pk The. i SH. DY ee he, —felbhe tel an Et Sy FLO. Awe’ SCIENCE AS A WAY OF KNOWING ace An Ongoing Project of the he pro Ase Committee on Education Ghke mee Ju pf WC ft ™ of the / JN setre - American Society of Zoologists ¢ top fKerrin : —_— Cosponsored by Mu fey The American Society of Naturalists Fae The Society for the Study of Evolution The Biological Sciences Curriculum Study LAD The American Institute of Biological Sciences MeN | The American Association for the Advancement of Science The Association for Biology Laboratory Education The National Association of Biology Teachers The Society for College Science Teachers . The Genetic Society of America and the University of California, Riverside co3 SCIENCE AS A WAY OF KNOWING III—GENETICS John A. Moore Department of Biology. University of California Riverside, California 92521 Annual Meeting. December 1985. Baltimore. Copyright 1985 by the American Society of Zoologists GENETICS genetics—which shortly became the cen- tral paradigm of the biological sciences. WuatT Do GENEs Do? Nevertheless the crude probes available before 1953 made possible important dis- coveries in gene function. Among the probes were those developed for studying enzymes. During the first half of the 20th century one of the most vigorous fields of cell biology and biochemistry was the study of enzymes. Enzymes were viewed as one of the major factors making life possible. The sorts of reactions that were known or suspected to occur in cells simply could not take place without these organic catalysts. In one of those strange episodes in the history of ideas, genes and enzymes were first linked at a time when very little was known about either. An English physician, Archibald E. Gar- rod (1857-1936), had a patient, a baby, with a rare disease—alkaptonuria. It was so named because the urine of patients has alkapton bodies, which consist largely of homogentisic acid. That substance becomes dark red or black when oxidized. A clue to the patient’s problem was stains on its diapers (or, since the baby was British, its nappys). Garrod knew that the baby’s parents were first cousins and he wondered if alkapto- nuria might be an inherited disease. In 1902 () he consulted Bateson, who suggested that the disease might be due to recessive alleles. Garrod (1908a, 19084; Harris, 1963) spoke of alkaptonuria and similar ailments as “inborn errors of metabolism.’ Bateson continued to be interested and wrote in 1913a (p. 233): Alkaptonuria must be regarded as due to the absence ofa certain ferment which has the power of decomposing the sub- stance alkapton. In a normal body that substance is not present in the urine, because it has been broken up by the responsible ferment; but when the organism is deficient in the power to pro- duce that ferment, then the alkapton is excreted undecomposed and the urine is coloured by it. The hypothesis, then, is ‘‘one allelomorph, 145 one ferment.” Thirty years later, with the terminology brought up to date, this was to become one of the most important hypotheses guiding genetic research. Neither Garrod nor alkaptonuria is men- tioned in any of the books written by the Morgan school in the years of active dis- covery. Even if Morgan knew of Garrod’s hypothesis he may have ignored it. Morgan was sO pro experimental science and anti all else—including non-experimental sci- ence—that he would have viewed Garrod’s hypothesis as useless, for he had written: It is the perogative of science, in com- parison with the speculative procedures of philosophy and metaphysics, to cher- ish those theories that can be given an experimental verification and to disre- gard the rest, not because they are wrong, but because they are useless. Sturtevant in his history (1965a, p. 134) notes, There are other examples of a wide- spread failure to appreciate first-rate dis- coveries in genetics, and it is perhaps worthwhile to examine some of these briefly. Perhaps the most remarkable examples are the work of . . . and of Gar- rod on biochemical genetics .... Garrod was concerned with biochemical processes, and few geneticists were well enough grounded in biochemistry to be willing to make the moderate effort required to understand what he was talk- ing about. But possibly an important part of the answer lies elsewhere. When research pro- grams were developing rapidly and pro- ductively, as they were for the Drosophila workers, there is little stimulus to look for new things to do. It was not until the 1930s, with transmission genetics satisfactorily explained, that geneticists began an inten- sive study of the sorts of problems that interested Garrod. METABOLIC PATHWAYS IN CELLS George W. Beadle (born 1903), Edward L. Tatum (1909-1975), and Boris Ephrussi (1901-1979) were leaders in the quest for information on how genes act. By the late 146 Hows Thar For CARC POL Paco & RErD tore. . . 