DNA The Discovery of the Double Helix

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DNA

The Discovery of the Double Helix

Friedrich Miescher

Deoxyribonucleic acid was discovered in 1869 by Friedrich Miescher

Miescher was a Swiss Physician.

After learning organic chemistry at the University of Tübingen, he began to work in the laboratory of Hoppe-Seyler, the first laboratory devoted entirely to Biochemistry.

It was located in an ancient castle overlooking the Neckar River.

It was here that Miescher made the discovery of nuclein, or deoxyribonucleic acid (DNA).

Miescher’ Laboratory

Miescher’s objective was to analyze the chemical inside the nucleus of the cell.

Miescher used white blood cells for his experiments.

To obtain the white blood cells, Miescher used pus, which he extracted from bandages obtained from the Tübingen surgical clinic.

Miescher began to analyze the chemical composition of the intact white blood cells by treating them with various solutions, including salt solutions, acid, alkali and alcohol.

When he treated the pus cells with alkali, he obtained a substance that he ascribed to the nucleus.

Miescher began to work on a method to isolate the nuclei of the white blood cells.

Miescher found that if he treated the cells with dilute hydrochloric acid, that he could dissolve most of the materials of the cell, leaving the nuclei behind.

When Miescher examined the isolated nuclei under the microscope, he saw contamination, which he attributed to protein.

To remove the protein Miescher treated the nuclei

with the protein-digesting enzyme pepsin.

The next step was to extract the substance in the

isolated nuclei (rather than the whole cells) with

dilute alkali.

Miescher analyzed the substance chemically and concluded that he had discovered a new compound.

Miescher called the new compound nuclein.

Today we know this compound as DNA.

Albrecht Kossel

Born on September 16, 1853
Specialized in the chemistry of the cell including nucleins
Kossel worked in the laboratory Hoppe-Seyler
Kossel broke down Nuclein into its basic components

Kossel discovered the purine adenine and the pyrimidine thymine
Kossel was awarded the 1910 Nobel Prize in Physiology or Medicine for research in cell biology, especially proteins and nucleic acids

Phoebus Levene

Phoebus Levene was a biochemist who worked out key structural features of DNA

Phoebus Levene was born in Sagor Russia, and grew up in St. Petersburg
Levene studied medicine at the Imperial Military Academy in St. Petersburg
Phoebus Levene was a former student of the Russian chemist and composer Alexander Borodin
Levene received his M.D. degree in 1891

In 1891, Levene and his family fled Russia because of anti-semitism
They emigrated to the United States and arrived in New York City on the symbolic day of July 4
Levene began to practice medicine on the Lower East Side

Levene was interested in research. He enrolled in Columbia University and began to work in the Department of Physiology
In 1896, he was appointed as an Associate in the Pathological Institute of the New York Hospitals
In 1905, Levene was hired by the Rockefeller Institute of Medical Research as the head of the biochemical laboratory

By 1900, it was known that the basic building blocks of DNA were phosphate, a sugar, and four nitrogenous bases
The nitrogenous bases included adenine (A), thymine (T), cytosine and guanine (G)
The adenine and guanine were large double-ring molecules known as purines
The thymine and cytosine were smaller single-ring molecules known as pyrimidines

In 1909, Levene found that the carbohydrate present in nucleic acid from yeast (later called ribonucleic acid) is the pentose sugar ribose
In 1929, he succeeded in identifying the carbohydrate in the nucleic acid from the thymus of an animal (later called deoxyribonucleic acid). It is also a pentose sugar but lacks one oxygen atom of ribose and was therefore called deoxyribose

The basic building block of the DNA molecule is a nucleotide, which is made up of a phosphate group, a deoxyribose sugar and one of four nitrogenous bases: adenine, thymine, cytosine, or guanine
Levene determined how the nucleic acid components combine to form the nucleotides
He also proposed that nucleic acids were composed of linear chains of purines and pyrimidines linked to each other through the sugar-phosphate-ester backbone

Tetranucleotide Hypothesis

Although Levene contributed much to our knowledge of the chemistry of nucleic acids, he is best remembered for an incorrect hypothesis of the structure of DNA known as the Tetranucleotide Hypothesis

Levene was convinced that the amounts of the four bases, adenine, thymine, cytosine, and guanine were the same in all DNA molecules, whatever their origin
The nucleotides were believed to be arranged in a repetitive, unvarying pattern
Levene also erroneously concluded that DNA was a rather small molecule with a molecular weight of about 1500

Implications of the Tetranucleotide Hypothesis

Those who accepted the tetranucleotide hypothesis were misled into viewing DNA as a small molecule that lacked the structural complexity and diversity that were required of a molecule if it was to serve as the genetic material

Dissuaded by the Tetranucleotide Hypothesis that DNA was the genetic material, most scientists at that time turned instead to proteins as the most likely candidate for the genetic material
Proteins was large molecules with a complex, varied structure. They are composed of long chains of units known as amino acids. The amino acids, of which at least 10 were known at that time (there are 20), could be linked in many different combinations to produce a great variety of proteins

