jan 1, 1944 - 1944: Oswald Avery identifies DNA as the 'transforming principle'

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By the 1940s, scientists' understanding of the principles of inheritance had moved on considerably - genes were known to be the discrete units of heredity, as well as generating the enzymes which controlled metabolic functions. However, it wasn't until 1944 that deoxyribonucleic acid (DNA) was identified as the 'transforming principle'.

The man who made the breakthrough was Oswald Avery, an immunochemist at the Hospital of the Rockefeller Institute for Medical Research. Avery had worked for many years with the bacterium responsible for pneumonia, pneumococcus, and had discovered that if a live but harmless form of pneumococcus was mixed with an inert but lethal form, the harmless bacteria would soon become deadly.

Determined to find out which substance was responsible for the transformation, he combined forces with Colin MacLeod and Maclyn McCarty and began to purify twenty gallons of bacteria. He soon noted that the substance did not seem to be a protein or carbohydrate but rather a nucleic acid, and with further analysis, it was revealed to be DNA.

In 1944, after much deliberation, Avery and his colleagues published a paper in the Journal of Experimental Medicine, in which they outlined the nature of DNA as the 'transforming principle'. Although the paper was not widely read by geneticists at the time, it did inspire further research, paving the way for one of the biggest discoveries of the 20th century.

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