1930s there was considerable information about cell metabolism. That fundamental reaction of all life, It Le Cc C,H,20,4 + 60, - 60, + 60% had been resolved into several dozen sep- arate reactions, each controlled by a spe- cific enzyme. The elucidation of this one metabolic pathway had required the efforts of many scientists for many years. One of the major problems was the speed of the reactions, often requiring a fraction of a second. How was one to study a reaction that would be over before the investigator knew it had started? The standard way was to use chem- ical substances (‘enzyme poisons”) that would block the action of a specific enzyme. The result would be that the substrate for that enzyme would then accumulate in the cell and could possibly be detected and identified. Assume, for example, that one metabolic pathway in cells involves molecule A being changed into molecule B and then B into molecule C and then down the alphabet to molecule Z. We will assume that the change from A to B is controlled by enzyme A-ase and from B to C by B-ase and from Y to Z by Y-ase. All we know at first is that the cell changes molecule A to molecule Z. That is, the conversion may be accom- plished by a single enzyme in a single reac- tion. One of the first enzyme poisons we try is cyanide. We observe that no Z is formed and, instead, a previously undetected mol- ecule, M, is found. What can we conclude? Can we say that the cell converts A to Z in two steps: A is converted to M and then M to Z? That may have been said a few gen- erations earlier but, as the complexity of intracellular metabolism came to be under- stood in the 1930s, the conclusion would be no more than “‘there are at least two intermediary steps from A to Z.” Other poisons could be tried, and with time more and more could be learned about normal metabolism by throwing these chemical wrenches into the biochemical gears of the cell. Some early studies of Beadle and Ephrussi on the way that eye color genes Joun A. Moore of Drosophila produce their effects had indicated that gene action might be mediated by enzymes. Enough was discov- ered to suggest that the hypothesis “one gene, one enzyme” might be a fruitful approach. The biochemistry of Drosophila proved to be too complex to test that hypothesis and for the first time that noble animal let a geneticist down. So a long-standing experimental tech- nique was invoked: if the experiment can- not be done with one organism, search for another one that is suitable. By this time Beadle was at the California Institute of . Technology with Morgan. Before Morgan left Columbia, Bernard Dodge of the New York Botanical Garden gave him a culture of the red bread mold, Neurospora crassa, in the belief that it might be of use in genetic experiments. Morgan never used Neuro- spora but it was still being cultured in his laboratory when Beadle and Tatum sought an organism for their research. NEUROSPORA CRASSA Beadle and Tatum (1941) reasoned that lethal mutations change alleles so that they are incapable of producing an enzyme essential for the life of the organism. Thus they intended to induce lethal mutations with radiations and to study their biochem- ical effects. This might appear to your stu- dents to be a considerable problem since, if the lethal kills the individual, there would not appear to be much to investigate. But Beadle and Tatum solved that problem in what was surely one of the most innovative and productive lines of experimentation in the late 1930s and 1940s. Others must have thought so too because Beadle and Tatum shared a Nobel Prize for this work. For reasons that will shortly become apparent they first had to determine exactly the minimum variety of molecules required for normal growth—the minimal medium. The menu was surprisingly simple: air, water, inorganic salts, sucrose, and the vitamin biotin. Neurospora is, of course, composed of innumerable organic com- pounds, all interacting as the life of that organism. Yet from those few raw mate- rials it is able to synthesize all of the amino acids, proteins, fats, carbohydrates, nucleic GENETICS acids, vitamins, and other substances of its body. As an example of the many experiments done by Beadle and Tatum, we will discuss those concerned with the synthesis of the amino acid arginine. The working hypoth- esis was that specific genes control the pro- duction of specific enzymes that catalyze the reactions that lead to the formation of arginine. Presumably these genes could mutate to allelic forms that would either be unable to make the enzyme or not be able to make it in sufficient quantity. Since arginine is essential for the life of Neuro- spora, such mutations would be lethal. Beadle and Tatum then devised a method for the production of these lethal muta- tions, for identifying them as related to the synthesis of arginine, and for maintaining them in culture in order to work out the metabolic pathway of arginine synthesis. This may sound impossible, especially when we realize that for most of its life cycle Neurospora is monoploid and hence any lethal mutations could not be carried as heterozygotes. This was their game plan. First, X-rays were used to induce mutations. They assumed that all sorts of mutations would be produced, but by chance some might be involved with the production of arginine. When we remember how rare any specific mutation would be, the chance of obtain- ing the desired mutations would be exceed- ingly small. Spores from the irradiated Neurospora were then placed on the minimal growth medium. Most of them grew, showing that whatever mutations may have occurred none was so serious as to prevent the Neu- rospora from synthesizing all of its sub- stance from the few chemicals in the min- imal medium. Other spores did not germinate, and among these might be some biochemical mutants that could not pro- duce the enzymes necessary for normal