1984 marks the fortieth anniversary of the publica- tion of the classic work of Oswald Avery and his coworkers (Avery, et al. 1944) proving that DNA is the hereditary molecule. Few biological discoveries rival that of Avery's. He paved the way for the many molecular biologists who followed. Indeed, 1944 is often cited as the beginning of molecular Oswald Avery biology. Having been briefed on the experiments a year before their publication, Sir MacFarlane Burnet and DNA wrote home to his wife that Avery "has just made an extremely exciting discovery which, put rather crudely, is nothing less than the isolation of a pure Charles L. Vigue gene in the form of desoxyribonucleic acid" (Olby 1974). Recalling Avery's discovery, Ernst Mayr said "the impact of Avery's finding was electrifying. I Downloaded from http://online.ucpress.edu/abt/article-pdf/46/4/207/41261/4447817.pdf by guest on 23 September 2021 can confirm this on the basis of my own personal experience . . . My friends and I were all convinced that it was now conclusively demonstrated that DNA was the genetic material" (Mayr 1982). Scientific dogma is established in many ways. Dis- coveries such as that of the planet Uranus are quickly accepted because the evidence for them is so compel- ling. Some scientific pronouncements are immedi- ately accepted but later found to be erroneous. For example, it was widely accepted in the 1930s, 1940s, and early 1950s that humans had 48 chromosomes; in 1956 it was proven that we have only 46. Some find- ings are not accepted even though, in retrospect, the evidence was compelling. Such is the case with Avery's discovery. Although Avery convinced many, including Burnet, Mayr, and his friends, he did not convince the majority of scientists. Even fif- teen years after the discovery, some were still unwill- ing to accept DNA as the universal hereditary mole- cule. Avery was interested in the phenomenon of bac- terial transformation first described by Frederick Griffith in the 1920s. Griffith succeeded in transform- ing a nonvirulent, non-encapsulated, Type II variant of Streptococcus pneumoniae (formally Pneumococcus) into a virulent form with a Type III capsule. Since the capsule protects the bacterium from the pha- gocytic cells of the host's immune system, it cannot be easily destroyed. Thus, the encapsulated bacteria are virulent. Griffith found that if he injected the non-virulent Type II form into mice along with a large number of heat-killed, Type III, virulent bac- teria, the mice frequently succumbed to pneumonia. Upon autopsy the mice's blood revealed the pres- Charles L. Vigue is associate professor of biology at the University ence of living, encapsulated Type III bacteria. Thus, of New Haven, West Haven, CT 06516. He is also coordinator of the non-virulent, non-encapsulated form had been the Program in Environmental Science and chairman of the fac- ulty. He holds a B.A. and an M.S. in zoology from the University transformed into the Type III capsulated and there- of Maine and a Ph.D. in genetics from North Carolina State Uni- fore virulent form of Streptococcus. versity. He has published several articles on the functional and Avery and his coworkers, Colin MacLeod and evolutional significance of isoenzymes (in BiochemicalGenetics and GeneticalResearch) and articles in ABT, The Biologist, and The Jour- Maclyn McCarty, were interested in isolating the nal of College Science Teaching. transforming factor and characterizing it chemically.

