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note: because important web-sites are frequently "here today but gone tomorrow", the following was archived from http://www.pbs.org/wgbh/nova/photo51/before.html on November 15, 2006 . This is NOT an attempt to divert readers from the aforementioned web-site. Indeed, the reader should only read this back-up copy if it cannot be found at the original author's site.
Before Watson and Crick
by Brenda Maddox
James Watson and Francis Crick stumbled upon the structure of DNA, but only by making use of the discoveries of many scientists who came before them including Rosalind Franklin.
The first half of 20th Century science belonged to Physics with the General Theory of Relativity, Quantum Mechanics, and nuclear fission. The second half would belong to Biology. In the post-war world, the secret of the gene -- how hereditary characteristics pass from one generation to another -- was the hottest topic in science.
For a number of physicists who had worked on the Manhattan Project to develop the atomic bomb, the post-war shift into biology was a stark exchange of the science of Death for the science of Life. But their conversion was as much intellectual as ideological. Biology was now where the action lay. The war had interrupted a line of investigation leading towards understanding the chemical basis of heredity.
That physical features are passed on by discrete units (later called "gene") had been discovered in 1865 by the Austrian monk Gregor Mendel in his experiments with garden peas. Each gene determined a single characteristic -- such as height or color -- in the next generation of plant. By 1905, it had been learned that within living cells the genes are strung together like beads on the chromosomes, which copy themselves and separate. But how does the genetic information get from the old chromosome to the new?
Protein was the obvious candidate. By the 1920s, it was thought that genes were made of protein. The other main ingredient in the chromosome is deoxyribonucleic acid (or DNA). A substance of high molecular weight, DNA was identified in 1871 by a young Swiss scientist, Friedrich Miescher. (There is, in fact, a second kind of nucleic acid in the cell called RNA, with a slightly different chemical composition.) The "D" in DNA stands for "deoxy" -- a prefix often spelled as "des" in Rosalind's day, a usage now obsolete -- which identifies it as the ribonucleic acid with one fewer hydroxyl group. But as RNA exists in cells mainly outside the nucleus, it was unlikely to be the genetic vehicle.
Protein was far more interesting to geneticists than DNA because there was a lot more of it. And also because each protein molecule is a long chain of chemicals, of which 20 kinds occur in living things. DNA, in contrast, contains only 4 kinds of the repeating units called nucleotides. Hence it seemed too simple to carry the complex instructions required to specify the distinct form of each of the infinite variety of cells that constitute living matter.
In 1936, at the Rockefeller Institute on the Upper East Side of Manhattan, a microbiologist called Oswald Avery wondered aloud if the "transforming principle" -- that is, the carrier of the genetic information from old chromosomes to new -- might not be the nucleic acid, DNA. No one took much notice. DNA seemed just a boring binding agent for the protein in the cell.
During the pre-war years, in Britain, J.D. Bernal at Cambridge and William Astbury at Leeds -- both crystallographers -- began using X-rays to determine the structure of molecules in crystals. Astbury -- interested in very large biological molecules -- had taken hundreds of X-ray diffraction pictures of fibers prepared from DNA. From the diffraction patterns obtained, Astbury tried building a model of DNA. With metal plates and rods, he put together a Meccano-like model suggesting how DNA's components -- bases, sugars, phosphates -- might fit together.
Astbury concluded -- correctly, as it turned out -- that the bases lay flat, stacked on each other like a pile of pennies spaced 3.4 Ångströms apart. [An Ångström equals one ten-billionth of a meter.] This "3.4 Å" was no gratuituous detail. Published with other measurements in an Astbury paper in Nature in 1938, it was to remain constant throughout all the attempts to solve DNA's structure that were to come.
Avery’s discovery has been called worth 2 Nobel Prizes. But he never got even one.
But Astbury made serious errors. His work was tentative, and he had no clear idea of the way forward. By the time of the Second World War, no one knew that genes were composed entirely of DNA.
In 1943, Avery at 67 was too old for military service. Still working at the Rockefeller Institute and building on an experiment with pneumococcus (bacteria that cause pneumonia) done by the English physician Frederick Griffith in 1928, he made a revolutionary discovery. He found that when DNA was transferred from a dead strain of pneumoccocus to a living strain, it brought with it the hereditary attributes of the donor.
