Transformation is the genetic alteration of a cell resulting from the direct uptake and incorporation of extracellular DNA from its surroundings. This process allows a cell to acquire new genetic traits without cell-to-cell contact.
Explanation
In bacteria, natural transformation is mediated by proteins that bind and import extracellular DNA into the cytoplasm. Only certain species are naturally competent; they express specific receptors and DNA translocases that allow them to capture DNA fragments. Gram‑positive bacteria such as Streptococcus pneumoniae and Bacillus subtilis develop competence during particular growth phases. The DNA is taken up as single strands and integrated into the chromosome by homologous recombination. Gram‑negative species such as Neisseria gonorrhoeae and Haemophilus influenzae also undergo transformation but often require sequence‑specific uptake motifs. Transformation contributes to genetic diversity, adaptation and the dissemination of antibiotic resistance. The phenomenon was first observed by Frederick Griffith in 1928, who showed that non‑virulent pneumococci could become virulent after exposure to heat‑killed virulent strains. Avery, MacLeod and McCarty later demonstrated that DNA was the transforming principle, establishing DNA as the hereditary material. In laboratory settings, transformation refers to the introduction of plasmid DNA or other constructs into bacteria or yeast using chemical treatments or electroporation, enabling genetic engineering and cloning.
Notable Examples and Practical Uses
Streptococcus pneumoniae becomes competent in early exponential phase and can take up exogenous DNA that carries genes for capsule synthesis, changing its serotype. Bacillus subtilis uses a DNA uptake apparatus composed of Com proteins and imports environmental DNA as a source of genetic variation. Neisseria gonorrhoeae employs a type IV pilus for transformation and discriminates DNA containing its 10‑base‑pair uptake sequence. Artificial transformation of Escherichia coli with plasmids is induced by CaCl2 and heat shock or by electroporation; this technique is fundamental in cloning and recombinant protein production. In yeast, transformation by lithium acetate and polyethylene glycol permits insertion of linear DNA through homologous recombination. Clinically, transformation has contributed to the spread of penicillin resistance in Streptococcus pneumoniae through uptake of altered penicillin‑binding protein genes.
Transformation is a key mechanism of horizontal gene transfer and an indispensable tool in molecular biology. By importing DNA from the environment, cells can acquire new traits that enhance survival. The study of transformation has provided foundational insights into genetics and continues to underpin genetic manipulation techniques.
Related Terms: Transduction, Conjugation, Competence, Plasmid, Horizontal gene transfer