NWR Presentation, May 2nd, 2013
On the 28th of February, 1953 Francis Crick, a physicist, walked into the Eagle pub in Cambridge, England with fellow scientist, James Watson, a biologist from Indiana University, and announced, “We have found the secret of life.”
He was bragging – not because of the ‘secret of life’ aspect of that proclamation, as you might think, but because Crick and Watson were only two of a much larger group of researchers who contributed to the discovery of the molecular structure of the DNA-helix, the molecule that carries genetic information from one generation to the other.
Nevertheless, to this day a plaque exists on the wall of the Eagle pub that this was where Crick and Watson announced “their discovery of how DNA carries genetic information”.
On April 25th, 1953 (hence the 60th anniversary in 2013) Watson and Crick presented their work in the journal Nature with these words, “This structure has novel features which are of considerable biological interest…” which is perhaps one of science’s most famous understatements.
Nine years later, in 1962, they shared the Nobel Prize in Medicine with Maurice Wilkins, a New Zealand physicist, who was the deputy director of the King’s College biophysics lab, for solving one of the most important of all biological riddles. Sixty years later, important new implications of this contribution to science are still coming to light.
Worthy of a BBC drama
From the mid-1850s to nearly a century later DNA research was a progression of scientific discoveries each one building on the last, but in the 1950s the story behind DNA discovery was worthy of a BBC drama with everyone trying to win the race and the ‘femme fatale’ dying before her worth was recognised.
However, without the scientific foundation provided by early scientific pioneers and their contemporary researchers, Watson and Crick may never have reached their ground-breaking conclusion of 1953.
The Czech monk, Gregor Mendel, also known as the “Father of Genetics” entered the Roman Catholic Church in 1843, studied at the University of Vienna where he mastered mathematics, and then later performed many scientific experiments.
The greatest experiment that Mendel performed involved growing thousands of pea plants for 8 years. Mendel was able to show that certain traits in the peas, such as their shape or colour, were inherited in different packages. These packages are what we now call ‘genes’.
In 1869 Swiss physiological chemist Friedrich Miescher first identified what he called “nuclein” inside the nuclei of human white blood cells. The term “nuclein” was later changed to “nucleic acid” and eventually to “deoxyribonucleic acid,” or DNA.
Miescher’s plan was to isolate and characterize the protein components in white blood cells. Miescher made arrangements for a local surgical clinic to send him used, pus-coated patient bandages; once he received the bandages, he planned to wash them, filter out the white blood cells, and extract and identify the various proteins within the white blood cells. But when he came across a substance from the cell nuclei that had chemical properties unlike any protein, Miescher realized that he had discovered a new substance.
Other scientists continued to investigate the chemical nature of nuclein. One of these other scientists was Russian biochemist Phoebus Levene. A physician turned chemist, Levene was a prolific researcher, publishing more than 700 papers on the chemistry of biological molecules over the course of his career.
Levene nor any other scientist of the time knew how the individual nucleotide components of DNA were arranged in space. The large number of molecular groups made available for binding by each nucleotide component meant that there were numerous alternate ways that the components could combine. Several scientists put forth suggestions for how this might occur, but it was Levene’s “polynucleotide” model proposed in 1919 that proved to be the correct one.
Based upon years of work using hydrolysis to break down and analyse yeast nucleic acids, Levene proposed that nucleic acids were composed of a series of nucleotides, and that each nucleotide was in turn composed of just one of four nitrogen-containing bases, a sugar molecule, and a phosphate group.
In 1928 a scientist named Frederick Griffith was working on a project that enabled others to point out that DNA was the molecule of inheritance.
Griffith’s experiment involved mice and two types of pneumonia, a virulent and a non-virulent kind.
- He injected the non-virulent pneumonia into a mouse and the mouse continued to live.
- Next he injected the virulent pneumonia into a mouse and the mouse died.
- After this, he heated up the virulent disease to kill it and then injected it into a mouse. The mouse lived.
- Last he injected non-virulent pneumonia and the virulent pneumonia that had been heated and killed into a mouse. This mouse died.
Why? Griffith thought that the killed virulent bacteria had passed on a characteristic to the non-virulent one to make it virulent. He thought that this characteristic was in the inheritance molecule.
Oswald Avery & Erwin Chargaff – 1940s research
For a long time the connection between nucleic acid and genes was not known. But in 1944 the American scientist Oswald Avery managed to transfer the ability to cause disease from one strain of bacteria to another. But not only that: the previously harmless bacteria could also pass the trait along to the next generation. What Avery had moved was nucleic acid. This proved that genes were made up of nucleic acid.
Erwin Chargaff was a professor of biochemistry at Columbia University researching whether there were any differences in DNA among different species. After developing a new paper chromatography method for separating and identifying small amounts of organic material, Chargaff reached two major conclusions in 1950:
- Nucleotide composition of DNA varies among species
- Almost all DNA–no matter what organism or tissue type it comes from–maintains certain properties, even as its composition varies. In particular, the amount of adenine (A) is usually similar to the amount of thymine (T), and the amount of guanine (G) usually approximates the amount of cytosine (C). This second major conclusion is now known as specific base pairing or Chargaff’s rule
Solving the puzzle
While Miescher found DNA, it wasn’t until 84 years later, that scientists recognized it as genetic material, however, with only four bases adenine, thymine, guanine and cytosine to vary the composition wasn’t it just too simple to be the building blocks of life? Also nobody had the slightest idea of what the molecule might look like.
