Horizontal gene transfer and plasmid conjugation
Horizontal gene transfer moves genes between organisms of the same generation; conjugation copies a plasmid (a small circle of DNA) between living bacteria through direct contact.
Remember
A plasmid carrying genes for antibiotic resistance passes directly between living bacteria, even of different species, by conjugation, which is a form of horizontal gene transfer.
Memory aid
genes moving sideways between neighbours rather than down a family tree
Picture horizontal gene transfer as genes passing sideways between neighbours, whereas inheriting genes from a parent cell is the path down a family tree.
Genes can be inherited from a parent cell when it divides. That is vertical transfer, from parent to daughter cell (one of the two cells formed at division). Horizontal gene transfer works differently: a bacterium acquires genes from another organism that is not its parent, so genes move sideways between neighbours of the same generation rather than down a family tree.
Genes often travel on a plasmid, a small circle of DNA that sits apart from the main chromosome and often carries extra, non-essential genes such as those for antibiotic resistance.
A plasmid passes between living bacteria by conjugation. A donor cell (the cell giving the plasmid) builds a hollow tube called a pilus, which touches a recipient cell (the cell receiving it), and a copy of the plasmid is passed across. This needs direct cell-to-cell contact.
Horizontal transfer can occur even between distantly related species that live together, such as the many species on a hospital ward. One plasmid can carry several resistance genes together, so a single transfer can arm a bacterium with several of them at once.
The other two routes are transformation and transduction. In transformation, a cell takes up naked DNA, which is free DNA usually released by dead cells. In transduction, a bacteriophage (a virus that infects bacteria) carries DNA from one cell to another. Mutation is not horizontal gene transfer: it is a change within one cell's own DNA, not the acquisition of genetic material from another organism.
Key facts
- Horizontal gene transfer
- movement of genes between organisms of the same generation, not from parent to offspring.
- Plasmid
- a small circle of DNA apart from the main chromosome, often carrying resistance genes.
- Conjugation
- a donor cell's pilus (a hollow tube) touches a recipient cell and a copy of the plasmid is passed across.
- Transformation
- a cell takes up naked DNA, usually released by dead cells.
- Transduction
- a bacteriophage, which is a virus that infects bacteria, carries DNA from one cell to another.
- Reach
- horizontal transfer can cross between distantly related species that live together.
Exam traps
- Transformation is a genuine form of horizontal gene transfer, but a cell takes up free DNA from its surroundings, usually after other cells have died, so it does not describe a plasmid passing between living bacteria by contact.
- Passing genes from a parent cell to its daughter cells is vertical inheritance, and it cannot spread a gene between unrelated species.
- Transduction does move genes between bacteria, but it relies on a bacteriophage packaging DNA by accident, and bacteriophages usually infect only a narrow range of hosts, so it is not the best explanation for a plasmid passing directly between species.
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