growth and development. And somewhere among them might be genes involved in the synthesis of arginine. How could one find them? The spores were not germinat- ing, so they were for practical purposes “dead.” The solution of this apparently insolva- 147 ble difficulty was elegant in its simplicity and effectiveness. If the spores could not grow because they could not synthesize . their own arginine, why not give it to them? And that is precisely what Beadle and Tatum did. Again most of the spores did not grow but a precious few did. Among these precious few might be mutants of genes involved in arginine synthesis. The next, and critical, step in the anal- ysis was to make sure that whatever was wrong with the spores was inherited. It could not be concluded that, just because the otherwise “lethal” spores could grow on arginine, that a mutational event was the cause. The life cycle of Neurospora makes it ideal for some sorts of genetic analysis. The col- onies are monoploid for nearly their entire life. There are two mating types, A and a, which cannot be distinguished except by their mating behavior. If colonies of A and a are grown together, parts of each will fuse and A nuclei will unite (‘fertilize’) with a nuclei to form diploid zygotes. Mei- osis occurs immediately and 4 monoploid spores are formed. These divide, by mito- sis, to produce 8 monoploid spores. These 8 spores are enclosed in an elongate spore sac (ascus). They are arranged in the sac in a linear order that reflects the two meiotic divisions and the single mitosis. The spore sacs can be opened under a micro- scope and the individual spores removed and placed in culture media. Thus one can obtain all of the preducts of meiosis of a single zygote. The presumed mutant strains were crossed to normal strains. Meiosis occurred immediately afterwards and monoploid spores were formed. These were then iso- lated. Half were found to grow on the min- imal medium and half only if arginine was added. These results were consistent with the hypothesis that the wild-type Neuro- spora had a gene A, which was necessary for the synthesis of arginine. The radiation treatment had caused a mutation of A to a and a was unable to play some essential role in arginine synthesis. The experimental procedure appeared to be working and numerous genetic strains were isolated that required arginine for 148 growth. Were all the genetic strains alike or had different genes mutated to alleles that could not synthesize arginine? Can your students suggest how one could go about answering that question? There were two possible answers: First, all of the mutant strains could be due to changes at a single gene locus. Second, many different loci could have mutated. In this case one would suspect that many genes are involved in arginine synthesis: A,, Aj, As, A,, etc. Any one of these could have mutated to a,, ao, etc. In all these mutants the same phenotype would be observed— inability to grow on minimal medium without arginine. Crosses could test the alternatives. If a single locus is involved, a cross of two strains would produce spores unable to grow without arginine. Alternatively, if dif- ferent loci are involved, some of the spores will grow as wild-type colonies for the fol- lowing reason. Assume that different genes are involved and we are crossing a, X ay. If a mutation had occurred at only one locus in each strain, which is overwhelm- ingly probable (why?), the mutated strain would have a normal allele at the other locus. Thus, mutant strain a, would be expected to have Ay. Strain a, would be expected to have A,. Thusa cross of a,A, X A, a, would produce diploid zygotes with a genotype A,a, A,a.. Meiosis then occurs and the monoploid spores are produced. If the two loci are on different chromo- somes the isolated spores should give these results: ‘4 should be A, A, and grow on minimal medium. ‘A should be A,a, and will require argi- nine since a, cannot function. ‘4 should be a,A, and require arginine since a, is not functioning. ‘4 should be a,a, and require arginine since neither allele can function. If the loci are on the same chromosome, the frequency of the four genotypes will depend on the amount of crossing-over. Early on in the experiments, Beadle and Tatum discovered seven genetically differ- ent mutants, each requiring supplemental Joun A. Moore arginine if it was to grow normally. Various interpretations of the data were possible but Beadle and Tatum preferred the hypothesis that the synthesis of arginine required that at least seven normal genes be present—each producing an essential enzyme. When any one of these genes mutated in such a way that its specific enzyme could not be produced, the syn- thesis of arginine was blocked. There was no reason to believe, of course, that there are only seven steps in the synthesis of argi- nine in Neurospora. We can conclude only that seven was the minimum number. It was possible to extend the analysis by taking advantage of what was already known about the synthesis of arginine. In 1932 the biochemist Hans A. Krebs had discovered that in some vertebrate cells arginine is formed from citrulline, citrul- line from ornithine, and ornithine from an unknown precursor. A specific enzyme is required for each transformation. If Neurospora has a similar metabolic pathway, one should be able to determine how the seven mutant strains