Editing Rosalind Franklin (section)

===King's College London===
{{Single strand DNA discovery}}

In 1950 Franklin was granted a three-year [[Turner & Newall]] Fellowship to work at [[King's College London]]. In January 1951 she started working as a research associate in the [[Medical Research Council (UK)|Medical Research Council's]] (MRC) Biophysics Unit, directed by [[John Randall (physicist)|John Randall]].<ref>Maddox, p. 124.</ref> She was originally appointed to work on X-ray diffraction of [[protein]]s and [[lipid]]s in solution, but Randall redirected Franklin's work to DNA fibres<ref>Williams, p. 282.</ref> because of new developments in the field, and she was to be the only experienced experimental diffraction researcher at King's at the time.<ref>Maddox, p. 114.</ref><ref>Wilkins, Wilkins, M., The Third Man of the Double Helix, an autobiography (2003) Oxford University Press, Oxford. pp. 143–144.</ref> Randall made this reassignment, even before Franklin started working at King's, because of the pioneering work by DNA researcher [[Maurice Wilkins]], and he reassigned [[Raymond Gosling]], the graduate student who had been working with Wilkins, to be her assistant.<ref>Wilkins, p. 121.</ref>

In 1950 Swiss chemist [[Rudolf Signer]] in Berne prepared a highly purified DNA sample from calf [[thymus]]. He freely distributed the DNA sample, later referred to as the Signer DNA, in early May 1950 at the meeting of the [[Faraday Society]] in London, and Wilkins was one of the recipients.<ref>{{Cite journal|last=Meili|first=Matthias|year=2003|title=Signer's Gift – Rudolf Signer and DNA|journal=CHIMIA International Journal for Chemistry|language=en|volume=57|issue=11|pages=735–740|doi=10.2533/000942903777678623|doi-access=free}}</ref> Even using crude equipment, Wilkins and Gosling had obtained a good-quality diffraction picture of the DNA sample which sparked further interest in this molecule.<ref name="Gosling_obit">"[https://www.telegraph.co.uk/news/obituaries/11624246/Professor-Raymond-Gosling-DNA-scientist-obituary.html Professor Raymond Gosling, DNA scientist – obituary]". ''[[The Daily Telegraph]]''. 22 May 2015. Retrieved 3 September 2019.</ref> But Randall had not indicated to them that he had asked Franklin to take over both the DNA diffraction work and guidance of Gosling's thesis.<ref>Maddox, pp. 149–150, Elkin, p 45. Elkin, L.O. Rosalind Franklin and the Double Helix. Physics Today, March 2003(available free on-line, see references). Olby, R. ''The Path to the Double Helix'' (London: Macmillan, 1974).</ref> It was while Wilkins was away on holiday that Randall, in a letter in December 1950, assured Franklin that "as far as the experimental X-ray effort there would be for the moment only yourself and Gosling."<ref name="Klug-2004">{{Cite journal|last=Klug|first=Aaron|year=2004|title=The discovery of the DNA double helix|journal=Journal of Molecular Biology|volume=335|issue=1|pages=3–26|doi=10.1016/j.jmb.2003.11.015|pmid=14659736|s2cid=9670051 }}</ref> Randall's lack of communication about this reassignment significantly contributed to the well-documented friction that developed between Wilkins and Franklin.<ref name="Creager-2008" /> When Wilkins returned, he handed over the Signer DNA and Gosling to Franklin.<ref name="Klug-2004" />

Franklin, now working with Gosling,<ref>Maddox, p. 129.</ref> started to apply her expertise in X-ray diffraction techniques to the structure of DNA. She used a new fine-focus X-ray tube and microcamera ordered by Wilkins, but which she refined, adjusted and focused carefully. Drawing upon her physical chemistry background, a critical innovation Franklin applied was making the camera chamber that could be controlled for its [[humidity]] using different saturated salt solutions.<ref name="Klug-2004" /> When Wilkins enquired about this improved technique, she replied in terms which offended him as she had "an air of cool superiority".<ref>Wilkins, p. 155.</ref>