Frederick Griffith

The Discovery of Bacterial Transformation in Pneumococcus

Frederick Griffith was a British Microbiologist

He work at the Ministry of Health in London

Griffith was trying to develop a vaccine against pneumonia

The disease Pneumonia is caused either by a virus or by Pneumococcus bacteria
Pneumococcus bacteria exist in two forms, rough and smooth
The terms rough and smooth describe the appearance of the bacteria when grown on Petri plates
Bacteria on Petri plates grow as colonies, which are rounded masses composed of thousands of bacterial cells

The smooth organisms produce colonies that have a smooth outline, are dome-shaped, and regular
The rough organisms produce colonies that are granular, flat and irregular
The smooth organisms are virulent; the rough organisms are avirulent

Griffith’s Experiment

First, Griffith injected live R-strain bacteria into mice. The mice survived.
Next, Griffith injected live S-strain bacteria into mice. The mice caught pneumonia and died.
Then Griffith injected heat-treated S-strain bacteria into mice. The mice lived.
Finally, Griffith injected a mixture of heat-treated S-strain bacteria and live R-strain bacteria into mice. The mice caught pneumonia and died. Griffith removed blood samples from the infected mice. They contained live S-strain bacteria!

Griffith explained his results by saying that the R-strain bacteria had been transformed into S-strain bacteria.
Some substance had diffused from the heat-treated S-strain cell into the live R-strain cells. This substance conferred upon the R-strain cells the ability to produce a capsule. The ability to produce a capsule is a genetic characteristic.

Avery, MacLeod and McCarty
The Identity of the Transforming Substance

Oswald Avery

In the early 1940s, Avery, MacLeod, and McCarty began experiments in the Rockefeller Institute Hospital to identify the transforming principle.
Avery, MacLeod and McCarty’s experiments involving the progressive characterization of the transforming agent led to the first evidence for the genetic role of DNA.

The approach used by Avery, MacLeod and McCarty to identify the transforming substance was to employ enzymatic destruction of its activity.
Avery’s work focused first on purifying the transforming substance. Using refined versions of Colin M. MacLeod’s preparation techniques, Avery and McCarty isolated biologically active “transforming principle” from samples of pneumococci.

Avery and co-workers first tried to inactivate the transforming principle using protein-digesting enzymes. Trypsin and chymotrypsin did not inactivate it. This indicated that the transforming principle was not protein.
The enzyme ribonuclease failed to inactivate the transforming principle. The transforming principle was not RNA.

Evidence that the transforming principle was not a carbohydrate was produced from experiments showing that the transforming principle precipitated in ethyl alcohol. Carbohydrates are not precipitated by alcohol.
Lipases also failed to inactivate the transforming principle.

However, when Avery tried DNAase, the enzyme that digests DNA, the transforming principle was inactivated.
Additional support for the finding that the transforming principle was DNA came from experiments showing that the principle precipitated in alcohol. Fractionating the substance by adding alcohol drop by drop until a critical concentration was reached caused the substance to separate out in the form of a fibrous substance that wound around the stirring rod.

When this fibrous substance was analyzed it revealed the presence of phosphorus, it absorbed ultraviolet light at a maximum in the region 2600 Å, and it had a molecular weight of at least half a million.
These characteristics are all consistent with DNA.

Avery, MacLeod, and McCarty published their discovery that the transforming principle was DNA in 1944 in the Journal of Experimental Medicine.
Avery’s work was crucial in identifying DNA as the genetic material.

Erwin Chargaff

Erwin Chargaff was an Austrian born biochemist who worked at Columbia University in New York.

Chargaff investigated the proportions of the nitrogenous bases of DNA.

He discovered correspondences between pairs of these bases, adenine and thymine and cytosine and guanine.

This proved to be a crucial contribution that helped Watson and Crick to develop the base-pairing rule.
Chargaff is also credited with falsifying the tetranucleotide hypothesis.
Chargaff’s analysis of DNA from various species produced evidence of variation. This provided support for a genetic role for DNA.

Biographical Information

Chargaff was born on Aug. 11, 1905, in Czernowitz, Austria.
After receiving a doctorate from the University of Vienna in 1928, he accepted a position at Yale University, where he became the Milton Campbell Research Fellow in Organic Chemistry from 1928 to 1930.
From Yale, Chargaff returned to Europe. He spent 1930-1933 as Assistant in Charge of Chemistry for the Department of Bacteriology and Public Health at the University of Berlin.
He then moved to France and from 1933-1934 served as Research Associate at the Pasteur Institute in Paris.

In 1935, Chargaff returned to the United States and became an Assistant Professor at Columbia University.
Chargaff became interested in DNA following the publication of the paper on the transforming principle of pneumococci in 1944 by Avery, MacLeod and McCarty.
It was then that he decided to devote his laboratory almost completely to the chemistry of nucleic acids and nucleoproteins.

In 1944 Chargaff realized that genetically significant differences determined by DNA might be reflected in analytically detectable differences in the content and order of the DNA bases, i.e., the purines and pyrimidines.

Acting on a suggestion from Aaron Bendich, Chargaff decided to investigate the structure of nucleic acids using paper chromatography.
To assist him in this research. Chargaff selected a postdoctoral student highly trained in organic chemistry, Ernst Vischer.
Chargaff suggested a number of projects to Vischer. Vischer choose to work on the analysis of purines and pyrimidines in nucleic acids.