OSWALD AVERY 207 To this end, they partially purified the transforming claimed a role for protein (Olby 1974) are cases in principle from heat-killed Type III bacteria. DNA point. The large majority of the scientific commu- tests on the partially purified factor were positive. nity, however, was blind to Avery's discovery. RNA tests were only weakly positive. More impor- Severalreasons may be cited for this blindness. tantly, however, protein tests were negative. Avery, Humanity, and scientists are no exception, resists et al. (1944)concluded that "if the results of the pres- change. Scientists often resist new discoveries if they ent study on the chemical nature of the transform- do not conform to their preconceived notions. New ing principle are confirmed, then nucleic acids must ideas resulting from scientific discoveries that break be regardedas possessing biological specificity...." existing paradigms are difficult to establish. Pollock Thomas Kuhn said that "until the scientist has (1970)said that great discoveries are often "hated" learned to see nature in a new way-the new fact is because they require an "immense emotional (and not quite a scientific fact at all" (Brannigan1981). If therefore intellectual) effort of abandoning precon- Avery's work did convince scientists to look at ceived dogmata." Beadle and Stent believe that new Nature in a new way, it is not as obvious as many ideas will be accepted provided they can be fitted have said. Wyatt (1972)was quite accuratewhen he into established dogma without too much difficulty said that the history of the discovery and its subse- (Wyatt 1972). Scientists, often because of a healthy quent acceptance "has been mellowed by skepticism, will not accept new ideas until they can Downloaded from http://online.ucpress.edu/abt/article-pdf/46/4/207/41261/4447817.pdf by guest on 23 September 2021 hindsight." A survey of general genetics textbooks be proved beyond a reasonable doubt by repeated published from 1945to 1950clearly demonstrates that investigationin a variety of systems. Many scientists geneticistswere not particularlytaken by Avery's dis- were reluctantto accept DNA as the hereditarymol- covery. Laurence Snyder (1946) in The Principlesof ecule because its function had not yet been clearly Hereditaryand Sinnott, et al. (1950) in Principlesof described. Geneticsnever mention Avery's work. Snyder, with- Wyatt (1972) has demonstrated that the title of out saying that the hereditary material is protein, Avery's paper ("Studies on the Chemical Nature of says that "all estimates agree that the gene is of sub- the Substance Inducing Transformation of Pneu- microscopic volume, probably within the range of mococcal Types: Induction of Transformation by a size of large protein molecules." He also suggests that DesoxyribonucleicAcid FractionIsolated from Pneu- genes are large complex molecules and that nucleic mococcusType III") contained no key words (gene, acids are such complex molecules. Sinnott, et al. state for example) that would tip off the geneticist as to that "it is interesting that computations of sensitive the genetical implications of the work. Since geneti- values for various genes in Drosophilahave given cists were not trained as chemists, the biochemical dimensions of the order of magnitude of large pro- nature of Avery's work was foreign to most of them. tein molecules. . . . Genes would have sizes within Transformationwas a phenomenon studied in bac- the range of smaller viruses and medium-to-large- teria, an organism not commonly studied by geneti- sized protein molecules." cists and whose status as "living" was questioned. At the 1946 Cold Spring Harbor Symposium on Experimentsdescribing the phenomenon were pub- Heredity and Variation in Microorganisms there lished in medical science journals (The Journal of were 27 papers presented. Only eight, one of which ExperimentalMedicine, for example) not normally was by Avery himself, made reference to the classic read by geneticists. World War II certainlyrestricted work. At the 1951 Cold Spring Harbor Symposium the dissemination and flow of scientific knowledge. on Genes and Mutations only three papers made ref- And, the paper itself did not make clear the work's erence to Avery. The Cold Spring Harbor Sym- genetical implications. Although Avery knew the posium on Viruses held in 1953 saw 42 papers pre- genetical implications, he was "unreasonably cau- sented. Only three mentioned Avery. One cannot tious and too modest in his conclusions" (Pollock escape the conclusions that Avery's work was viewed 1970).In a 1943letter to his brother, however, Avery with some skepticism by the general scientific com- was not as cautious: "Nucleic acids are not merely munity. structurally important but functionally active sub- Certainly,scientists such as RollinHotchkiss, Andre stances in determining biological activities and spe- Boivin, and Harriett Taylor, who were working in cific characteristics of cells . . . Sounds like a virus- the DNA field, accepted Avery's finding. McCarty's maybe a gene" (Olby 1974). statement that "the accumulatedevidence has estab- What were the preconceived ideas that afflicted lished beyond a reasonable doubt that the active sub- the scientific community with myopia? Although stance responsible for transformation is a specific FrederickMeishner, the discoverer of DNA in 1869, nucleic acid of the desoxyribose type" (Mayr 1982) and many others prior to Avery suggested that DNA and Boivin's assertion that the burden of proof no would be a good candidate for the hereditary mole- longer lay with those who believed that DNA was cule, most scientists rejected the hypothesis after it the hereditary molecule, but with those who was erroneously demonstrated that DNA was a