Franklin's habit of intensely looking people in the eye while being concise, impatient and direct unnerved many of her colleagues. In stark contrast, Wilkins was very shy, and slowly calculating in speech while he avoided looking anyone directly in the eye.<ref name="Elkin 45">Elkin p. 45.</ref> With the ingenious humidity-controlling camera, Franklin was soon able to produce X-ray images of better quality than those of Wilkins. She immediately discovered that the DNA sample could exist in two forms: at a relative humidity higher than 75%, the DNA fibre became long and thin; when it was drier, it became short and fat. She originally referred to the former as "wet" and the latter as "crystalline."<ref name="Klug-2004" />

On the structure of the crystalline DNA, Franklin first recorded the analysis in her notebook, which reads: "Evidence for spiral [meaning helical] structure. Straight chain untwisted is highly improbable. Absence of reflections on [[Meridian arc|meridian]] in χtalline [crystalline] form suggests spiral structure."<ref name="Klug-2004" /> An immediate discovery from this was that the [[phosphate group]] lies outside the main DNA chain; Franklin, however could not make out whether there could be two or three chains.<ref name="Klug-1968" /> She presented their data at a lecture in November 1951, in King's College London. In her lecture notes, Franklin wrote the following:<blockquote>The results suggest a helical structure (which must be very closely packed) containing 2, 3 or 4 [[Coaxial|co-axial]] nucleic acid chains per helical unit, and having the phosphate groups near the outside.<ref>{{Cite journal|last1=Braun|first1=Gregory|last2=Tierney|first2=Dennis|last3=Schmitzer|first3=Heidrun|year=2011|title=How Rosalind Franklin Discovered the Helical Structure of DNA: Experiments in Diffraction|url=http://aapt.scitation.org/doi/10.1119/1.3555496|journal=The Physics Teacher|language=en|volume=49|issue=3|pages=140–143|doi=10.1119/1.3555496|bibcode=2011PhTea..49..140B}}</ref></blockquote>Franklin then named "[[A-DNA|A]]" and "B" respectively for the "crystalline" and "wet" forms. (The biological functions of A-DNA were discovered only 60 years later.<ref>{{cite journal |last1=Wood |first1=Bayden R. |title=The importance of hydration and DNA conformation in interpreting infrared spectra of cells and tissues |journal=Chemical Society Reviews |year=2016 |volume= 45|issue=7 |doi=10.1039/C5CS00511F |pmid=26403652 |pages=1980–98|s2cid=24571043 }}</ref>) Because of the intense personality conflict developing between Franklin and Wilkins, Randall divided the work on DNA. Franklin chose the data rich "A" form while Wilkins selected the "B" form.<ref name="Maddox 155">Maddox, p. 155.</ref><ref name="Wilkins, p. 158">Wilkins, p. 158.</ref> 

[[File:In memoriam card Wellcome L0043312 (cropped).jpg|thumb|A satirical death note of A-DNA helix by Franklin and Gosling.]]

By the end of 1951 it became generally accepted at King's that the B-DNA was a [[helix]], but after Franklin had recorded an asymmetrical image in May 1952, Franklin became unconvinced that the A-DNA was a helix.<ref name="Wilkins, p. 176">Wilkins, p. 176.</ref> In July 1952, as a practical joke on Wilkins (who frequently expressed his view that both forms of DNA were helical), Franklin and Gosling produced a funeral notice regretting the 'death' of helical A-DNA, which runs:<blockquote>It is with great regret that we have to announce the death, on Friday 18th July 1952 of DNA helix (crystalline). Death followed a protracted illness which an intensive course of Besselised [referring to [[Bessel function]] that was used to analyse the X-ray diffraction patterns<ref>{{Cite journal|last=Fuller|first=Watson|year=2003|title=Who said 'helix'?|journal=Nature|volume=424|issue=6951|pages=876–878|doi=10.1038/424876a|pmid=12931159|bibcode=2003Natur.424..876F|s2cid=4414783}}</ref>] injections had failed to relieve. A memorial service will be held next Monday or Tuesday. It is hoped that Dr M H F Wilkins will speak in memory of the late helix. [Signed Rosalind Franklin and Raymond Gosling.]<ref name="Schindler-2008">{{Cite journal|last=Schindler|first=Samuel|year=2008|title=Model, Theory, and Evidence in the Discovery of the DNA Structure|url=https://www.journals.uchicago.edu/doi/full/10.1093/bjps/axn030|journal=The British Journal for the Philosophy of Science|volume=59|issue=4|pages=619–658|doi=10.1093/bjps/axn030|jstor=40072305}}</ref></blockquote>During 1952 they worked at applying the [[Patterson function]] to the X-ray pictures of DNA they had produced. This was a long and labour-intensive approach but would yield significant insight into the structure of the molecule.<ref name="Klug-1974">{{Cite journal|last=Klug|first=A.|year=1974|title=Rosalind Franklin and the double helix|journal=Nature|volume=248|issue=5451|pages=787–788|doi=10.1038/248787a0|pmid=4599085|bibcode=1974Natur.248..787K|s2cid=4299246}}</ref> Franklin was fully committed to experimental data and was sternly against theoretical or model buildings, as she said, "We are not going to speculate, we are going to wait, we are going to let the spots on this photograph tell us what the [DNA] structure is."<ref name="Schindler-2008" /> The X-ray diffraction pictures, including the landmark ''[[Photo 51]]'' taken by Gosling at this time,<ref name="Gosling_obit" /> have been called by [[John Desmond Bernal]] as "amongst the most beautiful X-ray photographs of any substance ever taken".<ref name="Maddox, p. 153">Maddox, p. 153.</ref>

