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Three sets of experiments performed in the 20th century led to the conclusion that DNA is the genetic material. In the 1920s, Frederick Griffith injected mice with encapsulated and non-encapsulated stains of Streptococcus pneumonia. The encapsulated strain of the bacteria was virulent, allowing the bacteria to escape the mouse immune system, whereas the nonencapsulated strain was nonvirulent.

Mice injected with the virulent strain died, and those receiving the nonvirulent bacteria survived. Griffith then heat-killed the virulent strain and injected those into mice, and the mice lived. Finally, he injected the rodents with a mixture of the nonvirulent strain and the heat-killed virulent bacteria, and the mice died.

What did Griffith correctly conclude from this work?


All of the virulent bacteria were not killed by heating.


The live, nonvirulent bacteria acquired the ability to develop a capsule (the virulence factor) from the dead virulent strain of bacteria.


The nonvirulent strain developed a new virulence factor that was lethal for the mice and was not influenced by the killed virulent strain.


The virulent strain of bacteria shed their capsules, which were incorporated into the bacterial cell of the nonvirulent strain.

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