Avery, MacLeod, and McCarty Determine That DNA Carries Hereditary Information

Date 1944

Oswald T. Avery and colleagues demonstrated that the genetic transformation of bacteria was caused by deoxyribonucleic acid (DNA), providing direct evidence about the chemical nature of hereditary information. Their discovery, doubted at first, eventually led geneticists to understand that DNA carried life’s genetic blueprints.

Also known as “Studies on the Chemical Transformation of Pneumococcal Types”

Locale New York, New York

Key Figures

• Oswald T. Avery (1877-1955), American bacteriologist
• Frederick Griffith (1881-1941), American microbiologist
• Colin Munro MacLeod (1909-1972), American microbiologist
• Maclyn McCarty (1911-2005), American microbiologist

Summary of Event

In the 1920’s, the field of genetics had progressed to the point of locating hereditary information within the cell. Genes, which were as-yet-uncharacterized elements responsible for the inheritable traits of organisms, had been localized to the chromosomes of cells. In the cells of higher organisms, multiple chromosomes are found within the membrane-bounded compartment called the nucleus; within the simpler cells of bacteria, a single chromosome is found without a specialized compartment. In both cases, these chromosomes were known to be made up of two major chemical components: protein and a form of nucleic acid (an acid named for its nuclear location). Thus, it was known that protein and deoxyribonucleic acid (DNA) were in chromosomes, and chromosomes had something to do with the characteristic traits of organisms, but beyond that nothing was known about the physical nature of genetic information.

Oswald T. Avery was a bacteriologist at the hospital of the Rockefeller Institute in New York City. He was studying pneumonia, a disease caused by bacteria and a major cause of death in the late nineteenth and early twentieth centuries. Several different strains of pneumococci, the class of bacteria causing pneumonia, were known to exist; some strains in this class were nonpathogenic (that is, they did not cause disease).

Avery had demonstrated in 1917 that the blood and urine of patients infected by different pathogenic strains contained distinct soluble substances, specific for each strain. Later, experiments suggested that these specific substances were polysaccharides, starchlike molecules derived from the distinct cell coatings or capsules of these bacteria. Nonvirulent pneumococci were unencapsulated, and the differences in the coats of the encapsulated forms reflected the strain differences among the virulent pneumococci. Avery’s early work helped form the foundation of the scientific study of immunology.

In 1928, Frederick Griffith, an English public health officer, reported the results of his experiments using different strains of pneumococci to infect mice. Griffith had observed the following: Mice injected with a nonpathogenic (unencapsulated) strain of pneumococci did not contract pneumonia; mice injected with encapsulated pathogenic bacteria that had first been killed by heating also did not contract pneumonia. So far, there were no surprises for bacteriologists. However, when Griffith inoculated mice with a combination of nonpathogenic bacteria and heat-killed pathogenic pneumococci, many of those mice contracted pneumonia and died. Moreover, live bacteria recovered from these animals were encapsulated.

Griffith had somehow transferred the virulence and capsule-forming traits of one strain of bacteria to a formerly nonvirulent, unencapsulated strain. This acquired pathogenicity was maintained in subsequent generations of these bacteria, and the phenomenon was dubbed “genetic transformation.” Soon after they were reported, Griffith’s experiments were repeated with similar findings in several laboratories.

One of the scientists who confirmed Griffith’s work was Martin Dawson, a Canadian scientist then working in Avery’s laboratory at the Rockefeller Institute. Dawson took Griffith’s findings a step further and demonstrated that genetic transformation did not require the infection of a host animal—unencapsulated bacteria mixed with killed encapsulated forms gave rise to encapsulated, virulent colonies in bacterial cultures in the laboratory. Avery’s name did not appear on Dawson’s paper confirming Griffith’s findings; he was skeptical about transformation. This is perhaps not surprising, considering that Avery’s main contribution up to that time had been in establishing the existence of distinct, stable forms of pneumococci, recognizable by just such characteristics as Griffith’s work suggested could be transferred from strain to strain.

James Lionel Alloway, another scientist in Avery’s laboratory, later produced cell-free extracts of broken encapsulated bacteria and showed that such extracts were as effective as heat-killed cells in transforming nonvirulent strains. Alloway described precipitation that he observed in his extracts after adding alcohol to them. In a few years, these descriptions would be recognized as corresponding to the behavior of nucleic acids. At the time, however, the identity of the transforming agent in these extracts was unknown, and Alloway—like a substantial number of biologists—believed it was most likely to be protein.

Avery’s interest in the pursuit of the transforming principle appears to have been engaged at this point. The combined weight of evidence, much of it from his own laboratory, was irresistible. Together with two new collaborators in his laboratory, Colin Munro MacLeod and Maclyn McCarty, Avery performed the key experiments that first identified DNA as the active transforming material in genetic transformation.

Avery, MacLeod, and McCarty exhaustively fractionated transforming extracts, removing polysaccharides, lipids, and proteins by physical, chemical, and enzymatic treatments without removing the ability to transform. Enzymes that degraded ribonucleic acid (RNA) were also unable to interfere with transformation, but even trace amounts of DNA-degrading enzymes destroyed the transforming principle. They tested and retested their extracts, using different methods of measurement and different sources of enzymes; their results continued to show that the transforming principle behaved like DNA. Furthermore, their extract was extraordinarily potent: It continued to transform even when diluted to exceedingly low concentrations, down to “1 part in 600,000,000” from the starting material.