By January 1953 Franklin had reconciled her conflicting data, concluding that both DNA forms had two helices, and had started to write a series of three draft manuscripts, two of which included a double-helical DNA backbone (see below). Franklin's two A-DNA manuscripts reached ''[[Acta Crystallographica]]'' in [[Copenhagen]] on 6 March 1953, the day before Crick and Watson had completed their model on B-DNA. Franklin must have mailed them while the Cambridge team was building their model, and certainly had written them before she knew of their work.<ref name="Maddox 199">Maddox, p. 199.</ref> On 8 July 1953 Franklin modified one of these "in proof" ''Acta'' articles, "in light of recent work" by the King's and Cambridge research teams.<ref>Franklin and Gosling (1953). ''Acta Crystallographica'', 6, 673–677.</ref>

The third draft paper was on the B-DNA, dated 17 March 1953, which was discovered years later amongst her papers, by Franklin's Birkbeck colleague, [[Aaron Klug]].<ref>{{Cite web|title=Wellcome Library Encore – [The Papers of Rosalind Franklin] [archive material]|url=http://search.wellcomelibrary.org/iii/encore/record/C__Rb1983228?lang=eng|access-date=11 October 2016|website=search.wellcomelibrary.org|archive-date=5 June 2021|archive-url=https://web.archive.org/web/20210605105047/https://search.wellcomelibrary.org/iii/encore/record/C__Rb1983228?lang=eng}}</ref> He then published in 1974 an evaluation of the draft's close correlation with the third of the original trio of 25 April 1953 ''Nature'' DNA articles.<ref name="Klug-1974"/> Klug designed this paper to complement the first article he had written in 1968 defending Franklin's significant contribution to DNA structure. Klug had written this first article in response to the incomplete picture of Franklin's work depicted in [[James Watson]]'s 1968 memoir, ''[[The Double Helix]]''.<ref name="Klug-1968">{{Cite journal|last=Klug|first=A.|year=1968|title=Rosalind Franklin and the discovery of the structure of DNA|journal=Nature|volume=219|issue=5156|pages=808–810|doi=10.1038/219808a0|pmid=4876935|bibcode=1968Natur.219..808K|s2cid=4200811}}</ref>

As vividly described by Watson, he travelled to King's on 30 January 1953 carrying a preprint of [[Linus Pauling]]'s incorrect proposal for DNA structure. Since Wilkins was not in his office, Watson went to Franklin's lab with his urgent message that they should all collaborate before Pauling discovered his error. The unimpressed Franklin became angry when Watson suggested she did not know how to interpret her own data. Watson hastily retreated, backing into Wilkins who had been attracted by the commotion. Wilkins commiserated with his harried friend and then showed Watson Franklin's DNA X-ray image.<ref>{{cite book |last1=Hubbard |first1=Ruth |title=Women, Science, and Technology |date=2013 |publisher=Taylor and Francis |location=Hoboken |isbn=978-1-135-05542-4 |page=269 |edition=3rd |chapter-url=https://books.google.com/books?id=unO_AAAAQBAJ&q=Wilkins%20Watson%20Franklin%20X-ray&pg=PA269 |chapter=Science, Power, Gender: How DNA Became the Book of Life}}</ref> Watson, in turn, showed Wilkins a prepublication manuscript by Pauling and [[Robert Corey]], which contained a DNA structure remarkably like their first incorrect model.<ref>Yockey, pp. 9–10.</ref>