At a time when many scientists thought proteins must ferry heritable attributes to the next generation, microbiologist Oswald Avery discovered that the genetic carrier was in fact the humble nucleic acid DNA.
as the "transforming principle" so simple then -- purely DNA? In Science where many grab for glory, there are some who thrust glory from them. Avery -- a shy bachelor who wore a pince-nez -- was one of those too modest for his own good. His discovery has been called worth 2 Nobel Prizes, but he never got even one. Perhaps because rather than rushing into print, he put his findings in a letter to his brother Roy - a medical bacteriologist at Vanderbilt University Medical School in Nashville. "I have not published anything about it -- indeed have discussed it only with a few," he said, "because I am not yet convinced that we have (as yet) sufficient evidence."
A year later, however, Avery with 2 colleagues wrote out their research. In what became a classic paper, they described an intricate series of experiments using the 2 forms of pneumococcus -- virulent and nonvirulent. When they freed a purified form of DNA from heat-killed virulent pneumococcus bacteria and injected it into a live, nonvirulent strain, they found that it produced a permanent heritable change in the DNA of the recipient cells. Thus the fact was established -- at least for the readers of The Journal of Experimental Medicine -- that the nucleic acid DNA -- and not the protein -- was the genetic message-carrier.
The essential mystery remained: how could a monotonous substance such as DNA -- like an alphabet with only 4 letters -- convey enough specific information to produce the enormous variety of living things from daisies to dinosaurs? The answer must lie in the way the molecule was put together. Avery and his co-authors -- Colin MacLeod and Maclyn McCarty -- could say no more than that "nucleic acids must be regarded as possessing biological specificity the chemical basis of which is as yet undetermined."
In 1943, another scientist at one remove from the world conflict (because he had been offered a haven in neutral Ireland) gave a series of lectures in Dublin called provocatively "What is Life?" An audience of 400 for every lecture suggested that his supposedly difficult subject was of great general interest.
rwin Schrödinger -- a Viennese -- had shared the Nobel Prize in physics in 1933 for laying the foundations of wave mechanics. That same year he left Berlin where he had been working because -- although not himself Jewish -- he would not remain in Germany when persecution of the Jews became national policy. A long odyssey through Europe brought him in 1940 to Dublin at the invitation of Eamon de Valera, Ireland's premier. De Valera had been a mathematician before he became a revolutionary, then a politician. In 1940 he set up the Dublin Institute of Advanced Studies. Schrödinger found Ireland "paradise", not least because it allowed him the detachment to think about a very big question.
In his Dublin lectures, Schrödinger addressed what puzzled many students: why bBiology was treated as a subject completely separate from Physics and Chemistry. Frogs, fruit flies, and cells on one side with atoms and molecules, electricity, and magnetism on the other. The time had come -- Schrödinger declared from his Irish platform -- to think of living organisms in terms of their molecular and atomic structure. There was no great divide between the living and nonliving. They all obey the same laws of Physics and Chemistry.
He put a physicist's question to Biology. If entropy is (according to the Second Law of Thermodynamics) things falling apart -- the natural disintegration of order into disorder -- why don't genes decay? Why are they instead passed intact from generation to generation?
^ was the Uncle Tom’s Cabin of Biology -- a small book that started a revolution.
He gave his own answer. "Life" is matter that is doing something. The technical term is metabolism -- "eating, drinking, breathing, assimilating, replicating, avoiding entropy." To Schrödinger, Life could be defined as "negative entropy" -- something not falling into chaos and approaching "the dangerous state of maximum entropy which is Death." Genes preserve their structure because the chromosome that carries them is an irregular crystal. The arrangement of units within the crystal constitutes the hereditary code.
The lectures were published as a book the following year, ready for physicists to read as the War ended and they looked for new frontiers to explore. To the molecular biologist and scientific historian Gunther Stent of the University of California at Berkeley, What Is Life? was the Uncle Tom's Cabin of biology -- a small book that started a revolution. For post-War physicists suffering from professional malaise, "When one of the inventors of Quantum Mechanics [could] ask 'What is Life?,'" Stent declared, "they were confronted with a fundamental problem worthy of their mettle." Biological problems could now be tackled with their own language -- Physics.
Erwin Chargaff's discovery in 1949 that the total number of two of DNA's 4 base chemicals always equaled the total number of the other two helped set the stage for Watson and Crick's brilliant insight into DNA's structure just 4 years later.
esearch into the new field of biophysics inched forward in the late 1940s. In 1949, another Austrian refugee scientist -- Erwin Chargaff, working at the Columbia College of Physicians and Surgeons in New York -- was one of the very few who took Avery's results to heart and changed his research program in consequence. He analyzed the proportions of the 4 bases of DNA and found a curious correspondence. The numbers of molecules present of the 2 bases -- adenine and guanine, called purines -- were always equal to the total amount of thymine and cytosine -- the other 2 bases called pyrimidines. This neat ratio -- found in all forms of DNA -- cried out for explanation. But Chargaff could not think what it might be.
