Transformation was first observed by Oswald Avery in 1944 and provided the earliest strong evidence that purified DNA carries genetic information and, therefore, that genes are made of DNA. Pneumococcus pneumoniae (now renamed Streptococcus pneumoniae) has two variants, one forms smooth colonies when plated on nutrient agar, the other has a rough appearance. The smooth variant has a capsule that surrounds the bacterial cell wall, whereas the bacteria in the rough colonies are missing the capsule. The ability to make a capsule affects both colony shape and virulence as the capsule protects bacteria from the animal immune system. Thus, if smooth isolates of S. pneumoniae are injected into a live mouse, it dies of bacterial pneumonia. In contrast, rough strains are non-virulent. Avery exploited this difference to prove that DNA from one strain could "transform" or change the other strain. Avery used DNA extracted from virulent strains of S. pneumonia. He purified it and added it to harmless strains of the same bacterial species. Some of the harmless bacteria took up the DNA and were transformed into virulent strains. Hence Avery named this process transformation (Fig. 18.04). [Strictly speaking, Avery's transforming DNA could have electroporator Device that uses a high voltage discharge to make cells competent to take up DNA
Transformation as Proof that DNA is the Genetic Material
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