====Discovery of DNA structure====
In November 1951 James Watson and Francis Crick of the [[Cavendish Laboratory]] in [[Cambridge University]] had started to build a [[molecular model]] of the B-DNA using data similar to that available to both teams at King's. Based on Franklin's lecture in November 1951 that DNA was helical with either two or three stands, they constructed a triple-helix model, which was immediately proven to be flawed.<ref name="Klug-2004" /> In particular, the model had the phosphate backbone of the molecules forming a central core.  But Franklin pointed out that the progressive solubility of DNA crystals in water meant that the strongly [[hydrophilic]] phosphate groups were likely to be on the outside of the structure; while the experimental failure to titrate the CO- and NH<sub>2</sub> groups of the bases meant that these were more likely to be inaccessible in the interior of the structure. This initial setback led Watson and Crick to focus on other topics for most of the next year.

Model building had been applied successfully in the elucidation of the structure of the [[alpha helix]] by Linus Pauling in 1951,<ref name="Wilkins, p. 158" /><ref>Maddox, p. 147.</ref> but Franklin was opposed to prematurely building theoretical models, until sufficient data were obtained to properly guide the model building. She took the view that building a model was to be undertaken only after enough of the structure was known.<ref name="Wilkins, p. 176" /><ref>Maddox, p. 161.</ref> Franklin's conviction was only reinforced when Pauling and Corey also came up with an erroneous triple-helix model<ref name="Schindler-2008" /> in their late-1952 paper (published in February 1953<ref>{{Cite journal|last1=Pauling|first1=L.|last2=Corey|first2=R. B.|year=1953|title=A Proposed Structure For The Nucleic Acids|journal=Proceedings of the National Academy of Sciences of the United States of America|volume=39|issue=2|pages=84–97|doi=10.1073/pnas.39.2.84|pmc=1063734|pmid=16578429|bibcode=1953PNAS...39...84P|doi-access=free}}</ref>). Ever cautious, Franklin wanted to eliminate misleading possibilities. Photographs of her Birkbeck work table show that Franklin routinely used small molecular models of DNA, although certainly not ones on the grand scale successfully used at Cambridge.

The arrival in Cambridge of Linus Pauling's flawed paper in January 1953 prompted the head of the Cavendish Laboratory, [[Lawrence Bragg]], to encourage Watson and Crick to resume their own model building.<ref name="CobbComfort2023">[[Matthew Cobb|Cobb, Matthew]] and [[Nathaniel C. Comfort|Comfort, Nathaniel]] (25 April 2023), [https://www.nature.com/articles/d41586-023-01313-5 What Rosalind Franklin truly contributed to the discovery of DNA's structure], ''[[Nature (journal)|Nature]]'', '''616''' 657–660<br />See also [https://twitter.com/matthewcobb/status/1650877644529975296 twitter thread] by [[Matthew Cobb]], 25 April 2023 / [https://twitter.com/nccomfort/status/1650883271478132741 duplicate thread] by [[Nathaniel C. Comfort]], 25 April 2023</ref>  Six weeks of intense efforts followed, as they tried to guess how the nucleotide bases pack into the core of the DNA structure, within the broad parameters set by the experimental data from the team at King's, that the structure should contain one or more helices with a repeat distance of 34 Angstroms, with probably ten elements in each repeat; and that the hydrophilic phosphate groups should be on the outside (though as Watson and Crick struggled to come up with a structure they at times departed from each of these assumptions during the process).<ref name="CobbComfort2023" />

Crick and Watson received a further impetus in the middle of February 1953 when Crick's thesis advisor, [[Max Perutz]], gave Crick a copy of a report written for a [[Medical Research Council (United Kingdom)|Medical Research Council]] biophysics committee visit to King's in December 1952, containing many of Franklin's crystallographic calculations.<ref>{{cite book |last=Hubbard |first=Ruth |title=The Politics of Women's Biology |date=1990 |publisher=Rutgers State University |isbn=0-8135-1490-8 |page=[https://archive.org/details/politicsofwomens00hubb/page/60 60] |url=https://archive.org/details/politicsofwomens00hubb/page/60 }}</ref>  This decisively confirmed the 34 Angstrom repeat distance; and established that the structure had C2 symmetry, immediately confirming to Crick that it must contain an equal number of parallel and anti-parallel strands running in opposite directions.<ref name="CobbComfort2023" />