We come now to the researchers working on the DNA puzzle in the 1950s. Crick, Watson and Wilkins have already been introduced but interweave the work of Rosalind Franklin and Linus Pauling into the mix and you get a dramatic tale of jealousy and competition.
Rosalind Elise Franklin was born in 1920 in London and attended St. Paul’s Girls School where she excelled in science, mathematics, and athletics.
In 1938, she was awarded a scholarship in physics and chemistry to attend Cambridge University where she undertook studies in X-ray crystallography. After earning her Ph.D. and publishing seminal papers on coal she took a job offer in one of the best labs in Paris. She was a good experimenter, perfected her X-ray techniques, published and spoke at conferences, and was well liked by her peers. It has been reported that she was a fashion-minded lady of Paris wearing Dior and socializing as a chef for her friends.
After 4 years in Paris she decided at 30 years of age to return to London. She was hired by J.T. Randall, Director of King’s College biophysics labs, to create an X-ray unit and work on DNA. She arrived at the King’s College lab in 1951 to work on an x-ray picture of DNA taken by a graduate student Raymond Gosling.
The working relationship at King’s college did not get off to a good start because of a misunderstanding. New Zealander Maurice Wilkins who had been working on nucleic acids and x-ray diffraction of DNA at King’s College, was away on holiday when Franklin arrived. Gosling, his second in command, stood in for him at the first office meeting and, since no work had been done on DNA for several months, it was all given to Franklin.
When Wilkins returned he thought that Franklin was a high class technical assistant who was to supply the team with experimental data for it to analyse. This was not to be. Relationships were strained between Wilkins and Franklin. It didn’t help that the atmosphere at King’s was akin to an old boy’s club (the lunch room was for men only) which lead to conflict. Wilkins also insisted on calling Franklin “Rosy”, which she hated.
Franklin soon found out that by bundling super thin strands of DNA and zapping them with a super fine x-ray beam there were two forms of hydration: the A form (easy to photograph) that is dry and the B form (hard to photograph) that is wet. Her B form photographs showed a fuzzy cross which meant a helix but she didn’t know how many strands were involved.
However she did draw the important conclusion that since the water would be attracted to the phosphates in the backbone, and the DNA was easily hydrated and dehydrated, the backbone was on the outside and the bases were on the inside.
In November of 1951 Franklin gave a departmental seminar to bring the unit up to date on what she had achieved so far. In it she presented the A and B form data.
In the audience was American James Watson. After the seminar Watson caught the train back to Cambridge and, based on what he had heard and seen, he and Francis Crick built their first model of DNA. Watson had not taken notes, knew nothing about crystallography, and had only remembered bits of Franklin’s talk. So, when Watson and Crick invited Franklin and Wilkins to view the model, Franklin tore its construction to shreds as the model was a triple helix with bases on the outside.
It was Franklin’s famous “Photograph 51” that finally revealed all the answers. The photo was taken in May, 1952, by long exposure started the previous day. Franklin got the first good photograph of the B form of DNA. The photograph clearly showed a double helix. Franklin, though, was a perfectionist when it came to research and would not release any data until she had more information on the A form.
Not releasing the information on the B form proved to be Franklin’s downfall, for she got bogged down with calculations and obsessed in trying to determine whether the A form was also helical. She did meet with Crick who tried to offer advice, but, since his character was typically patronising, she rejected an opportunity to collaborate. An opportunity missed, for they would most certainly have solved the puzzle much sooner.
Franklin became fed up with the scientific atmosphere at King’s College and accepted a position at Brikbeck College, London, with crystallographer J.D. Bernal to work on the structure of viruses. In her final seminar at King’s College, she never showed photo 51.
Meanwhile, in America, Linus Pauling was working on the structure of DNA and was very close to solving the puzzle. He was a physical chemist with an interest in biological chemistry, who in 1950 constructed the first satisfactory model of a protein molecule.
In January 1953, Linus Pauling sent his son Peter, who was studying at Cambridge, a draft copy of his paper on DNA for comment. Watson knew Peter and somehow got a sight of the DNA paper. Watson took the paper to Franklin for comment, but she dismissed it as rubbish, being further annoyed as she had written to Pauling for information, but none had been sent.
The penny drops
By 1953, Watson had obtained photo 51, either from Franklin herself, her lab assistant, Raymond Gosling, or Wilkins showed him Franklin’s photo 51 (some have called this unethical on Wilkins part). Watson immediately recognized the significance of the “X” in photo-51 showing the double helix shape of the DNA molecule. Watson sketches a copy of the photo on a newspaper and returns to Crick.
