Answer: In this experiment, strain III-S bacteria were heated until they died, and their remains were added to strain II-R bacteria. Although mice will not die if exposed to either bacterial remains of strain III-S (which has already died) or strain II-R separately, the combination of the two results in the death of the host mouse. Griffith succeeded in isolating both live II-R and live III-S strains of pneumococcus from the blood of these dead mice. Griffith concluded that type II-R bacteria had been transformed into strain III-S by a transformational principle that somehow became part of the dead strain III-S bacteria.
We now know that the transforming principle observed by Griffith was the DNA of strain III-S bacteria. Even though the bacteria were dead, their DNA survived the heating process and was taken up by strain II-R bacteria. The III-S strain DNA contains genes that form a protective capsule. Equipped with this gene, II-R strain bacteria become protected from the host's immune system and can kill it. Verification of DNA as the transforming principle was carried out in experiments by Avery, McLeod and McCarty and by Hershey and Chase.
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