are involved. This could be done by seeing which, if any, of the seven would grow if either citrulline or ornithine was used to replace arginine. Your students should be able to predict what conclusions could be drawn ifa mutant strain, normally requiring supplemental arginine, wquld grow if citrulline was sub- situted or if ornithine was substituted. Many experiments were done. Four of the mutant strains would grow if either ornithine, citrulline, or arginine was added. This suggested that these four mutants were involved in reactions before the orni- thine stage. If ornithine was added, the remaining enzymatic steps, being normal, could carry the reactions to arginine. Two of the strains would not grow if only ornithine was added but they would grow if either citrulline or arginine was added. In these cases the block was between orni- thine and citrulline. Since two genetically different strains were both blocked between ornithine and citrulline, it is reasonable to conclude that there are at least two steps between these molecules. Finally, one strain was found that would GENETICS grow only if arginine was added. This sug- gests that some enzyme between citrulline and arginine was deficient or defective. Thus, Beadle and Tatum were able to conclude that, for Neurospora to synthesize arginine, a minimum of seven enzyme-con- trolled reactions are required and a mini- mum of seven kinds of molecules are involved. Two of these are known: orni- thine and citrulline. The hypothesis that a function of genes is to control the production of specific enzymes was supported. One could not conclude that this is the only thing genes do. Beadle and Tatum had designed their experiments solely to detect enzymes involved in metabolic pathways. Much as Sutton had linked cytology and genetics in the early 1900s, Beadle and Tatum effectively linked genetics and bio- chemistry in the early 1940s. Their type of experimentation was used immediately by numerous other investigators on other molds, yeasts, and bacteria. This approach led directly to the molecular biology of today. While all this was going on still another attempt to study genetics at the molecular level was underway. This was a line of investigation that began in the 1920s and ultimately led to the positive identification of the gene as DNA. That will be our final topic, bringing us to the formulation of the current paradigm of genetics by Watson and Crick in 1953. THE SUBSTANCE OF INHERITANCE The dynamics of scientific discovery elude us to this day. There is no way of predicting the who?, the what?, and the where? Important discoveries are nearly always made by scientists active in the field. The breakthrough may be made by an out- standing scientist or by a novice. Neither Mendel, Sutton, Morgan, Watson nor Crick was a leader in the field of inheritance to which each made such notable contribu- tions. The revolution in biology that fol- lowed from Watson and Crick (1953a, 1953) was due in part to scientists from other fields (mainly physics) deciding that the problems in biology were more excit- 149 ing than their own (Fleming, 1968; Judson, 1979), Many prominent molecular genet- icists of today remember being made aware of new possibilities for genetic research by a slender book written by Schrédinger (1945), himself a physicist. It could be that it is easier for those not steeped in the data and traditions of a field to see problems and solutions clearly than for those fully engaged in their Kuhnian normal science. As Hanson says (1965, p. 30): Physical science is not just a systematic exposure of the senses to the world; it is also a way of thinking about the world, a way of forming conceptions. The par- adigm observer is not the man who sees and reports what all normal observers see and report, but the man who sees in familiar objects what no one else has seen before. Some important discoveries are the out- come of deliberate attempts to find answers to specific questions. In other cases discov- ery is more of an accident. The elegant experiments of Beadle and Tatum are ex- amples of experiments planned to test a specific hypothesis. The road to DNA was not nearly so straight. The zero milestone cannot be identified but we can start in 1928 with some observations in another field that were to lead, a quarter of a cen- tury later, to the description of the chem- ical structure of DNA. a TRANSFORMATION IN PNEUMOCOCCUS Pneumonia in human beings and many other mammals is caused by the pneumo- coccus bacterium (properly known as Diplococcus pneumoniae). As in many dis- ease-causing microorganisms, there are numerous genetic strains. These are called Type I, Type II, etc. The specificity is based on the chemical composition of the bac- terium’s polysaccharide coat. The strains are identified immunologically. If they are injected into rabbits, antibodies are formed against the polysaccharide antigens. If capsulated cells are grown on culture plates, they form colonies that are smooth and shiny. Some of the colonies may have 150 a different appearance—they are rough. These changes were observed long before the cause was known—the change from smooth to rough is the result of a gene muta- tion. There was considerable medical interest in this phenomenon because the smooth cells cause pneumonia but the rough mutant does not. It was discovered that the smooth cells have