Question
Transformation
Answer
Transformation is a mechanism of horizontal gene transfer in which a bacterium takes up free (naked) DNA directly from its surrounding environment — released by lysis of other bacterial cells — and incorporates it into its own genome, without requiring direct cell-to-cell contact (unlike conjugation) or a viral vector (unlike transduction).
Requirements: the recipient bacterium must be in a physiological state called competence, in which it expresses surface proteins/receptors capable of binding and taking up extracellular DNA; competence occurs naturally in certain bacterial species (e.g., Streptococcus pneumoniae, Neisseria, Haemophilus influenzae) under specific growth conditions, or can be induced artificially in the laboratory (e.g., by calcium chloride treatment or electroporation) for genetic engineering purposes.
Mechanism: double-stranded DNA fragments released from lysed donor bacteria bind to the recipient cell surface; one strand is degraded as the DNA is taken up, and the remaining single strand integrates into the recipient’s chromosome by homologous recombination (replacing the corresponding region of the recipient’s own DNA), or, in the case of plasmid DNA, can re-circularize and replicate independently.
Historical/biological significance: transformation was the mechanism demonstrated in Griffith’s classical experiment (1928), in which heat-killed virulent (encapsulated) pneumococci transferred the genetic information for capsule production to living non-virulent (unencapsulated) pneumococci, converting them to virulent, encapsulated organisms — the first experimental evidence that DNA (later confirmed by Avery, MacLeod, and McCarty) is the genetic material responsible for heritable transfer of traits.
Clinical relevance: transformation contributes to the spread of antibiotic resistance genes and can also drive antigenic variation in naturally competent pathogens (e.g., Neisseria gonorrhoeae).