Bacterial transformation involves uptake of naked exogenous DNA. For a bacterium to be 'competent' for natural transformation, it must express a type IV pilus-like apparatus (in Gram-negative organisms like Neisseria) or a ComFA-like system (in Gram-positive organisms). The key feature of transformation that distinguishes it from transduction is:

• A In transformation, DNA is transferred as naked free DNA without a bacteriophage or pilus vector, and is susceptible to DNase degradation unless protected by receptor-mediated uptake during the competent state ✓
• B Transformation requires physical cell-to-cell contact mediated by a sex pilus
• C Transformation occurs only at elevated temperature and relies on heat-shock protein expression for DNA import
• D Transformation exclusively transfers chromosomal genes, while transduction can transfer only plasmid DNA

Explanation

Transformation is distinguished from conjugation and transduction by: (1) the DNA transferred is free/naked in the environment (not inside a phage particle or pilus), (2) DNase in the medium destroys non-competent transforming DNA confirming it is not virus-protected, and (3) the recipient cell must be in a physiological state of competence to actively take up and import the DNA. Conjugation requires direct cell-cell contact via F-pilus. Transduction requires a bacteriophage as the DNA vector. Transformation can transfer either chromosomal or plasmid DNA. Heat-shock is used to induce artificial competence in laboratory E. coli (CaCl2 method), but natural competence is genetically programmed.

Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.

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