the polysaccharide cap- sules but the rough cells do not. The road to DNA begins in 1928 with F. Griffith, a Medical Officer with the Brit- ish Ministry of Health. His publications give no evidence of an interest in genetics; he was a medical bacteriologist concerned with diseases of human beings. He knew that if he injected mice with capsulated Type II smooth (capsulated) cells, they would die. Type II rough (non-capsulated) cells would not cause the death of his mice. However, heat-killed smooth cells did not kill the mice. Therefore, it was not the polysaccharide coat that was the cause of death. The next experiment is the crucial one for us. Griffith gave four mice a double injection of Type II cells: living rough cells plus dead smooth cells. Survival was expected, since the rough cells are not pathogenic and the pathogenic smooth cells had been killed. Nevertheless, all four mice died after five days. Type II smooth cells were found in their blood. Thirty control mice injected only with living rough cells remained healthy. This was an unbelievable result—but the experiment was repeated and confirmed. It appeared that the ability to synthesize a capsule had been transferred from the dead capsulated cells to the living non-capsu- lated cells. Any geneticist of 1928 who might have known of these experiments would have shuddered and rededicated himself to Drosophila melanogaster. During those years geneticists ignored microorganisms almost entirely and micro- biologists ignored genetics. It was not sus- pected by either group that microorgan- isms possessed a genetic system remotely similar to that of higher organisms. Joshua Lederberg, who asa young student worked in the Zoology Department at Columbia University and who was to find that ‘“‘adap- Joun A. Moore tation” in bacteria is a mutational event, was far in the future. A later generation of geneticists might have suspected that a mutation from rough to smooth had occurred but another exper- iment by Griffith showed this not to be so. This time the living and the dead cells were of different Types. The living cells without capsules (rough) were Type II and the killed cells with capsules (smooth) were Type I. Eight mice were injected and two died. Their blood was found to contain virulent capsulated cells of Type I. Somehow the Type II non-capsulated cells had been transformed to Type I. This was not a tran- sitory change. They were cultured and thereafter remained Type I. The change was permanent, and hence in a broad sense genetic. In today’s terms we also might sus- pect the transformation to virulence to be due to mutation. But this second experi- ment rules out that possibility since, had the living Type II cells mutated from cap- sule-less to capsulated, they would still have been Type II. However, the capsulated cells were like the dead cells, Type I. This line of research was taken up by many bacteriologists, including M. H. Dawson and Oswald T. Avery of the Rock- efeller Institute in New York. They became convinced that transformation must be due to some chemical substance and it was rea- sonable to suspect the polysaccharide of the capsule. Nevertheless that proved not to be so. Alloway, another member of the Rockefeller group, summed up the prob- lem in 1932 as follows (with my paraphras- ing): The polysaccharide when added in chemically purified form, has not been found effective in causing transforma- tion of non-capsulated organisms derived from Diplococcus of one Type into cap- sulated forms of the other Type. When non-capsulated cells change into the cap- sulated form they always acquire the property of producing the specific cap- sular substance. The immunological specificity of the encapsulated cell depends upon the chemical constitution of the particular polysaccharide in the GENETICS 151 capsule. The synthesis of this specific polysaccharide is a function peculiar to cells with capsules. However, since the non-capsulated cells under suitable con- ditions have been found to develop again the capacity of elaborating the specific material, it appears in them this function is potentially present, but that it remains without effect. However, a then available crude deoxyribonuclease destroyed the activity of the purified transforming sub- stance. What does this all mean? This is how Avery, MacLeod, and McCarty inter- preted their experiments (see also McCarty, 1985): latent until activated by specific environ- mental conditions. The fact that a non- capsulated strain derived from one Type of Diplococcus, under the conditions defined in this paper, may be caused to acquire the specific characters of the capsulated forms of a Type other than that from which it was originally derived, implies that the activating stimulus is of a specific nature. There is nothing in this quotation, or in the writings of other bacteriologists of the period, to suggest that transformation might be a genetic phenomenon. It seemed more probable that some sort of physio- logical modification had occurred. Many bacteriologists at the time suspected that some sort of Lamarckian evolution was responsible for this phenomenon known as ‘‘adaptation.” It was much later that it was found that mutation and selection would account for the phenomena observed. DNA Is THE TRANSFORMING SUBSTANCE But if “the activating stimulus is of a spe- cific nature,” hard work and luck might