After hydrolyzing the DNA, Vischer employed the technique of partition chromatography and ultraviolet spectrophotometry to separate and measure the proportions of the bases in DNA.

Chargaff and his co-workers found that the amount of adenine in DNA equaled the amount of thymine and that the amount of cytosine equaled the amount of guanine. These findings are commonly referred to as Chargaff’s rules.
Chargaff also demonstrated that the proportions of the four nitrogenous bases vary in different species such as yeast, bacteria, ox, sheep, pig, and man.
The different DNAs of many cellular organisms contained significant differences in their ratios of the sum of adenine + thymine to cytosine + guanine.

The results overturned the tetranucleotide hypothesis: The bases were not present in equal quantities and they varied from organism to organism.

Hershey and Chase

In 1952 an experiment using bacteriophage were carried out by Alfred Hershey and Martha Chase that has since become known as the blender experiment.
The object of the experiment was to determine whether it was protein or DNA that was the genetic material.

Bacteriophage – a virus that infects a bacterial cell.
The first step in the infection of a bacterial cell by a bacteriophage virus is the attachment of the virus to the cell wall of the bacterial cell. The virus uses tail fibers to recognize the cell wall of the bacterium.
Next the virus creates a hole in the cell wall using enzymes. Next the virus injects its core material into the bacterial cell. In doing this the virus acts like a tiny hypodermic needle, contracting its tail and injecting its core into the bacterial cell.

When the core material enters the bacterial cell, it takes over the metabolic machinery of the cell. It directs the bacterial DNA to make viral proteins and DNA rather than its own components.

After new viral DNA and protein coats are made separately, they combine to form intact complete viruses. The viruses then break open the bacterial cell, and move out to locate other cells which they infect.

Hershey and Chase knew that whatever the genetic material was, protein or DNA, it had to enter the cell.
Hershey and Chase also knew that proteins contain sulfur but not phosphorous. DNA contains phosphorous but not sulfur.

Hershey and Chase performed two experiments.
In the first experiment they grew bacteriophage in the presence of radioactively-labeled sulfur (32S). This procedure labeled the protein coats of the virus.
They then allowed the labeled virus to infect bacterial cells.
After allowing enough time for the viruses to infect the cells, they used a blender to agitate the culture.
This procedure separate the protein coats of the virus from the cells containing the injected genetic material.

Hershey and Chase then placed the culture in a tube and used a centrifuge to spin the sample at high speed.
As centrifugal force acts on the particles and liquids in the mixture, the heavier particles fall to the bottom of the tube; the lighter particles remain near the top.
Following centrifugation, the material in the tube separates into a portion at the top called the supernatant, and a portion at the bottom of the tube called the pellet.
The supernatant contains the lighter particles (protein coats) and the pellet contains the heavier particles (cells).

Hershey and Chase then analyzed each faction of the tube for radioactivity.
The radioactively labeled sulfur was detected in the supernatant.
This finding suggested that protein was not the genetic material. Evidently, protein had not entered the bacterial cell.

In a second set of experiments, Hershey and Chase labeled the DNA of the viral core using radioactive phosphorous (32P).
They allowed the labeled virus to infect the bacterial cells.
They used a blender to separate the infected cells from the viral protein coats.
They centrifuged the culture to separate the lighter protein coats in the supernatant from the heavier cells in the pellet.

This time the radioactive phosphorous associated with DNA was concentrated in the pellet.
This indicated that DNA was the genetic material.

Solving the Structure of the Double Helix

The Principal Investigators involved in determining the structure of DNA were:

James Watson
Francis Crick
Maurice Wilkins
Rosalind Franklin

James Dewey Watson

James Dewey Watson
Biographical Information

The Watsons come to the United States in 1795 and settled in Camden NJ.
His father’s side was Scottish, his mother’s side was Irish.
His mother’s grandparents fled Ireland during the great potato famine of the 1840s and came to Indiana.
James Dewey Watson was born in Chicago, Ill, on April 6th, 1928, as the only son of James D. Watson, a businessman, and Jean Mitchell Young.

Watson attended the public school system in Chicago. After attending Horace Mann Grammar School, he graduated to South Shore High School.
As a boy, Watson appeared on the Chicago Quiz Kid radio program.
He then received a scholarship to the University of Chicago, which he entered at the age of 15.
While attending the University of Chicago, he read Erwin Schrödinger’s book What is Life? Reading this book motivated Watson to pursue the chemical nature of the gene.

In 1947, at the age of 19, he graduated from the University of Chicago with a Bachelor of Science degree in Zoology.
He then moved on to Indiana University at Bloomington, which he attended as a recipient of a fellowship for graduate study.
At Indiana, Watson’s Ph.D. advisor was S. E. Lauria. Under Lauria’s guidance, Watson completed a study on the effect of X-rays on bacteriophage multiplication.

Lauria, his close friend Max Delbrück, and Alfred Hershey formed the core of what became known as the phage group.
In 1966, Delbrück and his colleagues wrote a volume in which they reminisced about working with Watson. Watson entitled his contribution “Growing Up in the Phage Group”.
Watson spent the summer of 1948 at Cold Spring Harbor working on phage experiments.