208 THE AMERICAN BIOLOGY TEACHER, VOLUME 46, NO. 4, APRIL 1984 small, simple, uniform molecule-too small, too sim- a letter from Hershey, who was denied a visa, said ple, and too uniform to be the hereditary molecule. "almost no one in the audience of over four hun- Thus, the role of the hereditary molecule fell to pro- dred microbiologists seemed interested as I read long tein, a large, complex, and variable molecule which sections of Hershey's letter" describing the experi- also is a component of the chromosomes. When the ments (Wyatt 1972). Among the DNA skeptics was experiments of Avery et al. and the many who fol- Hershey himself who said "my own guess is that lowed began to implicate DNA, it was contrary to DNA will not prove to be a unique determiner of established dogma. Despite Avery's work showing genetic specificity" (Hershey 1953). More farsighted that the transforming molecule contained less than than Hershey, however, was Hattie Alexander. Con- 0.02% protein, that it was insensitive to protein sidering the transforming principle and the work denaturing agents and protein digesting enzymes with bacteriophage Alexander said "the possibility is but sensitive to DNase, an enzyme that digests DNA, therefore raised that a comparable control of inheri- the blindness persisted. tance may apply to cells in general" (Zinder 1953). New scientific discoveries are not transformed into Even as late as the 1950s two serious objections to general laws until the results are confirmed many DNA blocked its widespread acceptance as the times by independent researchers. The transforma- hereditary molecule. DNA, although now thought tion experiments were difficult to repeat. These early to be a large rather than a small molecule, was con- Downloaded from http://online.ucpress.edu/abt/article-pdf/46/4/207/41261/4447817.pdf by guest on 23 September 2021 failures discouraged many workers from pursuing sidered to be invariable, i.e., to have the same struc- DNA research (Ephrussi-Taylor 1951). Eventually, ture in all organisms, and not complex enough to be however, the phenomenon was demonstrated in the hereditary molecule. Although it was generally other bacteria. Not only were other capsular types accepted that the hereditary molecule, whatever it shown to be determined by DNA but so were traits was, directs the synthesis of protein, it was incon- such as drug resistance and sugar fermentation ceivable how DNA could perform this function. (Hotchkiss 1955). By 1955, thirty or more transforma- Three important developments occurred in the tions had been demonstrated (Hotchkiss 1955). 1950s: the demonstration that the composition of The repeated demonstration of the DNA nature of DNA varies from species to species, the elucidation of the transforming principle won many converts. It is the structure of DNA, and the discovery of the proc- not surprising that those scientists converted first were ess of information transfer from DNA to protein. those working with bacteria. Many biologists, how- These discoveries removed the objections to DNA. In ever, felt that the demonstration that DNA was the 1950 Erwin Chargaff demonstrated, contrary to pre- hereditary molecule in bacteria did not qualify it as vious thought, that DNA was not composed of equal a general principle applicable to all organisms. amounts of the four bases. Instead, the base composi- Scientists working with bacteriophage converted tion varies from species to species but remains many skeptics by showing that it was DNA and not roughly constant within a species. Chargaff con- protein that was involved with infection. As early as cluded "the presumption, therefore, is that there 1951 Roger Herriott said "that the virus may act like a little hypodermic needle full of transforming prin- ciple; that the virus as such never enters the cell; that only the tail contacts the host and perhaps enzymatically cuts a small hole through the outer membrane and then the nucleic acid of the virus head flows into the cell" (Olby 1974). The definitive proof was furnished by an experiment by Alfred Hershey and Martha Chase in 1952. Although the Hershey-Chase experiment was far from definitive, it demonstrated that during infection of E. coli by T2 bacteriophage, most of the phage DNA gets into the E. coli whereas most of the protein does not. Hershey and Chase concluded: "We infer that protein has no Owl T.Aey(8715):FomGntc 11(95. function in phage multiplication, and that DNA has some function" (Hershey and Chase 1952). Although the Hershey-Chase experiment won many over to the DNA side there were as many skeptics as believers. Mainstream geneticists consid- ered viruses less typical than bacteria. James Watson who attended the 1952 meeting of the Society for General Microbiology at Oxford, England and read

