page_title: Avery–MacLeod–McCarty experiment
section_title: Experimental work
hierarchical_section_title: Avery–MacLeod–McCarty experiment / Experimental work
caption_attribution_description: English: Colin MacCleod, geneticist http://history.nih.gov/exhibits/nirenberg/popup_htm/03_macleod.htm
context_page_description: The Avery–MacLeod–McCarty experiment was an experimental demonstration, reported in 1944 by Oswald Avery, Colin MacLeod, and Maclyn McCarty, that DNA is the substance that causes bacterial transformation, in an era when it had been widely believed that it was proteins that served the function of carrying genetic information. It was the culmination of research in the 1930s and early 20th century at the Rockefeller Institute for Medical Research to purify and characterize the "transforming principle" responsible for the transformation phenomenon first described in Griffith's experiment of 1928: killed Streptococcus pneumoniae of the virulent strain type III-S, when injected along with living but non-virulent type II-R pneumococci, resulted in a deadly infection of type III-S pneumococci.
context_section_description: Pneumococcus is characterized by smooth colonies which have a polysaccharide capsule that induces antibody formation; the different types are classified according to their immunological specificity.
  The purification procedure Avery  undertook consisted of first killing the bacteria with heat and extracting the saline-soluble components.  Next, the protein was precipitated out using chloroform and the polysaccharide capsules were hydrolyzed with an enzyme.  An immunological precipitation caused by type-specific antibodies was used to verify the complete destruction of the capsules.  Then, the active portion was precipitated out by alcohol fractionation, resulting in fibrous strands that could be removed with a stirring rod.
  Chemical analysis showed that the proportions of carbon, hydrogen, nitrogen, and phosphorus in this active portion were consistent with the chemical composition of DNA.  To show that it was DNA rather than some small amount of RNA, protein, or some other cell component that was responsible for transformation, Avery and his colleagues used a number of biochemical tests.  They found that trypsin, chymotrypsin and ribonuclease (enzymes that break apart proteins or RNA) did not affect it, but an enzyme preparation of "deoxyribonucleodepolymerase" (a crude preparation, obtainable from a number of animal sources, that could break down DNA) destroyed the extract's transforming power.
  Follow-up work in response to criticism and challenges included the purification and crystallization, by Moses Kunitz in 1948, of a DNA depolymerase (deoxyribonuclease I), and precise work by Rollin Hotchkiss showing that virtually all the detected nitrogen in the purified DNA came from glycine, a breakdown product of the nucleotide base adenine, and that undetected protein contamination was at most 0.02% by Hotchkiss's estimation.
  The experimental findings of the Avery–MacLeod–McCarty experiment were quickly confirmed, and extended to other hereditary characteristics besides polysaccharide capsules.  However, there was considerable reluctance to accept the conclusion that DNA was the genetic material.  According to Phoebus Levene's influential "tetranucleotide hypothesis", DNA consisted of repeating units of the four nucleotide bases and had little biological specificity.  DNA was therefore thought to be the structural component of chromosomes, whereas the genes were thought likely to be made of the protein component of chromosomes.  This line of thinking was reinforced by the 1935 crystallization of tobacco mosaic virus by Wendell Stanley, and the parallels among viruses, genes, and enzymes; many biologists thought genes might be a sort of "super-enzyme", and viruses were shown according to Stanley to be proteins and to share the property of autocatalysis with many enzymes.  Furthermore, few biologists thought that genetics could be applied to bacteria, since they lacked chromosomes and sexual reproduction.  In particular, many of the geneticists known informally as the phage group, which would become influential in the new discipline of molecular biology in the 1950s, were dismissive of DNA as the genetic material (and were inclined to avoid the "messy" biochemical approaches of Avery and his colleagues).  Some biologists, including fellow Rockefeller Institute Fellow Alfred Mirsky, challenged Avery's finding that the transforming principle was pure DNA, suggesting that protein contaminants were instead responsible.  Although transformation occurred in some kinds of bacteria, it could not be replicated in other bacteria (nor in any higher organisms), and its significance seemed limited primarily to medicine.
  Scientists looking back on the Avery–MacLeod–McCarty experiment have disagreed about just how influential it was in the 1940s and early 1950s.  Gunther Stent suggested that it was largely ignored, and only celebrated afterwards—similarly to Gregor Mendel's work decades before the rise of genetics.  Others, such as Joshua Lederberg and Leslie C. Dunn, attest to its early significance and cite the experiment as the beginning
caption: Colin MacLeod
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