Avery’s Influence

Workers at Cold Spring Harbor were aware of the work of Oswald Avery.
Lauria had visited Avery at the Rockefeller University.
Delbrück and Avery’s brother Roy were at Vanderbuilt University together. It was there that Roy Avery showed Delbrück a letter from Oswald that reviewed the importance of the research on the transforming principle.
Delbrück had visited Avery at Rockefeller University.

Watson received his Ph.D. degree in 1950 at the age of 22.
After receiving his Ph.D. Lauria arranged for Watson to do postgraduate work in Copenhagen under the direction of Herman Kalckar.
Lauria also helped Watson obtain a Merck Fellowship from the National Research Council for three thousand dollars a year and normally renewable.

Kalckar was studying the synthesis of the small molecules that make up DNA. However, Watson was unconvinced that this approach would lead to an understanding of the essence of the gene.
During April and May of 1951, Watson went to the Zoological Station at Naples. He attended a conference on X-ray diffraction methods for determining the structure of molecules.
There he heard a talk by Maurice Wilkins, a biophysicist from King’s College in London.
In his talk, Wilkins summarized the results of recent studies on DNA using the technique of X-ray diffraction.

Wilkins displayed a photograph of an X-ray diffraction pattern of DNA that he had taken recently.

Watson reported “Instantly I saw myself moving to London to help Wilkins find the structure. My attempts to converse with him after his talk, however, went nowhere. All I got for my efforts was a declaration of his conviction that much hard work lay ahead.”

On his way back to Copenhagen after visiting Naples, Watson stopped off in Geneva to visit the Swiss phage worker Jean Weigle.
Weigle told Watson that Linus Pauling had discovered the alpha helix, which explained the structure of certain proteins.

Watson wanted to learn how to use X-ray diffraction to solve the structure of large molecules.
He knew that Max Perutz, at Cambridge University, had successfully used this technique to investigate the structure of the protein hemoglobin.
Watson wrote Lauria and asked him to arrange his acceptance into the Cambridge Lab.
After meeting with Perutz’s coworker, John Kendrew who was visiting the United States, Lauria was able to arrange the transfer.
When the funding agency objected, Lauria arranged that Watson would work under Roy Markham, who worked with plant viruses.

The Director of the Cavendish Laboratory at Cambridge was Sir Lawrence Bragg.
Bragg invented X-ray crystallography and received the Nobel Prize in Physics in 1915. At the age of 25, he was the youngest person to receive the Nobel Prize.
Bragg developed Bragg’s Law: nλ = 2d sin θ.

At Cambridge, Watson met Francis Crick, a 35 year-old ex-physicists.
Watson and Crick shared a biochemistry room together.

Francis Crick

Biographical Information

Born June 8, 1916 in Northhampton, England.
Received a degree in physics from University College, London in 1937.
During World War II he worked for the British Admiralty designing circuits for mines and devising methods of sweeping mines.
During the war he married Ruth Dodd and the couple had a son, Michael.

Following the war, Crick and his wife divorced.
In 1947, Crick married Odile Speed, and the couple had two daughters, Gabrielle and Jacqueline.
In that same year, Crick began working at Strangeways Laboratory in Cambridge.
There he studied the physical properties of cytoplasm in cultured fibroblast cells.

Crick was originally a physicist but decided to switch to Biology after reading “What is Life?” by Erwin Schrödinger in 1946.
In 1949, Crick joined the Medical Research Council Research Group at Cambridge.
The group was headed by Max Perutz.
Crick’s Ph.D. project was to investigate proteins using X-ray diffraction.

Maurice Wilkins

Maurice Wilkins was born in Pongoroa, New Zealand on December 15, 1916.
Most of Maurice’s childhood in New Zealand was spent in Wellington, where his father was New Zealand Director of School Hygiene.
In 1923, his father decided to return with his family to his native Ireland.
The family eventually settled in Birmingham, where his father obtained a post as School Doctor

Maurice went to school at Wylde Greene College and later King Edward’s High School in Birmingham.
After graduating High School, Wilkins attended St. Johns College in Cambridge where he learned concepts relating to electricity and magnetism and quantum mechanics from Professor Oliphant.
He received his degree in 1938.
He then went to Birmingham University, where he became a research assistant to J.T. Randall in the Physics Department.

Working under Randall, Wilkins began a Ph.D. research program on thermoluminescence.
During the war, Randall invented a device that was very important in the development of radar. The device was the cavity magnetron which generated powerful microwaves.
During this time, Wilkins began a study showing how phosphorescence arose from electrons moving in and out of traps.
He received a Ph.D. degree in 1940.

Birmingham University was a leading centre for atomic physics research, and Oliphant had a team working on the atomic bomb (Wilkins, 2003, p 76). Wilkins decided to ask Oliphant if he could join the team.
Oliphant agreed and assigned Wilkins the task of separating the Uranium isotope needed to make the atomic bomb using evaporation and diffusion.
Oliphant’s group was then moved to the Manhattan Project at the University of California at Berkeley.
At Berkeley, Wilkins continued to work on vaporizing uranium metal. After following a suggestion from Lawrence to use an electric current, Wilkins was successful.