discover what it is. It was found that the transforming principle could be extracted from capsulated cells and that transfor- mation could occur in vitro—no need that mice be used. After a decade Avery, MacLeod, and McCarty (1944) reported that they had purified the transforming substance and that it was almost certainly DNA. The overall elemental composition of the transforming principle agreed closely with that of DNA. The molecular weight was judged to be about 500,000. The sub- stance was highly active—one part in 600 million was effective. Treatment with tryp- sin and chymotrypsin left activity intact indicating that it was not protein. Ribo- nuclease, which denatures RNA, was also Various hypotheses have been advanced in explanation of the nature of the changes induced. In his original descrip- tion of the phenomenon Griffith sug- gested that the dead bacteria in the inoc- ulum might furnish some specific protein that serves as a ‘pabulum’ and enables the [non-capsulated] form to manufac- ture a capsular carbohydrate. More recently the phenomenon has been interpreted from a genetic point of view. The inducing substance has been lik- ened to a gene, and the capsular antigen which is produced in response to it has been regarded as a gene product. In dis- cussing the phenomenon of transfor- mation Dobzhansky has stated that “If this transformation is described as a genetic mutation—and it is difficult to avoid so describing it—we are dealing with authentic cases of induction of specific mutations by specific treatments It is, of course, possible that the biolog- ical activity of the substance described is not an inherens property of the nucleic acid but is due to minute amounts of some other substance adsorbed to it or so intimately associated with it as to escape detection. If, however, the bio- logically active substance isolated in highly purified form as the sodium salt of deoxyribonucleic acid actually proves to be the transforming principle, as the available evidence strongly suggests, then nucleic acids of this type must be regarded not merely as structurally important [at the time biochemists could not discover any function for the nucleic acids} but as functionally active in deter- mining the biochemical activities and specific characteristics of [the bacterial] 152 cells. Assuming that the sodium deoxy- ribonucleate and the active principle are one and the same substance, then the transformation described represents a change that is chemically induced and specifically directed by a known chemical compound. If the results of the present study on the chemical nature of the transforming principle are confirmed, then nucleic acids must be regarded as possessing biological specificity the chemical basis of which is as yet unde- termined. Was DNA only an inducing agent or was it something else? Most geneticists would probably have agreed with Dobzhansky that DNA could not be the genetic material. The evidence was fairly convincing. Enough was known about DNA to realize that it was a rather simple molecule—com- posed of a few bases, a simple sugar, and phosphate. Presumably an extremely com- plex substance would be required to con- trol the life of cells. Proteins were a far more likely candidate than DNA to be the gene. They could be huge and were com- posed of a number of amino acids about equal to the number of letters in our alpha- bet. Just as the combinations of a few let- ters can give us the uncounted numbers of words in the languages of the world, that same number of amino acids should be ade- quate to supply all the genetic variation required. Core OR CoAT? The answer came in less than a decade: DNA is the gene, not a mutagenic agent. One of the more important experiments was done in 1952 by A. D. Hershey and Martha Chase. By that time much more sophisticated experimentation was possi- ble. In large part as a result of the work on the atom bomb in World War II many sorts of radioactive substances had been produced that could be used to study intra- cellular reactions. Methods were devel- oped for culturing many different sorts of microorganisms and, for many reasons, they were becoming the favorite experi- mental organisms for geneticists. There was also very much more research being done. JOHN A. Moore The extraordinary contributions of scien- tists to the war effort were recognized in Washington and the work of scientists began to be supported on a lavish scale. It was estimated that in the 1950s the num- ber of active scientists was equal to all the scientists who had ever lived. Big Science was national policy and a national activity. Hershey and Chase took advantage of the peculiar life cycle of bacteriophage to ascertain whether or not DNA contains the information for that organism. Bacterio- phages, or phages, are incapable of an independent life. They are parasites of bac- teria, upon which they depend for their own reproduction. If the bacterium Escherichia coli is infected with a phage called T,, the bacterium is killed in about 20 minutes. Before entrance of the phage, the bacterial cell was synthe- sizing its own specific molecules: bacterial proteins, bacterial nucleic acids, and so on. The phage changes all this. It assumes con- trol of the bacterial synthetic machinery and diverts it to producing phage mole- cules instead of E. coli molecules. About 100 phages are made in about 