That is where things stood when Rosalind Franklin arrived at King's College London on 5 January 1951. Leaving coal research to work on DNA, moving from the crystal structure of inanimate substances to that of biological molecules, she had crossed the border between nonliving and living. Coal does not make more coal; but genes make more genes.
Defending Franklin's Legacy
by Lynne Osman Elkin
When it comes to her place in the history of the double helix's delineation, Rosalind Franklin has not received fair treatment. Or so maintains Dr. Lynne Osman Elkin, a professor of biological sciences at California State University, Hayward who spends much of her time these days trying to clarify Franklin's significant role in one of the 20th Century's greatest scientific discoveries.
In March 2003, Elkin published a lengthy article on Franklin in ^ (see Further Reading), and she's hard at work on a biography. In this interview, hear what Elkin has to say about exactly where Franklin stands in her mind -- and where Photo 51's creator ought to stand in the history books.
[NOVA]: How close did Franklin actually come to deciphering the structure of DNA?
[Elkin]: She was very close. She had all the parameters of the helical backbone. She was the one who figured out that there were 2 forms of DNA, which made solving the whole structure possible. She had figured out that backbone of the 'A' form is anti-parallel. It wouldn't have been very long before she figured out that the 'B' form backbone was anti-parallel as well.
Franklin's notebooks reveal that she was intimately familiar with the work of others who had made key discoveries along the road to Watson and Crick's breakthrough.
he other thing was base-pairing -- which was Watson's brilliant idea -- made possible by chemical information supplied by Jerry Donohue. But if you look at her notebooks, she was very, very aware of hydrogen bonding. She was very, very aware of the difference between 'enol' and 'keto' forms, which were the key to base-pairing. She was aware of Chargaff's ratios. She was aware of Donohue's work. All the stuff that circled around base-pairing.
[NOVA]: How soon might she have worked it out if Watson and Crick hadn't gotten her data?
[Elkin]: Well, at one time Crick estimated that it would have taken her 3 months. I don't know how long it would have taken her, but I think the critical thing with the timing is that she was about to publish her paper on the 'B' form. That's the March 17th draft that Aaron Klug discovered. And that paper was written well before March 17th. And then after the Watson-Crick structure was figured out, she modified it very minimally and it became the third Nature paper.
There is no way without her data that Watson and Crick could have figured out the structure before [her March 17th draft] got published. Now, if that had gotten published first and then they figured it out -- remember, she talked about the double helix in that paper -- then even though they had figured out the actual structure, they would have had to incorporate her information and credit her properly, and she would not have been written out of history.
[NOVA]: What did Watson actually get out of 'Photo 51' beyond the idea that the "X" signified a helix?
[Elkin]: After Watson saw 'Photo 51', he went out to dinner with Wilkins and pressed him for the interpretation of it -- the 34-angstrom measurements and so on. At that early date, Watson didn't know how to interpret a diffraction photo other than that an "X" meant helix. In terms of getting measurements out of it, he hadn't the foggiest at that point. It was Wilkins who told him how to interpret it.
[NOVA]: What about the idea that the sugar-phosphate groups were on the outside? Did Watson get that from 'Photo 51'?
[Elkin]: No. That was from the MRC report. Watson and Crick got a tremendous amount of information from that MRC report. Now, they persisted in wanting to put the bases on the outside. And it's absurd! You don't put a hydrophobic thing on the outside of a structure in a cell. You put the hydrophobic stuff on the inside -- where it's protected -- and the hydrophilic phosphates and sugars on the outside.
As a chemist Franklin knew that automatically. And so did even a graduate student at King's, Bruce Fraser, when he tried building a model. But Watson and Crick -- being weak in their knowledge of chemistry -- kept putting it on the outside. And Wilkins said, "You know, Rosalind said it should be on the inside." So Wilkins once again was telling them information that he knew from Rosalind. They kept resisting, however, because to put it on the inside, it seemed very difficult to know how to pack things.
But when Crick saw the MRC report -- in which Franklin had not only said that the phosphates are on the outside but had also offered measurements of the interphosphate distances -- even he couldn't argue with that anymore. So when Watson once again was trying to build a model and it wasn't working, Crick said, "Why don't you put the phosphates on the outside like Rosalind said?"
