This is another anniversary in the history of genetics: it’s been seventy years since three investigators at Rockefeller Institute in New York City (now Rockefeller University, where I began grad school)—Oswald Avery, Colin MacLeod, and Maclyn McCarty (“AMM”)—discovered that DNA was the genetic material. Although their paper on this work was published in 1944 (reference and link below, full pdf here), May 26, 1943 was the day that Oswald Avery wrote to his brother announcing the discovery. (You can see Avery’s scrawled letter here.)
This finding truly was the beginning of the frenzy in molecular genetics that led directly to the discovery of the structure of DNA by Watson and Crick, and then to elucidating how DNA produced proteins. For it wasn’t until we knew what the genetic material actually was that we could determine its structure. And before the AMM paper, nobody knew for sure.
I’ve read the paper several times, and it’s a masterpiece of meticulous work and scientific exposition. What it showed was that one could change the hereditary nature of pneumonia-causing bacteria (that is, whether they could be virulent) by “transforming” nonvirulent strains with DNA. These strains had been killed, but could be revitalized (and made virulent) by replacing their DNA with that from nonvirulent cells. The virulence persisted (i.e., was inherited) after transformation. This transformation was not effected by protein, so that ruled out proteins as the genetic material. The way that AMM did this in those early days of molecular biology was masterful.
But I’ll let Matthew tell you about it in his new article in the Guardian, “Oswald T. Avery, the unsung hero of genetic science.“ Avery had two collaborators on this, Colin MacCleod and Maclyn McCarty, but Avery was the driving force behind the research.
Their discovery, which was soon replicated, certainly deserved a Nobel Prize, but none of the three men ever got it. I suppose that’s why Matthew calls Avery an “unsung hero.” If you’re not a biologist you probably won’t have heard of the three men and their paper. But those of us working in genetics all know the work simply as “Avery, MacCleod andMcCarty”, and it’s a good time for all of us to remember their contribution. Go read Matthew’s article now: it’s very good and clearly explains what they did and why it was important.
This is what scientific history looks like: the title and summary of their paper. Note the “conclusion”. You can read “hereditary material” for “fundamental unit of the transforming principle.
One side note: the authors cite my academic grandfather, Theodosius Dobzhansky, for recognizing that the phenomenon of transformation itself gives a clue that the genetic material might be involved. I’m chuffed that support for DNA as the hereditary molecule came from an evolutionary geneticist:
Here’s the part of Avery’s 14-page letter where he says that, after “many heartaches and heartbreaks,” they figured out that the transforming principle seemed to be DNA. “Who could have guessed it?”, he adds.
And here are the three unsung heroes:
You can read more about this experiment at Wikipedia, which has a more detailed description, or in Horace Freeland Judson’s masterful account of the history of molecular genetics, The Eighth Day of Creation.
And isn’t it amazing that a primate could figure this out?
Avery, Oswald T.; Colin M. MacLeod, Maclyn McCarty (1944). “Studies on the Chemical Nature of the Substance Inducing Transformation of Pneumococcal Types: Induction of Transformation by a Desoxyribonucleic Acid Fraction Isolated from Pneumococcus Type III”. Journal of Experimental Medicine 79 (2): 137–158. doi:10.1084/jem.79.2.137