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Griffith's 1928 experiment showed that something from dead bacteria could change living ones. It did not say what. As NCERT notes, before this work the genetic material was thought to be a protein. This post explains how a small team at the Rockefeller Institute in New York spent a decade isolating the "transforming principle", how they ruled out each candidate, and why their careful conclusion did not convince everyone at once.
Who were Avery, MacLeod and McCarty?
All three worked at the Hospital of the Rockefeller Institute for Medical Research, the address on their 1944 paper. A. W. Downie's 1972 Griffith Memorial Lecture explains that Oswald Avery's laboratory had been the centre of pneumococcus research in America for about fifteen years by the time of Griffith's paper. Avery at first refused to accept that transformation was real and suspected poor controls, which Downie calls understandable in someone who had devoted so much effort to the idea of fixed immunological types.
Colleagues in his laboratory confirmed and extended Griffith's result. Then, in 1934, Colin MacLeod joined as a junior member of staff and took up the problem. Maclyn McCarty, the third author, later prepared a fairly pure enzyme that breaks down DNA, which became important for answering critics.
Why did the work take ten years?
Downie's account shows how much of science is getting a method to work. MacLeod struggled to get extracts that were reliably active. Not every rough strain could be transformed, so he grew a smooth Type II strain through 36 successive cultures and picked out one rough colony, called R36A, that gave consistent results. Some batches of broth failed until they were treated with charcoal. Enzymes from the bacteria slowly destroyed the active substance until the team learned to heat the suspension first. Not until 1937 could they make active extracts consistently and measure how active they were.
Purification came next. The extracts contained protein, a non-specific sugar and the type-specific capsule polysaccharide. Protein was removed by repeated shaking with chloroform, without loss of activity. The capsule sugar was removed with a specific enzyme prepared by Dubos. The active material was then precipitated with alcohol and purified again and again.
What did the 1944 paper claim?
The paper's summary states that, within the limits of their methods, the active fraction "contains no demonstrable protein, unbound lipid, or serologically reactive polysaccharide and consists principally, if not solely, of a highly polymerized, viscous form of desoxyribonucleic acid." The evidence came from chemical, enzymatic and serological analyses, together with preliminary electrophoresis, ultracentrifugation and ultraviolet spectroscopy. The summary also notes that the active material worked in "exceedingly minute amounts" and that the change it caused was type-specific and passed on to later generations.
Worked reasoning: the enzyme test
The NCERT Class 12 chapter on molecular inheritance summarises the logic: proteases and RNases did not affect transformation, but DNase did. You can rebuild the argument as a set of predictions.
If the principle were protein, a protease would destroy it. Observed: transformation still happens. Protein is ruled out.
If it were RNA, RNase would destroy it. Observed: transformation still happens. RNA is ruled out.
If it were DNA, DNase would destroy it. Observed: transformation stops. DNA fits.
Downie adds that in 1946 McCarty and Avery showed a DNase from ox pancreas destroyed transforming activity under the same conditions in which it broke down purified DNA. Later, Hotchkiss, in the same department, gave further chemical evidence that the active preparations contained no contaminating protein.
Common mistakes: mixing up DNA and DNase (NCERT asks this directly; DNase is the enzyme that digests DNA), and thinking one enzyme result is enough on its own. The enzyme test only works because the purification had already removed most other substances.
Why were people not convinced at once?
NCERT notes that not all biologists were convinced. Downie writes that the importance of the work was not generally appreciated for several years, and that Avery himself never identified DNA as the genetic material in print. Some critics suspected that a trace of protein, too small to detect, did the real work. Younger colleagues were more willing to commit: Downie quotes Harriet Taylor, working in Avery's laboratory, writing in 1949 that it seemed justified "to visualize the transforming principle much as the geneticist pictures genes".
NCERT describes a different kind of experiment, with viruses and radioactive labels in 1952, as the unequivocal proof. Read about it in our post on the Hershey-Chase experiment, and for the experiment that started it all, see Frederick Griffith and the transforming principle.
What can a student take from their way of working?
Make the method reliable before you trust the answer. Three years went into getting reproducible extracts.
Remove candidates one by one. Each purification step and enzyme test took away one possible explanation.
Answer the objections with new experiments. The 1946 enzyme work was a reply to the protein-contaminant objection.
Say exactly what you showed. Their careful wording has held up.
If the chain of evidence in this chapter feels like a list to memorise, the Fermi Concept Library has every concept in biology with its own lesson, so you can follow the reasoning from transformation to DNA step by step.
Write the three enzyme predictions above from memory, then add a fourth: what would you expect if the principle were the capsule polysaccharide?