OSWALD AVERY 209 exists an enormous number of structurallydifferent stance of all cellular organisms" (Leslie 1961);"DNA nucleic acids" (Chargaff1950). appears to be the stuff of which genes are made" In 1953 Watson and Crick published their model (Srb, et al. 1965);"It seems safe to equate DNA with of DNA. The Watson and Crick model showed that the genetic material" (King 1965); "Transformation the molecule is very complex with the complexity provides direct and conclusive evidence that chro- residing in the sequence of bases: "The sequence of mosomal DNA is genetic material" (Herskowitz bases . . . does not appear to be restricted in any 1965); "The gene molecule is composed of nucleic way. It follows that in a long molecule many differ- acids" (White 1962). ent permutationsare possible, and it therefore seems In retrospect, the gradual acceptance of DNA as likely that the precise sequence of the bases is the the hereditarymolecule followed a logical sequence. code which carries the genetical information" (Wat- Microbiologists, who were most familiar with the son and Crick1953). The work of Chargaffand Wat- phenomenon of transformation, were the first to son and Crick clearly demonstrated that DNA was accept DNA as the hereditary molecule. Virologists, neither too simple nor invariable. The other major because of the experiments of Hershey and Chase development took place over a twenty-year period and others, were next. They were followed by from the 1940s to the 1960s and led to the elucida- eukaryoticgeneticists. Eventually, the idea spread to

tion of how the base sequence of DNA codes for the organismalbiologists. Downloaded from http://online.ucpress.edu/abt/article-pdf/46/4/207/41261/4447817.pdf by guest on 23 September 2021 amino acid sequence of proteins. In looking back at Avery's work and that of the One would expect that the Watson-Crick model others who followed, it is difficult to understand and the elucidation of the function of DNA would why it took so long for the role of DNA in the have finally won the case for DNA. This, however, hereditary process to be accepted by the scientific is not so. Most of the skeptics were eukaryoticgeneti- community. According to Thomas Kuhn it is anoma- cists who believed that discoveries in prokaryotesare lous data that makes a paradigm no longer tenable not necessarily applicable to eukaryotes. As late as and leads to new discovery by continuous explora- 1955 Goldschmidt said "we conclude . .. that it can- tion of the anomaly (Brannigan1981). Certainly, for not be stated as dogma or as a proven fact that DNA those believing protein was the hereditary material, is the hereditary material" (Mayr 1982). Adrian Srb Avery'sdata was anomalous. Continuous exploration and Ray Owen in GeneralGenetics recognized DNA confirmedAvery's discovery many times. But, it still as being the transformingmolecule but stopped short took over fifteen years for the scientific community of making general application to all organisms (Srb to universally accept the biological role of DNA in and Owen 1955). In Genetics,Edgar Altenberg said the hereditary process. Mahlon Hoagland stated it that "it is difficult to exclude the possibility that even best: "Skepticism, generally a healthy trait in scien- in purified DNA, some protein is still associated with tists, bordered on the absurd in this case" (Hoagland occasionalmolecules. Hence, the present experiments 1981). strongly 'suggest' rather than prove, that genes are Avery died in 1955. He never won the Nobel pure DNA" (Altenberg 1957). In the fifth edition of Prize, an award he so richly deserved. Scientificsuc- Principlesof GeneticsSinnott, et al. recognized that cess, however, is best reflected in the research one the transformingprinciple "appears" to be a nucleic stimulates. Few have stimulated more than Avery. acid but also stopped short of extending this idea to Avery's work bridged the gulf between biologists all organisms (Sinnott, et al. 1958). and chemists and gave birth to the field of molecular Definitive experiments indicating a role for DNA biology. His conviction that biological specificitywas in the hereditaryprocess in eukaryotes were few and reflected in chemical specificity (Olby 1974) finally far between. Eukaryotic geneticists had to rely on convinced the scientific community that DNA is the more indirect evidence such as the fact that DNA essence of life. content but not protein content of chromosomes cor- relates with ploidy, the fact that DNA is meta- References bolicallystable and that it is found in chromosomes, ALTENBURG,E. 1957. Genetics.London: Constable and the finding that mutagenic agents interact with Co., LTD. DNA, and the fact that the base ratios of DNA are AVERY, O.T., MACLEOD, C.M., and MCCARTY,M. consistentwithin a species. Nevertheless, by the early 1944. Studies on the chemical nature of the substance 1960sDNA won by default and the skeptics silenced. inducing transformationof pneumococcal types: Induc- tion of transformationby a desoxyribonucleicacid frac- Bernard Strauss was convinced. In An Outline of tion isolated from pneumococcus type III. J. Exp. Med. ChemicalGenetics he said, "the genetic informationis 79:137-158. carried . . . by a chemical substance-deoxy- BRANNIGAN, A. 1981. The social basis of scientific discov- ribonucleic acid (DNA)" (Strauss 1960). Other eries.New York:Cambridge University Press. geneticists were similarly convinced: "Deoxy- CHARGAFF,E. 1950. Chemical specificity of nucleic acids and mechanism of their enzymatic degradation. Experi- ribonucleic acid (DNA) is the primary genetic sub- entia 6:201-209.