20 minutes. The bacterium bursts and liberates the phages. They can then enter (they must if they are to live and reproduce) other bac- terial cells and repeat the process. There are many kinds of phages that maintain their genetic identity and other specific characteristics. Structurally they are simple, being composed of a protein coat and a DNA core. The protein of the phage coat is chemically very different from the DNA core. The coat contains sulfur but little or no phosphorus. The reverse is true for DNA. Radioactive isotopes of both phosphorus and sulfur were available to Hershey and Chase. The experiment was as follows. One group of bacteria was grown in a medium with *P, which became incorporated in the bacterial molecules. Later, phages were introduced. When the bacteria then began to synthesize new phages, the latter’s DNA became tagged with the **P. The protein coat would have little or no label. In a parallel experiment bacteria were grown in a medium containing *S. This became incorporated in some of the bac- GENETICS terial proteins. Later phages were intro- duced and in this case the protein coats of the phages became labelled with °5S. These two sorts of phages, one labelled for the protein coat and the other for the DNA, were then used in separate experi- ments. They were introduced into cultures of bacteria and Hershey and Chase found that the labelled DNA entered the bacte- rial cells. The labelled protein remained on the outside. These observations, together with others, suggested that the phage attaches itself to the cell wall of the bacterium and injects its DNA core, the coat remaining on the outside. The phages in both experiments repro- duced and destroyed the bacterial cells. The experiments had shown that the entire genetic information on “how to make phage” is contained in the phage DNA. The work surveyed in this chapter, together with a very much larger amount going on at the same time, leads to this tremendous thought: the once mysterious gene, which though invisible could be mapped and followed through the gener- ations with precision, is revealed as an iden- tifiable molecule—DNA. Just as E. B. 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THE END This essay, as part of the symposium Sci- ence as a Way of Knowing—Genetics, has sought to provide a background for the papers by the symposium speakers and to provide materials for teachers using the science as a way of knowing approach. For the most part, the speakers will be dealing with events that occurred after 1953. These events have been so staggering in their importance and different in their problems and procedures that we must recognize that a new paradigm now guides the investi- gators. The old paradigm of the Chromosomal Theory of Heredity, or transmission genetics, held the attention of geneticists to the mid-1930s but by then it was so well established thats geneticists sought new challenges. It was during the 1930s and 1940s the groundwork was laid for an attack at the molecular level on what genes are and what they do. Molecular genetics is very different from classical genetics, which is the concern of this essay. And that raises a difficult problem for what should be taught in the first-year biol- ogy course in the colleges and universities when the time available is severely limited. Can Mendel, Sutton, and Morgan hold the attention of students who live in a world where genetic engineering is about to per- form its miracles? Should students be taught about these classical experiments and con- cepts? I think they should and there is no need 154 for an either/or structuring of the curric- ulum. The basic argument of the Science as a Way of Knowing approach is that students are best served if they are provided with the conceptual framework of the field. Full appreciation of the events of today is pos- sible only if that conceptual framework is understood. There is a practical matter also. Few stu- dents in first-year courses have the back- ground necessary to understand the tre- mendously sophisticated experiments and data of modern molecular genetics. In many instances they may be able to mem- orize the material but I am talking about something else—understanding. Classical genetics, on the other hand, is approach- able to a considerable degree by students in first-year courses. They really can understand the questions, the data, and the reasons for the conclusions. This is another of our goals—having students understand how science works. Nevertheless we serve our students poorly if we leave them ignorant of the general results and especially the implica- tions of the science of the day. My rec- ommendation, therefore, is to emphasize classical genetics and then discuss the main conclusions of molecular genetics, stress- ing its implications for better health and better food. And, most certainly, there should be consideration of some of the more difficult ethical questions that are being raised by molecular genetics. Remember also that everything does not have to be included in a first-year course. Something of importance and interest should be left for the more advanced courses. Biologists, alone among scientists, seem to believe that all the cream has to come that first year. It really does not. My suggestions may not have much appeal for some university scientists for according to Sydney Brenner (Nature 