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... this experiment, the dead S bacteria were of type III. The live R bacteria had been derived from type II. The virulent bacteria recovered from the mixed infection had the smooth coat of type III. Thus some property of the dead type III S bacteria can transform the live R bacteria so that they make the type III capsular polysaccharide, and as a result become virulent. The component of the dead bacteria responsible for transformation was called the transforming principle. It was purified by adding extracts of dead S bacteria to live R bacteria before injection into the host. The studies of Avery showed chemically in 1944 that the transforming principle is deoxyribonucleic acid (DNA). This was a great surprise, because, at the time, DNA was not even recognised as a component of Pneumococcus. DNA was however recognised as a major component of eukaryotic chromosomes. This provided a link between inheritance ...