In 1944, Avery, MacLeod, and McCarty’s classic paper entitled “Studies on the Chemical Transformation of Pneumococcal Types” appeared in the Journal of Experimental Medicine, presenting their evidence that DNA was responsible for the transfer of genetic information. Far from being accepted as an elegant proof of DNA’s role, Avery’s paper met with resistance and disbelief for several reasons. One reason was the presumed simplicity (if not monotony) of DNA’s structure. It was thought to be a polymer of identical repeating units, similar to some starch molecules.

Such a structure for DNA seemed incompatible with the variety and specificity of genetic information. The molecule’s presumed uniformity was even more striking in comparison with the immense diversity that had been observed among protein molecules, which—like DNA—were known to be associated with chromosomes. The prevailing view held that proteins, not DNA, were probably the vectors of genetic information. Even though Avery, MacLeod, and McCarty had demonstrated that the transforming ability of their extract was highest in the most pure, most protein-free preparations, their claim for DNA’s role in genetic transformation still seemed implausible. Many thought that even minute traces of contaminating proteins could be responsible for transformation. Alternatively, some suggested that transformation of pneumococci was a special case: DNA might have an anomalous effect on these cells that caused them to begin making capsules and become virulent in a fashion unrelated to genetic material.

Significance

It took several years and two other studies to resolve the doubt about DNA. In 1949, Rollin Hotchkiss, who had begun work in Avery’s laboratory in 1935, demonstrated DNA-mediated transfer of an entirely different set of characteristics, related to antibiotic resistance, to a formerly nonresistant strain of pneumococci. This showed conclusively that capsule formation was not a special case.

In 1952, Alfred Hershey and Martha Chase grew virus cultures in the presence of two different radioactive compounds. These compounds were used to “tag” the two candidates for genetic vectors: One tag was incorporated into the proteins of the viral particles and another was incorporated into viral DNA. The viruses then infected bacteria, reproduced themselves inside, and burst the bacterial cells to release many progeny viruses.

Hershey and Chase showed that, in the viral infections they studied, virus proteins (identified by their specific radioactive tag) remained outside the bacterial cell, while the tagged viral DNA was injected into each bacterium. They thus showed that this DNA alone was responsible for the subsequent production of progeny, which in turn demonstrated that viral genetic information resided in the same chemical substance that had carried the genes of pneumococci in Avery’s work. The case for DNA as the genetic vector had become irrefutable.

In April, 1953, James D. Watson and Francis Crick published their model of the double, helical structure of DNA, a model that explained how complex genetic information could be carried by a polymer built from simple subunits and how this polymer could be replicated over and over in generation after generation. Watson and others, whose work formed the basis for the new field of molecular biology, traced their interest in nucleic acids to Avery’s experiments.

Bibliography

Dubos, Rene J. The Professor, the Institute, and DNA. New York: Rockefeller University Press, 1976. A biography of Oswald Avery. Illustrated, with bibliography and index.

Hotchkiss, Rollin D. “Gene, Transforming Principle, and DNA.” In Phage and the Origins of Molecular Biology, edited by John Cairns, Gunther S. Stent, and James D. Watson. Cold Spring Harbor, N.Y.: Cold Spring Harbor Laboratory, 1966. A collection of essays on their work by the principal players in the early days of molecular biology. Illustrated, with bibliographies.

Judson, Horace Freeland. The Eighth Day of Creation: Makers of the Revolution in Biology. New York: Simon & Schuster, 1979. A historical account of the development of ideas about the chemical nature of genes. A journalist, Judson writes with a fascination for both the science and the scientists involved. Illustrated, with bibliography and index.

Portugal, Franklin H., and Jack S. Cohen. “Genetic Transformation by DNA.” In A Century of DNA: A History of the Discovery of the Structure and Function of the Genetic Substance. Cambridge, Mass.: MIT Press, 1977. A discussion for the general scientific reader, with extensive treatment of Avery’s contributions. Illustrated, with bibliography and index.

‗‗‗‗‗‗‗. “The Mechanism of Gene Expression.” In A Century of DNA: A History of the Discovery of the Structure and Function of the Genetic Substance. Cambridge, Mass.: MIT Press, 1977. A discussion for the general scientific reader, with extensive treatment of Avery’s contributions. Illustrated, with bibliography and index.

Tudge, Colin. In Mendel’s Footnotes: An Introduction to the Science and Technologies of Genes and Genetics from the Nineteenth Century to the Twenty-Second. London: Jonathan Cape, 2000. Begins as a history of the development of genetics and genetic technologies and builds on that history to become a speculative text on the future of the science and its applications. Bibliographic references and index.

Watson, James D. The Double Helix: A Personal Account of the Discovery of the Structure of DNA. New York: Atheneum, 1968. An eccentric book by an eccentric man; the science is presented in a fairly approachable manner, and the larger story is accessible to anyone interested in a view from the inside. Illustrated.

‗‗‗‗‗‗‗. A Passion for DNA: Genes, Genomes, and Society. Cold Spring Harbor, N.Y.: Cold Spring Harbor Laboratory Press, 2000. Watson’s reflections on the social and scientific consequences of the discovery of DNA. Indexes.