317: 209, 1985): For most young molecular biologists, the history of their subject is divided into two epochs: the last two years and every- thing else before that. The present and very recent past are perceived in sharp detail but the rest is swathed in a leg- JouN A. Moore endary mist where Crick, Watson, Men- del, Darwin—perhaps even Aristotle— coexist as uneasy contemporaries. Too bad, if so. We have to do better for our students. ACKNOWLEDGMENTS Any degree of success that the Science as a Way of Knowing project may achieve is based on the support of the Carnegie Cor- poration of New York. They have pro- vided us with the funds to pay the travel expenses (small) of the symposium partic- ipants and for publication and distribution of the proceedings (large) for the first four years of our operations. The personnel of the Corporation have been understanding and sympathetic. This support in no way implies that the Corporation is responsible for any statements or views expressed but we would like to believe that our efforts are advancing the Corporation’s mission— the improvement of education. Many individuals have helped with the preparation of my essay. The first draft was read by Betty Moore and, after making the corrections she suggested, a revised draft went to Ingrith Deyrup-Olsen and William V. Mayer. Their suggestions were the basis of the final draft. Essential support has been provided by the Riverside Campus of the University of California, Not only do I rely on the help of many colleagues, but space, some office . supplies, much of the telephone costs, and some of the mailing expenses are provided. We depend also on the friendly coop- eration and superb professionalism of the staff of Allen Press. Very little time is avail- able for the preparation of these long essays and the Press allows me the maximum and themselves a minimum even when this makes their own schedule most difficult. Many individuals have helped me, in manners large and small, and to them | extend my sincere gratitude: William L. Belser, Jr., *Mark Chappell, Kenneth W. Cooper, Joe W. Crim, James F. Crow, W. M. Dugger, Tamir Ellis, Peter von Hippel, Leroy Hood, Oliver Johnson, Edward B. Lewis, R. C. Lewontin, Mary Price, Herb Quick, Rodolfo Ruibal, Vaughan Shoe- GENETICS maker, William V. Thomson, E. Peter Volpe, J. Giles Waines, Nickolas Waser, and Lewis G. Weathers. Oxford University Press has given me permission to use material from my Hered- ity and Development and Readings in Heredity and Development, both published by the Press in 1972. My thanks. REFERENCES Ackerknecht, E. H. 1953. Rudolf Virchow, doctor, statesman, anthropologist. Univ. of Wisconsin Press, Madison. Allen, G. E. 1966@a. Thomas Hunt Morgan and the problem of sex determination. Proc. Amer. Phil- osophical Soc. 110:48-57. Allen, G. E. 19666. T.H. Morgan and the emergence of a new American biology. Quart. Rev. Biol. 44: 168-188. Allen, G. E. 1969. Hugo de Vries and the reception of the “mutation theory.” Jour. History Biology 2:55-87. Allen, G. E. 1974a. Opposition to the Mendelian- chromosome theory: The physiological and developmental genetics of Richard Goldschmidt. Jour. History Biology 7:49-92. Allen, G. E. 19744. Morgan, Thomas Hunt. Diction- ary of Scientific Biography 9:515-526. Allen, G. E. 1975¢. 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A supplement to the account of a distempered skin, published in the 424th number of the Philosophical Transactions. Philosophical Transactions 49(Pt. 1):21-24. Baker, J. R. 1948-1955. The cell theory: A restate- ment, history, and critique. Quarterly Jour. Microscopic Science 89:103-125; 90:87-108; 93: 157-190; 94:407—440; 96:449-481. Balbiani, E. G. 1881. Sur la structure du noyau des cellules salivaires chez les larves de Chironomus. Zool. Anzeiger 4:637-641, 662-666. Baltzer, F. 1964. Theodor Boveri. Science 144:809— 815. Baltzer, Fritz. 1967. Theodor Boveri. Life and work of a great biologist, 1862-1915. Univ. of California Press, Berkeley. Barlow, Nora (ed.) 1958. The autobiography of Charles Darwin, 1809-1882; with original omissions restored; edited with appendix and notes by his grand-daughter. Collins, London. Barthelmess, Alfred. 1952. Vererbungswissenschaft. Karl Alber, Freiburg. Bateson, Beatrice. 1928. William Bateson, F.R.S. Nat- uralist. Cambridge Univ. Press, London. Bateson, William. 1894. Maierials for the study of vari- ation treated with especial regard to discontinuity in the origin of species. Macmillan, London. Bateson, W. 1900a. sHybridisation and cross-breed- ing as a method of scientific investigation. Jour. Royal Horticultural Soc. 24:59-66. Bateson, W. 19008. Problem of heredity as a subject for horticultural investigation. Jour. Royal Hor- ticultural Soc. 25:54-61. Bateson, William. 1902. Mendel's principles of heredity. A defence. Cambridge Univ. Press, Cambridge. Bateson, W. 1906. An address on Mendelian hered- ity and its application to man. Brain 29:157-179. Bateson, W. 1908. The methods and scope of genetics. An inaugural lecture delivered 23 October 1908. Cambridge Univ. Press, Cambridge. Bateson, William. 1909. Mendel’s principles of heredity. Cambridge Univ. Press, Cambridge. [Reprinted with additions, 1913.] Bateson, William. 1913a. Above with additions. Bateson, William. 19134. The problems of genetics. Yale Univ. 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