As the war ended, Wilkins read “What is Life? by Erwin Schrödinger.
He was attracted to Schrödinger’s idea of linking the biological idea of a gene with the idea of electrons moving in crystals. He wrote about a gene being an aperiodic crystal.
He decided to accept an offer by Randall to work at St. Andrews University in Scotland.
While still in California, Wilkins met Ruth, an Art student.
Wilkins married her but the marriage lasted only a few months.

In July 1945, Wilkins returned to England alone.
At St. Andrew’s University, Wilkins began working in the physics department headed by Randall. There Randall was organizing Biophysical studies.
Soon thereafter, Randall decided to accept a position as the head of the head of the physics department at King’s College in London.
The Medical Research Council made him Director of a Biophysics Research Unit.
Randall and Wilkins than began to work at King’s College.

Wilkins at that time was studying DNA using special reflecting microscopes.
At a big scientific conference in London, the biochemist Rudolph Signer offered very pure samples of DNA that he had extracted. Wilkins obtained a sample of it.
Wilkins decided to examine the DNA using X-ray diffraction.
Working together with his student Raymond Gosling, Wilkins obtained patterns of DNA that were sharper and more detailed than any before.

In the summer of 1950, Randall appointed a new X-ray specialist to work on protein solutions. Her name was Rosalind Franklin, and she had been working in Paris on the structure of coals.

Rosalind Franklin

Rosalind Franklin was born in London, England on July 25, 1920 to Muriel Waley and Ellis Franklin.
Elders of the Franklin clan claimed direct descent from King David.
Rosalind’s great-uncle was Herbert Samuel who was the first High Commissioner of Palestine.
Samuel wrote the memorandum that resulted in the Balfour Declaration in 1917.
He outlined a plan whereby after the war, Britain would assume a protectorate over Palestine and encourage Jewish immigration until a majority was reached and Palestine could be granted self-government.

Franklin excelled at science and attended classes at the academically rigorous St. Paul’s Girls’ School in London.
At St. Paul’s she studied physics and chemistry.
When she was 15, she decided to become a scientist.

In 1938 Rosalind enrolled at Newnham College, Cambridge.
At Cambridge she studied chemistry, physics, mathematics, mineralogy and scientific German.
While at Cambridge, she attended a meeting of the Association of Scientific workers presided over by William Lawrence Bragg.
Bragg was known as the father of Crystallography.
In 1915 William Lawrence Bragg received a Nobel Prize at the age of 25 for demonstrating the use of X-rays for revealing the structure of crystals.

Rosalind also studied under another brilliant crystallographer on the staff at Cambridge, J.D. Bernal.
Bernal refined the classification of “space groups”, the 230 forms into which the seven recognized crystal systems are organized.
Rosalind learned about the technique of X-ray diffraction which could be used to discover the atomic structure of crystals.

Rosalind graduated from Newnham College in 1941.
She held a graduate fellowship for a year, but quit in 1942 to work at the British Coal Utilization Research Association.
Rosalind studied the porosity of coals by passing helium through the coal at high temperatures.
She developed theories of “molecular sieves” in coal that would establish her international reputation.

This work was the basis of her doctorate in physical chemistry, which she earned from Cambridge University in 1945.

In February, 1947, Rosalind moved to Paris and began working at the Laboratoire Central Des Services Chimiques De L’Etat.
Rosalind worked under the direction of Jacques Mering.
Mering taught her how to employ X-ray diffraction to look at the internal organization of charcoal and clay.

Rosalind analyzed the structure of coal and graphite and their response to heating using the new techniques.
She found the work fascinating and the people she worked with kind and good-willed.
She was part of a group that went to a small restaurant for lunch every day and later gathered for coffee and conversation.

While in Paris she published a series of papers that established her international reputation as a researcher in the structure of coal and graphite.
In 1950, Rosalind realized that it was inevitable that she should return to England. It was there that she would return to her family and establish a scientific career.
She inquired about job openings in England and was directed by Professor Charles Coulson to King’s College in London.
King’s College offered her the opportunity to apply the techniques she had learned to the investigation of biological materials.

Rosalind received a fellowship for three years to work at King’s College under J.T. Randall.
Originally, she was to work on proteins in solution and the changes in structure which accompany the denaturing of proteins in solution.
Maurice Wilkins suggested to Randall that Franklin be assigned to work on DNA.
Wilkins, together with a graduate student, Raymond Gosling has previously obtained good X-ray diffraction pictures of DNA.

In a letter that Randall sent to Franklin, he gave her the impression that the DNA project was given to her. The critical section read as follows:
“This means that as far as the experimental X-ray effort is concerned there will be at the moment only yourself and Gosling, together with the temporary assistance of a graduate from Syracuse, Mrs Heller.

Gosling, working in conjunction with Wilkins, has already found that fibres of desoxyribose nucleic acid derived from material provided by Professor Signer of Berne give remarkably good fibre diagrams.”

King’s College London

In January, 1951 Rosalind Franklin arrived at King’s College, London.
Shortly thereafter, Randall called together a meeting attended by Rosalind Franklin, Alexander Stokes, and Raymond Gosling to discuss the DNA research.
Maurice Wilkins was not in attendance. He was on vacation.
Randall’s letter to Franklin, as well as Wilkins absence from the meeting may have contributed to a misunderstanding that developed between him and Franklin.