Since Franklin had decided to transfer to Birkbeck College and Randall had insisted that all DNA work must stay at King's, Wilkins was given copies of Franklin's diffraction photographs by Gosling. By 28 February 1953 Watson and Crick felt they had solved the problem enough for Crick to proclaim (in the local pub) that they had "found the secret of life".<ref>"The Double Helix", p. 115.</ref> However, they knew they must complete their model before they could be certain.<ref>"The Double Helix", p. 60.</ref> The closeness of fit to the experimental data from King's was an essential corroboration of the structure.<ref name="CobbComfort2023" /><ref name=zallen/>

Watson and Crick finished building their model on 7 March 1953, a day before they received a letter from Wilkins stating that Franklin was finally leaving and they could put "all hands to the pump".<ref>"All hands to the pump" letter is preserved in the Crick archives at the University of California, San Diego, and was posted as part of their Web collection. It is also quoted by both Maddox, p 204, and Olby.</ref> This was also one day after Franklin's two A-DNA papers had reached ''Acta Crystallographica''. Wilkins came to see the model the following week, according to Franklin's biographer [[Brenda Maddox]], on 12 March, and allegedly informed Gosling on his return to King's.<ref name="Maddox 207">Maddox, p. 207.</ref>

One of the most critical and overlooked moments in DNA research was how and when Franklin realised and conceded that B-DNA was a double-helical molecule. When Klug first examined Franklin's documents after her death, he initially came to an impression that Franklin was not convinced of the double-helical nature until the knowledge of the Cambridge model.<ref name="Klug-1968" /> But Klug later discovered the original draft of the manuscript (dated 17 March 1953) from which it became clear that Franklin had already resolved the correct structure. The news of Watson–Crick model reached King's the next day, 18 March,<ref name="Klug-1974" /> suggesting that Franklin would have learned of it much later since she had moved to Birkbeck. Further scrutiny of her notebook revealed that Franklin had already thought of the helical structure for B-DNA in February 1953 but was not sure of the number of strands, as she wrote: "Evidence for 2-chain (or 1-chain helix)."<ref name="Olby">Olby, p. 418.</ref> Her conclusion on the helical nature was evident, though she failed to understand the complete organisation of the DNA strands, as the possibility of two strands running in opposite directions did not occur to her.<ref name="Klug-1974" />

Towards the end of February Franklin began to work out the indications of double strands, as she noted: "Structure B does not fit single helical theory, even for low layer-lines." It soon dawned to her that the B-DNA and A-DNA were structurally similar,<ref name="Olby" /> and perceived A-DNA as an "unwound version" of B-DNA.<ref name="Klug-1974" /> Franklin and Gosling wrote a five-paged manuscript on 17 March titled "A Note on Molecular Configuration of Sodium Thymonucleate."<ref>{{Cite web|year=2013|title=J. Craig Venter Institute History of Molecular Biology Collection: MS 001|url=https://oac.cdlib.org/findaid/ark:/13030/c8k35xs6/entire_text/|access-date=15 September 2021|website=oac.cdlib.org|publisher=J. Craig Venter Institute Archives}}</ref> After the Watson–Crick model was known, there appeared to be only one (hand-written) modification after the typeset at the end of the text which states that their data was consistent with the model,<ref name="Klug-1974" /> and appeared as such in the trio of the 25th of April 1953 ''[[Nature (journal)|Nature]]'' articles; the other modification being a deletion of "A Note on" from the title.<ref>Olby, p. 474.</ref><ref name="autogenerated13">{{cite journal |author=Franklin, R. E. |author2=R. G. Gosling |date=April 1953|title=Molecular configuration in sodium thymonucleate|url=http://www.nature.com/nature/dna50/franklingosling.pdf|journal=Nature|volume=171|issue=4356|pages=740–741|bibcode=1953Natur.171..740F|doi=10.1038/171740a0|pmid=13054694|s2cid=4268222}} [Reproduction with interpretation: {{cite journal |last1=Franklin |first1=Rosalind E. |last2=Gosling |first2=R. G. |title=Molecular configuration in sodium thymonucleate |journal=Resonance |year=2004 |volume=9 |issue=3 |pages=84–88 |doi=10.1007/BF02834994 |s2cid=123270020 |url=https://www.ias.ac.in/article/fulltext/reso/009/03/0084-0088}}]</ref>

