Part 4

Most people believed that genes must be made of proteins because nucleic acids were considered too simple to carry genetic information. Avery worked all his life on Pneumococcus and bacterial pneumonia. Griffith showed that transformation of a non-virulent strain can be achieved in mice by coinjection of heat-killed virulent bacteria. Avery’s lab managed to obtain transformation in the test tube, but it took many years to establish a reliable assay and finally to purify the molecule responsible for this effect, which turned out to be DNA. Although this work was well known, most scientists were not convinced of the general implication of this phenomenon. Furthermore, many biochemists believed that even the purified DNA was contaminated with a protein. Finally, transformation was a very inefficient process and the mechanism of transformation remained mysterious for many years. The work of Hershey and Chase finally convinced the scientific community that genes are made of DNA. We now realize that exchange of DNA by transformation is very common, and participates to the horizontal transfer of DNA between at least bacterial species, and was a considerable accelerator of evolution

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