210 THE AMERICAN BIOLOGY TEACHER, VOLUME 46, NO. 4, APRIL 1984 EPHRUSSI-TAYLOR,H. 1951.Genetic mechanisms in bac- teriaand bacterialviruses III. Genetic aspects of transfor- mations by pneumococci. ColdSpring Harbor Symposium 16:445-446. HERSHEY,A.D. 1953. Functional differentiation within particles of bacteriophage T2. Cold SpringHarbor Sym- posium18:135-139. ., and CHASE, M. 1952. Independent functions The BiologyTest Makeris designed for biology teachers of viral protein and nucleic acids in growth of bac- to make quizzes and tests from a bank of almost 900 teriophage. J. Gen. Physiol. 36:39-56. HERSKOWITZ,I. 1965. Genetics. Boston: Little, Brown multiple choice questions. The questions are based on and Co. the most up to date concepts in biology courses. HOAGLAND, M. 1981. Discovery: The search for DNA's secrets.Boston: Houghton Mifflin Co. FEATURES HOTCHKISS,R. 1955. The biological role of the deoxy- pentose nucleic acids. In Chargaff, E., and Davidson, * MIXQUESTIONS FROM DIFFERENT UNITS ON THE J.N., (eds.), Thenucleic acids: Chemistry and biology.New SAME TEST* GENERATEANYSIZE TEST* PREVIEW York:Academic Press. THE QUESTIONS BEFORE CHOOSING 0 PREVIEW KING, R. 1965. Genetics. New York: Oxford University ON PAPER OR ON THE SCREEN 0 USES BOTH Press. UPPER AND LOWER CASE ON SCREEN AND PAPER Downloaded from http://online.ucpress.edu/abt/article-pdf/46/4/207/41261/4447817.pdf by guest on 23 September 2021 LESLIE, I. 1961. Biochemistry of heredity: A general * TESTS CAN BE SAVED ON COPYABLE DISKS . hypothesis. Nature189:260-268. ADD YOUR OWN QUESTIONS TO TEST BANK * MAYR, E. 1982. The growth of biological thought. MODIFYA TEST AT ANYTIME PRINT TEST AS TEST Cambridge,Mass.: The BelknapPress. FORM OR WITH ANSWERS * EASY TO CHANGE OLBY,R. 1974. Thepath to the doublehelix. Seattle:Univer- ORDER OF QUESTIONS * sity of WashingtonPress. POLLOCK,M.R. 1970. The discovery of DNA: An ironic tale of chance, prejudice and insight. J. Gen. Micro. 63:1-20. $75.00 (4 disks) Apple 11 SINNOTT, E., DUNN, L.C., and DOBZHANSKY, T. 1950. Principles of genetics (fourth ed.) New York: McGraw-HillBook Co. . 1958. Principles of genetics (fifth ed.) New York:McGraw-Hill Book Co. SNYDER,L. H. 1946. Theprinciples of heredity.Boston: D.C. Heath and Co. SRB, A.M., and OWEN, R.D. 1955. Generalgenetics. San Francisco:W.H. Freeman and Co. ., OWEN, R.D., and EDGAR, R. 1965. General genetics(second ed.) San Francisco:W.H. Freeman and Developed by Roy Alexander Co. Winner of the 1983 Grand Prize for the STRAUSS,B.S. 1960. An outlineof chemicalgenetics. Phila- American Heart Association Courseware contest. delphia:W.B. Saunders Co. WATSON, J.D., and CRICK, F.H.C. 1953. Molecular This program is an excellent learning tool for any structure of nucleic acids: A structure for deoxyribose student or individual who needs to learn more nucleic acid. Nature171:737-738. about his or her heart and the systems that relate to WHITE,G.E. 1962. Genetics.New York:Vantage Press. the heart. Health problems related to the heart are WYATT, H.V. 1972. How does information become also included in this program. This software is knowledge? Nature 235:86-89. recommended for use in grades 6-13, at home, and ZINDER, N.D. 1953. Infective heredity in bacteria. Cold in health institutions. Spring Harbor Symposium 18:261-269. The remarks to attributed to Hattie Alexander are appended this FEATURES paperas a discussion following Zinder'saddress. * ANIMATEDBLOOD FLOWTHROUGHAND AROUND THE HEART. MUCH USE OFTHE HIGH RESOLUTION SCREEN * TUTORIALFORMAT WITH QUIZZES AND REFERENCE UNITS . CAN BE USED INDIVIDUALLY OR IN GROUPS . INCLUDES A HEARTSAVERGAME * HAS FOUR REFERENCE UNITS * INCLUDES MORE THAN ONE GLOSSARY * ILLUSTRATED SUBJECT REFERENCE * TEACHES HEART TERMS AND HOW THE HEART WORKS.

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OSWALD AVERY 211