In the spring of 1951, Wilkins attended a conference in Naples.
It was there that he met James Watson.
At the conference, Wilkins showed the first really clear crystalline pattern of DNA that he had produced with Raymond Gosling.
Seeing the X-ray diffraction photograph of DNA excited Watson about X-ray work on DNA.
Later that summer, Watson’s advisor Salvador Lauria was able to arrange for Watson to begin working at Cambridge University.

By Easter of 1951, Franklin was busy rebuilding the X-ray set with the fine-focus Ehrenburg X-ray tube that had been set up by Stokes and Wilkins.
After returning from Naples, Wilkins carried out studies in which he compared the DNA from a number of species using X-ray diffraction.
He discussed these results in a talk in Cambridge in July, 1951.
Following the meeting Franklin approached Wilkins and told him that he should stop doing X-ray work on DNA. Her advice was “Go back to your microscopes.”

In the laboratory, Franklin began to work on a method to keep the humidity more stable. She passed hydrogen through a series of salt solutions before introducing it into the camera.
She first pulled the water out by placing the DNA fiber over a drying agent and then reintroduced the water by exposing the DNA to a range of humidity values.

Franklin learned that there were two forms of DNA.
When hydrated, the fiber became longer and thinner. This form of DNA was the “B” form.
When placed over a drying agent, DNA changed back to the “A” form.

At this time, Wilkins was in the United States attending conferences.
Wilkins returned with some DNA that he had obtained from Erwin Chargaff.
Wilkins suggested that Franklin, Wilkins, and Stokes should collaborate on DNA.
Franklin became angry and told Wilkins “How dare you interpret my data for me.”

In an attempt to lessen the difficulties, Wilkins gave Franklin exclusive use of the high-quality Signer DNA as well as the fine-focus Ehrenburg X-ray tube.
Wilkins had made a bad deal. Frustrated, he began to talk to his friend at the Cavendish Laboratory, Francis Crick.

Watson and Crick at Cambridge

Watson started to work at the Cavendish Laboratory in Cambridge in October, 1951.
Watson joined Max Perutz’s department to study crystallography and plant viruses.
Watson met Francis Crick at Cambridge. Crick was working on his pH thesis. His project was to study proteins using X-ray diffraction.
Nobody at the Cavendish was working on DNA. That work belonged to King’s.

The two men clicked.
They had complementary expertise.
Watson knew biology and genetics; Crick was a physicist who had taught himself X-ray crystallography.
Watson and Crick had learned of Linus Pauling’s discovery of the alpha helix which explained the structure of polypeptides.
Pauling’s approach to the problem of protein structure was to build models.
Watson and Crick became convinced that the structure of DNA could also be solved by building a model.

In November 1951, a colloquium was set up at Kings College to survey the DNA work going on in their lab.
James Watson attended the meeting.
At the meeting Rosalind Franklin reported on three states of DNA: wet, crystalline, and dry and showed slides of their diffraction patterns.
She provided estimates of the water content of DNA.
She provided a preliminary analysis of the unit cell as monoclinic, face-centered.
She described the phosphate groups as being on the outside of the molecule.

Watson recollected – “She spoke to an audience of about fifteen in a quick, nervous style that suited the unornamented old lecture hall in which we were seated. There was not a trace of warmth or frivolity in her words. And yet I could not regard her as totally uninteresting. Momentarily I wondered how she would look if she took off her glasses and did something novel with her hair. Then, however, my main concern was her description of the crystalline X-ray diffraction pattern.”

There is disagreement as to whether Franklin described DNA as helical.
Watson and Wilkins have reported that they did not recollect Franklin doing so.
However, Franklin’s notes for the colloquium have been preserved and they do include references to a helical structure.

Although Watson had attended the meeting at King’s, he did not, as was his usual practice, take notes.
As a result his reporting of the water content of DNA was not reliable.
However, a review of the available data suggested a number of possibilities.
Watson and Crick set to work on building a molecular model of DNA.
They put in an order for precisely made jigs for the structures of the molecules to be made in the machine shop at Cambridge.

Two weeks after the colloquium at King’s, Francis Crick telephoned Wilkins to say that he and Jim Watson had built a model of DNA and invited him to Cambridge to see it.
Wilkins gathered Franklin, Gosling, Bill Seeds, and Bruce Fraser and took a train to Cambridge.

Watson and Crick’s first model had three strands.
In order to explain how the chains were held together, Watson and Crick envisioned salt bridges between magnesium ions and two or more phosphate groups.

It wasn’t long before Rosalind Franklin began to raise serious objections to the model.
She pointed out that because the magnesium ions would be surrounded by tight shells of water molecules, it is unlikely that they could hold the structure together.
Also, during the discussion it became evident that Watson and Crick’s estimate of the water content of DNA was not accurate. This further weakened the credibility of the model.

Watson and Crick had suffered a serious setback.
News of the fiasco reached William Lawrence Bragg, the director of the laboratory.
Randall, the director of the laboratory at King’s pointed out that what Watson and Crick were doing was unnecessary duplication of the work at King’s.
As a result Bragg decided to forbid Watson and Crick to continue working on DNA.
Watson and Crick turned over their jigs and model-making equipment to the group at Kings.