NOVA]: So the big question is if Franklin had lived, would she or should she -- instead of Wilkins -- have received the Nobel Prize with Watson and Crick?
[Elkin]: There's a big difference between "would" and "should". Should she have? Absolutely. One of the things I proposed last year at AAAS [the American Association for the Advancement of Science annual meeting] is that I think it should be called the Watson-Crick-Franklin structure. As far as I'm concerned, she was a de facto collaborator. Maybe she didn't give them her information directly. But every time they hit a stumbling point, it was her information that they got from Wilkins that straightened it out. So do I think should she have? Absolutely.
Would she have? I'm not so sure. The Nobel Prize could be very political. And often the Nobel Committee would put great emphasis on those who started the research, which in this case was Wilkins. But even Watson begrudgingly says that she should have gotten it.
[NOVA]: Do you believe Franklin had good intuition and that her careful science held her back from making big leaps of imagination, such as might have brought her to the structure of DNA faster than Watson and Crick?
[Elkin]: I don't think she had the intuition of someone like Crick or Dorothy Hodgkin, maybe not even of Watson. But she had very good intuition, and she could interpret her own data. She could see things. But unless she could prove them, she wouldn't publish them.
Now, from my point-of-view, being careful before you publish is a good thing. I've never considered that a liability. I think that's an asset. For someone of her age [Franklin died at age 37], her publication record was phenomenal. It would be phenomenal if she had retired at the age of 75.
Franklin, seen here serving coffee in evaporating dishes at her Parisian laboratory in the late 1940s, went on to publish an impressive array of scientific papers in the decade she had remaining to her.
ou see, what Watson and Crick did was they guessed. They had intuition, their work was brilliant, they guessed. But there were things wrong with the structure they proposed that had to be refined by Wilkins. So it's a double-edged sword. I don't think she had intuition to the level of Crick. But I think very few people in the world ever have had intuition to the level of Crick. Crick and Linus Pauling were in a class by themselves.
[NOVA]: Franklin would be 79 now if she had lived. What might she have accomplished in her career? Was she on a trajectory to be a Nobel Prize-winning scientist in her own right?
[Elkin]: Oh, sure. I think she would have ended up doing the virus work. Probably with Klug. And they probably would have shared a Nobel Prize for that work. That would be the most obvious thing. Lord knows what else she would have done!
If one looks in the records, in all the archives, the way people talked about her was phenomenal. One of the coal researchers wrote a letter about her to Bernal after she died. And in that letter he said, "I considered her a genius. And I don't use that word lightly." She was so young and so brilliant. And she wasn't the kind of person who pooped out. She just gained acceleration and quality in her work as she got older.
[NOVA]: Do you think her X-ray work contributed to the ovarian cancer that killed her?
[Elkin]: I think for sure, though there's no knowing one way or the other. For one thing, she probably had the ovarian cancer gene. I know that there's cancer in the family. But she was also in the X-ray beam a lot. Gosling talked to me about that. When they were physically putting the X-ray diffraction device together, they had practical difficulties. He said he would be lying on the floor holding the apparatus together, and she would spend hours adjusting the specimen. He thought, "Gee, she's in that X-ray beam an awfully long time" because the beam had to be on to do the adjustments.
So is there proof of it? No. Do all X-ray crystallographers end up with cancer? No. I don't think most of them spend as much time aligning the specimen as she did. And she didn't wear lead aprons. It was just beginning to be thought that you ought to do stuff like that.
[NOVA]: So did she know that it was potentially dangerous?
[Elkin]: It was beginning to be thought that it would be, but I don't think a lot of them took it seriously. I got to know Anne and David Sayre very well. Anne told me an anecdote about how she came to have lunch with David one day. There was this X-ray source sitting on his desk. Anne was very, very well-read, and she said, "You put that thing in a lead box or I'm divorcing you tomorrow." David is healthy as a horse today.
At any rate, I know a lot of old crystallographers, so it's not an automatic thing. It depends on how much you handle the specimen. I think the combination of the specimens and the gene did Rosalind Franklin in.
"Rosalind Franklin and the Double Helix" by Lynne Osman Elkin, Physics Today, March 2003 (available at www.physicstoday.org/vol-56/iss-3/p42.html)
Rosalind Franklin: The Dark Lady of DNA by Brenda Maddox (HarperCollins, 2002)
[StealthSkater note: more on Rosalind Franklin from PBS/NOVA is archived at doc pdf URL ]
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