Plasmids and Transposons
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Direct answer
Plasmids are extrachromosomal, usually circular DNA molecules that replicate independently of the bacterial chromosome and often carry resistance genes. Transposons are mobile DNA segments that move between sites within or between DNA molecules using a transposase enzyme. Together they let resistance genes jump between plasmids, chromosomes and bacterial species.
What you must remember
- F plasmid: about 100 kb, carrying the transfer genes for the sex pilus that drive conjugation.
- R factor: a resistance plasmid made of a resistance transfer factor, which carries conjugation genes, and resistance determinants.
- Col plasmid: encodes colicins, bacteriocins that kill closely related E. coli strains.
- Virulence plasmid: the Yersinia pYV plasmid of about 70 kb carries the Yop effector genes of its type III secretion system.
- Transposon: a DNA segment that moves using transposase, by cut-and-paste or replicative transposition.
- Insertion sequence: the simplest mobile element, carrying only a transposase gene flanked by inverted repeats.
- Integron: a gene capture platform with an integrase and a site for gene cassettes; class 1 integrons are common in Gram-negative bacilli.
- SCCmec: a staphylococcal mobile element carrying mecA, which confers methicillin resistance.
Mobile elements compared
Mobile genetic elements differ in size, enzyme and carrier, but all of them can move without a homologous template. The table compares the elements most often asked in examinations. Each row names the mechanism that moves the element and a clinical example of its effect.
| Element | Structure | How it moves | Clinical example |
|---|---|---|---|
| F plasmid | Circular, about 100 kb | Conjugation | Donor ability in E. coli |
| R plasmid | Transfer and resistance genes | Conjugation | Multidrug resistance in Enterobacterales |
| Transposon Tn3 | Carries the bla gene | Transposase | Ampicillin resistance |
| Insertion sequence | Transposase gene, inverted repeats | Transposase | Disruption of nearby genes |
| Class 1 integron | Integrase and cassette site | Integrase captures cassettes | Multidrug resistance in Gram-negative bacilli |
| SCCmec | Large staphylococcal cassette | Site-specific recombination | Methicillin resistance in S. aureus |
A plasmid carries whole sets of genes between cells, while a transposon moves single genes within a cell's DNA. Integrons and SCCmec combine gene capture or cassette movement, so one element can carry a complete resistance profile. This is why a resistant strain can acquire several drugs in one transfer event.
Mobile elements can also cause disease by altering genes. An insertion sequence landing inside a gene can switch it off, which is one way bacteria lose or gain a phenotype.
Where the exam tries to catch you
Plasmid and transposon questions test whether the element is essential, how it moves and what it carries.
- Trap: plasmids are essential for bacterial survival Correct: plasmids are dispensable accessory DNA, so a cell survives without them, although they confer advantages such as resistance.
- Trap: transposons insert only at sites with matching sequence Correct: transposons insert at many sites through transposase, which is why they spread widely across the genome.
- Trap: integrons are a type of plasmid Correct: integrons are gene capture platforms that can sit on plasmids or chromosomes and collect gene cassettes via integrase.
Frequently asked questions
What is an R factor in bacterial genetics?
An R factor is a resistance plasmid made of a resistance transfer factor, which carries conjugation genes, and resistance determinants that encode antibiotic resistance. It can transfer several resistance genes to other bacteria in one conjugation event.
Why is the F plasmid important in conjugation?
The F plasmid carries the transfer genes that build the sex pilus and allow its host to act as a donor. Cells carrying it are F+, while cells lacking it are F- and act as recipients.
How do transposons differ from plasmids?
Transposons are mobile DNA segments that jump between sites using transposase, and they do not replicate as independent molecules. Plasmids are circular molecules that replicate from their own origin, and they often carry transposons.
What is a class 1 integron?
A class 1 integron is a gene capture system built on an integrase gene and a site for gene cassettes. It is common in Gram-negative bacilli, where it collects and expresses multiple resistance genes.