Watson and Crick continued to think about DNA.
They turned their attention to the nitrogenous bases in DNA.
Erwin Chargaff had shown that in DNA samples, the amount of adenine equaled the amount of thymine and the amount of cytosine equaled the amount of guanine.
Also the proportion of adenine and thymine groups varied in the DNA of different organisms. Some had an excess of A and T, while others had an excess of G and C.

A number of interesting ideas resulted from a conversation between Francis Crick and a theoretical chemist John Griffith.
Their conversation followed a talk by the astronomer Tommy Gold on “the perfect cosmological principle.”
Crick began to wonder if there was a “perfect biological principle”.
He suggested that the perfect biological principle was the self-replication of the gene.

Griffith favored a scheme in which gene copying was based upon the alternative formation of complementary surfaces.
The argument went that gene duplication required the formation of a complementary (negative) image where shape was related to the original (positive) surface like a lock to a key. The complementary negative image would then function as the mold (template) for the synthesis of a new positive image.

What were the attractive forces between the bases?
At this point, Francis did not believe that they were caused by hydrogen bonds, believing that they were not specific enough.
Instead, Francis envisioned attractive forces between the flat surfaces of the bases.
Griffith was encouraged to calculate the attractive forces between bases with different structures.

Griffith suggested that adenine and thymine should stick to each other by their flat surfaces. A similar argument could be put forward for attractive forces between cytosine and guanine.
Francis remembered that these were the same bases that Chargaff had shown to occur in equal amounts.

Shortly thereafter Chargaff visited Cambridge and a meeting was arranged with Watson and Crick.
Chargaff developed an unfavorable impression of the pair, made worse by Crick’s failure to remember the chemical structure of the nitrogenous bases.

During this time, Watson had been reading biochemical papers on the interrelations of DNA, RNA, and protein synthesis.
He began to believe that DNA was the template upon which RNA chains were made. In turn, RNA chains were the templates for protein synthesis.
On the wall above his desk he taped a paper sheet saying:
DNA → RNA → Protein

Watson and Crick shared an office with Peter Pauling, son of the famous chemist Linus Pauling.
Peter received a letter from his father informing him that he had a structure for DNA.
A manuscript had been written which would soon be sent to Peter.
Two copies, in fact, were sent to Cambridge – one to Sir Lawrence, the other to Peter.

When Peter arrived in the lab with the manuscript, Watson pulled it out of his coat pocket and began to read it.
Watson wrote: “At once I felt something was not right. I could pinpoint the mistake, however, until I looked at the illustrations for several minutes. Then I realized that the phosphate groups in Linus’ model were not ionized, but that each group contained a bound hydrogen atom and so had no net charge. Pauling’s nucleic acid in a sense was not an acid at all.”

The hydrogens were part of the hydrogen bonds that held together the three intertwined chains. Without the hydrogen atoms, the chains would immediately fly apart and the structure vanish.
Somehow Linus, unquestionably the world’s most astute chemist, had made a major blunder.
Watson decided to inform Maurice Wilkins and Rosalind Franklin by bringing them the manuscript.

When Watson arrived at King’s, Wilkins was busy so he went down the corridor to Franklin’s lab.
Watson gave her the manuscript and pointed out Pauling’s mistake.
Franklin became annoyed at Watsons references to helical structures.
The argument became more heated.
At this point, Watson implied that she was incompetent in interpreting X-ray pictures.

Watson reported what happened next: “Suddenly Rosy came from behind the lab bench that separated us and began moving toward me. Fearing that in her hot anger she might strike me, I grabbed up the Pauling manuscript and hastily retreated to the open door.”

As Watson retreated, Maurice Wilkins came upon the scene.
As Franklin firmly shut the door, Watson and Wilkins made their way down the corridor.
Wilkins began to open up to Watson.
From an adjacent room, Wilkins retrieved a photograph of DNA that had been taken by Rosalind Franklin.
It was a photograph of the “B” structure of DNA.

Watson recalled: “The instant I saw the picture my mouth fell open and my pulse began to race. The pattern was unbelievably simpler than those obtained previously (“A” form). Moreover, the black cross of reflections which dominated the picture could arise only from a helical structure. … Conceivably, after only a few minutes’ calculations, the number of chains in the molecule could be fixed.”

On the train back home, Watson sketched what he remembered of the B pattern on the blank edge of his newspaper.
He decided to build two-chain models.
The next day Watson met with Bragg and told him what he knew of the B form.
Bragg was concerned that Linus Pauling might get to the DNA structure first.
He gave the go-ahead for Watson and Crick to resume model-building.

Watson and Crick ordered an new set of molecular models and set back to work.
It took several days for the models to be completed.
When phosphorous atoms were ready, Watson began to construct sections of the sugar-phosphate backbone.
At first Watson returned to building models in which the sugar-phosphate backbone was in the center.
After failing to produce a satisfactory model with the sugar-phosphate backbone in the center, Watson decided to try putting the sugars and phosphates on the outside.

Because the machine shop would not complete making the nitrogenous bases for another week, Watson could not continue by trying to fit the bases into the model.
He proceeded by checking the configuration of the backbone against Rosalind Franklins measurements.