Then with Chargaff’s Rule at hand Watson was able to figure out the base pairing rules. Chargaff, if you recall, in 1949, showed that even though different organisms have different amounts of DNA, the amount of the base adenine always equals the amount of the base thymine. The same goes for the base pair guanine and cytosine. For example, human DNA contains about 30 percent each of adenine and thymine, and 20 percent each of guanine and cytosine.
On the 21st of February 1953 Watson had the key insight, when he saw that the adenine-thymine bond was exactly as long as the cytosine-guanine bond. If the bases were paired in this way, each rung of the twisted ladder in the helix would be of equal length, and the sugar-phosphate backbone would be smooth.
The specific base pairing underlies the perfect copying of the molecule, which is essential for heredity. During cell division, the DNA molecule is able to “unzip” into two pieces. One new molecule is formed from each half-ladder, and due to the specific pairing this gives rise to two identical daughter copies from each parent molecule.
Together Watson and Crick modelled the structure of DNA as a 2 chain helical, with antiparallel properties and the bases facing inward paired to hold the molecule together. Within 2 months they had published their model.
Franklin, in fact, came to Cambridge to see their model, and readily accepted it. But save for a small acknowledgement at the end of their paper in Nature, Rosalind Franklin is not recognized for her contribution to the discovery of the structure of DNA.
The Nobel Prize was awarded a few years after the presentation of the model to Watson, Crick, and Maurice Wilkins. Rosalind Franklin did not receive the prize because she had died of ovarian cancer by this time, probably due to constant exposure to X-rays. In addition, only living people can win a Nobel Prize. Maurice Wilkins was able to share the prize with Watson and Crick, though, because of his work with Franklin, however, her accomplishment was never fully recognised.
The discord surrounding who deserved credit for each component of the discovery of the structure of DNA is thought by some to have slowed the progress of how DNA works for at least a decade by limiting collaborations and the open sharing of ideas.
2013 and beyond
The stunning find of the structure of the DNA molecule made possible the era of “new biology” that led to the biotechnology industry and, most recently, the deciphering of the human genetic blueprint.
Methods have been developed to purify DNA from organisms and to manipulate it in the laboratory to create a recombinant or man-made DNA sequence. The genetically modified organisms produced can be used to produce products such as recombinant proteins, used in medical research or grown in agriculture.
Uses in forensic science
The development of forensic science and the ability to now obtain genetic matching on minute samples of blood, skin, saliva or hair has led to a re-examination of a number of cases. Evidence can now be uncovered that was not scientifically possible at the time of the original examination.
People charged with serious crimes may be required to provide a sample of DNA for matching purposes. The most obvious defence to DNA matches obtained forensically is to claim that cross-contamination of evidence has taken place. This has resulted in meticulous strict handling procedures with new cases of serious crime.
DNA profiling is also used to identify victims of mass casualty incidents. As well as positively identifying bodies or body parts in serious accidents, DNA profiling is being successfully used to identify individual victims in mass war graves – matching to family members.
Scientists have also been able to insert new bits of DNA into cells that lack particular pieces of genes or whole genes. With this new DNA, the cell becomes capable of producing gene products it could not make before. The hope is that, in the future, diseases that arise due to the lack of a particular protein could be treated by this kind of gene therapy.
History and anthropology
DNA collects mutations over time, which are then inherited. DNA, therefore, contains historical information, and, by comparing DNA sequences, geneticists can infer the evolutionary history of organisms, or their phylogeny. This field of phylogenetics is a powerful tool in evolutionary biology. If DNA sequences within a species are compared, population geneticists can learn the history of particular populations.
DNA has also been used to look at modern family relationships, such as establishing family relationships between the descendants of Thomas Jefferson and Sally Hemings Jefferson, a president of the US and Hemings, a black slave were obviously not meant to be having children together.
English King Richard III’s bones were recently found and identified under a car park in Leicester thanks to DNA profiling. The bones match the descendants from the king’s family and were carbon dated back to 1455-1540 which confirms the history as the king died in a battle in 1485.
Adolf Hitler is likely to have had Jewish and African roots, DNA tests have shown. Saliva samples taken from 39 relatives of the Nazi leader show he may have had biological links to the “subhuman” races that he tried to exterminate during the Holocaust.
A chromosome called Haplogroup E1b1b1 which showed up in samples is rare in Western Europe and is most commonly found in the Berbers of Morocco, Algeria and Tunisia, as well as among Ashkenazi and Sephardic Jews.
Bioinformatics involves the manipulation, searching, and data mining of biological data, and this includes DNA sequence data. The development of techniques to store and search DNA sequences have led to widely applied advances in computer science.
In a paper published in Nature in January, 2013, scientists from the European Bioinformatics Institute proposed a mechanism to use DNA’s ability to code information as a means of digital data storage. The group was able to encode 739 kilobytes of data into DNA code, synthesize the actual DNA, then sequence the DNA and decode the information back to its original form, with a reported 100% accuracy. The encoded information consisted of text files and audio files.
They acknowledge that the costs involved in synthesizing the molecule in the lab make this type of information storage “breathtakingly expensive” at the moment, but argue that newer, faster technologies will soon make it much more affordable, especially for long-term archiving.