As Franklin considered the double helix, she also realised that the structure would not depend on the detailed order of the bases, and noted that "an infinite variety of nucleotide sequences would be possible to explain the biological specificity of DNA".<ref name="CobbComfort2023" />  However she did not yet see the complementarity of the [[base-pair]]ing – Crick and Watson's breakthrough of 28 February, with all its biological significance; nor indeed at this point did she have the correct structures of the bases, so even if she had tried, she would not have been able to make a satisfactory structure.<ref name="CobbComfort2023" /> Science historians [[Nathaniel C. Comfort]], of [[Johns Hopkins University]], and [[Matthew Cobb]], of the [[University of Manchester]], explained that "She did not have time to make these final leaps, because Watson and Crick beat her to the answer."<ref name="CobbComfort2023" />

Weeks later, on 10 April, Franklin wrote to Crick for permission to see their model.<ref>10 April 1953, Franklin postcard to Crick asking permission to view model. The original is in the Crick archives at the University of California, San Diego.</ref> Franklin retained her scepticism for premature model building even after seeing the Watson–Crick model, and remained unimpressed. She is reported to have commented, "It's very pretty, but how are they going to prove it?" As an experimental scientist, Franklin seems to have been interested in producing far greater evidence before publishing-as-proven a proposed model. Accordingly, her response to the Watson–Crick model was in keeping with her cautious approach to science.<ref>Holt, J. (2002).</ref>

Crick and Watson published their model in ''Nature'' on 25 April 1953, in an article describing the double-helical structure of DNA with only a footnote acknowledging "having been stimulated by a general knowledge of Franklin and Wilkins' 'unpublished' contribution."<ref name="autogenerated1">{{cite journal|author=Watson, J. D. |author2=Crick, F. H. |date=April 1953|title=Molecular structure of nucleic acids; a structure for deoxyribose nucleic acid|url=http://www.nature.com/nature/dna50/watsoncrick.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://www.nature.com/nature/dna50/watsoncrick.pdf |archive-date=2022-10-09 |url-status=live|journal=Nature|volume=171|issue=4356|pages=737–738|bibcode=1953Natur.171..737W|doi=10.1038/171737a0|pmid=13054692|s2cid=4253007}}</ref> Actually, although it was the bare minimum, they had just enough specific knowledge of Franklin and Gosling's data upon which to base their model. As a result of a deal struck by the two laboratory directors, articles by Wilkins and Franklin, which included their X-ray diffraction data, were modified and then published second and third in the same issue of ''Nature'', seemingly only in support of the Crick and Watson theoretical paper which proposed a model for the B-DNA.<ref name="autogenerated12">{{cite journal |author=Wilkins, M. H. |author2=A. R. Stokes |author3=H. R. Wilson |date=April 1953|title=Molecular structure of deoxypentose nucleic acids|url=http://www.nature.com/nature/dna50/wilkins.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://www.nature.com/nature/dna50/wilkins.pdf |archive-date=2022-10-09 |url-status=live|journal=Nature|volume=171|issue=4356|pages=738–740|bibcode=1953Natur.171..738W|doi=10.1038/171738a0|pmid=13054693|s2cid=4280080}}</ref><ref name="autogenerated13" /> Most of the scientific community hesitated several years before accepting the double-helix proposal. At first mainly geneticists embraced the model because of its obvious genetic implications.<ref>{{cite journal |last1=Rich |first1=Alexander |title=The double helix: a tale of two puckers |journal=Nature Structural Biology |year=2003 |volume=10 |issue=4 |pages=247–249 |doi=10.1038/nsb0403-247 |pmid=12660721|s2cid=6089989 }}</ref><ref>{{cite journal |last1=Scher |first1=Stanley |title=Was Watson and Crick's model truly self-evident? |journal=Nature |year=2004 |volume=427 |issue=6975 |page=584 |doi=10.1038/427584c |pmid=14961092 |bibcode=2004Natur.427..584S|doi-access=free }}</ref><ref>{{cite journal |last1=Arnott |first1=Struther |title=Historical article: DNA polymorphism and the early history of the double helix |journal=Trends in Biochemical Sciences |year=2006 |volume=31 |issue=6 |pages=349–354 |doi=10.1016/j.tibs.2006.04.004 |pmid=16678428}}</ref>