Watson and Crick had obtained this data from Max Perutz.
Perutz was a member of a committee set up to coordinate Biophysics research within its laboratories.
Randall had prepared a summary of the accomplishments of his workers and supplied it to the committee members and Perutz passed it along to Watson and Crick.

Watson began to concentrate on the nitrogenous bases.
He used the formulas for the bases as they were written out in J.N. Davidson’s book The Biochemistry of the Nucleic Acids.
For a long time, Watson and Crick had dismissed the possibility that the chains might be held together by hydrogen bonds between the bases.
One of the difficulties was that the hydrogen atoms on each of the bases could move from one location to another (a tautomeric shift).

Watson learned from reading biochemical papers that the bases do join by forming hydrogen bonds.
He began to try pairing of the bases using drawings that he made on paper.
He saw that two adenines could join to one another by forming hydrogen bonds. Hydrogen bonds could also hold together pairs of guanine, cytosine, or thymine.
He then proposed a DNA structure in which the two chains with identical base sequences were held together by hydrogen bonds between pairs of identical bases.

There was a problem with Watson’s proposed structure.
Such a structure could not have a regular backbone in which the spacing between the two chains remained constant.
The purines (adenine and guanine) and the pyrimidines (thymine and cytosine) have different shapes.
The purines are larger double-ring structures and the pyrimidines are smaller single-ring structures.
The distance across the molecule would be less for two pyrimidines connected together than for two purines.

It soon became apparent that Watson’s scheme would not work.
Jerry Donohue, an American crystallographer and an expert on hydrogen bonds pointed out the error.
Watson had chosen the wrong tautomeric forms for the bases.
Even though the forms that Watson used were the ones given in the textbooks, Donohue felt that the bases were present in the keto form rather than the enol form.

Another difficulty was that Watson’s scheme provided no explanation for Chargaff’s rules.
Watson tried again. He constructed accurate representations of the nitrogenous bases using cardboard. This time he used the keto forms of the bases.
He began to try various pairing possibilities.

As Watson remembered: “Suddenly I became aware that an adenine-thymine pair held together by two hydrogen bonds was identical in shape to a guanine-cytosine pair held together by at least two hydrogen bonds. All the hydrogen bonds seemed to form naturally; no fudging was required to made the two types of base pairs identical in shape.”

Watson realized that his scheme in which adenine paired with thymine and cytosine paired with guanine explained Chargaff’s rules.
In this scheme a purine would pair with a pyrimidine.
The hydrogen-bonding requirement meant that adenine would always pair with thymine, while guanine could pair only with cytosine.

Always pairing adenine with thymine and cytosine with guanine meant that the base sequences of the intertwined chains were complementary to one another.
“Given the base sequence of one chain, that of its partner was automatically determined.”
“Conceptually, it was thus very easy to visualize how a single chain could be the template for the synthesis of a chain with the complementary sequence.”

When Francis Crick came into the room, he realized that the two glycosidic bonds (joining base and sugar) of each base pair were systematically related by a diad axis perpendicular to the helical axis.
Thus, both pairs could by flipflopped over and still have their glycosidic bonds facing in the same direction.
This had the important consequence that a given chain could contain both purines and pyrimidines.
At the same time, it strongly suggested that the backbones of the two chains must run in opposite directions.

At lunch at a pub called the Eagle, Francis Crick told everyone that he and Watson had found the secret of life.
Soon, Watson and Crick had all of the components of the models they needed.
Watson began putting the component together to construct a model of DNA.
Their model was composed of two strands wound around one another in a helix.
The two strands were connected by pairs of nitrogenous bases.

In the base pairs connecting the two chains, an adenine always joined to a thymine and a cytosine always joined to a thymine.
The nitrogenous bases were joined by hydrogen bonds, which were weak chemical bonds.
The sequence of nitrogenous bases on one strand was complementary to the sequence of nitrogenous bases on the opposite strand.
The model that they had constructed became the famous “Double Helix”.

It was arranged that John Kendrew would call up Maurice Wilikins to invite him to see the model.
The next step was to compare the experimental X-ray data with the diffraction pattern predicted by the model.
Wilkins returned to London to measure the critical reflections.
Two days later, Wilkins reported that he and Rosalind Franklin found that their X-ray data strongly supported the double helix.

The discovery was to be reported in the Journal Nature.
It was decided that in addition to Watson and Crick’s paper from Cambridge, that there would be two papers from Kings.
One paper would be written by Wilkins and his collaborators and the other would be written by Rosalind Franklin and her assistant Raymond Gosling.
The papers appeared in the April 25, 1953 issue of Nature. The order in which the papers appeared was first Watson and Crick’s, then the paper by Wilkins, Stokes, and Wilson, and last Franklin and Gosling’s.

The 1962 Nobel Prize in chemistry was awarded to Perutz and Kendrew for the method of solving the structures of globular proteins by isomorphous replacement with heavy atoms, and for the solutions of the first two proteins, myoglobin to high resolution and hemoglobin to low.
The prize in physiology or medicine went to Crick, Watson, and Wilkins in common for the solution of the structure of deoxyribonucleic acid.