
- Col-plasmids (Colicinogenic): Produce colicins, proteins toxic to other bacteria.
- Virulence plasmids: Carry genes that turn a bacterium into a pathogen (e.g., encoding toxins, adhesins).
⭐ Many R-plasmids are conjugative, allowing for rapid spread of resistance genes between bacteria-even across different species-through horizontal gene transfer.
Transposons - Jumping Gene Jamboree
Transposons are mobile DNA segments (“jumping genes”) that move within or between genomes, including plasmids and chromosomes, via the enzyme transposase. They are a major mechanism for transferring genes like those for antibiotic resistance.
- Types of Transposons:
- Simple (Insertion Sequence): Code only for transposase.
- Composite: Carry additional genes (e.g., antibiotic resistance) flanked by insertion sequences.
- Mechanisms of Transposition:
| Feature | Replicative ('Copy & Paste') | Conservative ('Cut & Paste') |
|---|---|---|
| Copies | Transposon is duplicated. One copy remains at the original site, one moves to the new site (↑ copy number). | The original transposon is excised from the donor site and moves to the target site (no change in copy number). |
Integrons & Transduction - Genetic Hijackers
Integrons are genetic elements that capture and express gene 'cassettes', particularly for antibiotic resistance. They are not self-mobile but are often carried by plasmids or transposons.
- Key Components:
- intI gene: Codes for a site-specific recombinase (integrase).
- attI site: The primary recombination site for gene cassettes.
- Pc promoter: Drives expression of integrated cassette genes.
Transduction is bacteriophage-mediated DNA transfer.
Lysogenic conversion is when a prophage confers new properties to the host, like toxin production (e.g., Diphtheria toxin, Cholera toxin, Shiga toxin).
⭐ In specialized transduction, phages carry only specific portions of the bacterial chromosome, whereas in generalized transduction, any part of the chromosome may be transferred.
High-Yield Points - ⚡ Biggest Takeaways
- Plasmids are self-replicating, extrachromosomal DNA circles that often carry genes for antibiotic resistance (R-factors) and virulence factors.
- Transposons ("jumping genes") are mobile DNA segments that move within or between genomes, frequently carrying resistance genes.
- Bacterial conjugation, mediated by plasmids like the F-factor, is the primary mechanism of horizontal gene transfer.
- This transfer is a major driver for the rapid spread of multidrug resistance among bacterial populations.
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