It is 60 years since the structure of DNA was unravelled – the most important scientific breakthrough of the 20th century.
X-ray diffraction images, generated at King’s College London in the early 1950s, enabled James Watson and Francis Crick to model the molecule's double helix structure. Understanding how DNA is chemically arranged revealed the processes of self-duplication, which allows genetic information to be passed on to newly formed cells, and also demonstrated how variation can occur within species. This discovery revolutionized biology and has enabled great advancements in medicine, agriculture and crime investigation.
In 1962, Watson, Crick and Maurice Wilkins (a former molecular biologist at King’s College) were awarded the Nobel Prize for Medicine. The scientist's hypothesis was based on X-ray diffraction images made by Rosalind Franklin, a King’s College research assistant. Yet in the male-dominated world of post-war science, Franklin's contribution remained out the limelight. Her paper, released in the journal Nature, followed Watson and Crick's published work, which only hinted of Franklin's data, despite Crick stating her images were actually used. Franklin died of ovarian cancer in 1958, aged only 37, and could not receive the Nobel Prize. Six decades on, the importance of this discovery should not be forgotten, including Franklin’s involvement.
Franklin’s x-ray diffraction images, made in collaboration with several other scientists, confirmed the helical structure of DNA. Crick had limited knowledge of x-ray crystallography and did not fully understand Franklin’s description of the molecule's structural symmetry. Yet Crick was aware of helical structures between other atomic arrangements, depicted through x-ray diffraction patterns, and interpreted Franklin’s work as a double helix with two sugar-phosphate chains. In 1953, the molecular model of DNA, proposed by Watson and Crick, was published. Unknown to Franklin, there are reports that Watson and Crick had access to her data prior to releasing their theory.
Rosalind Franklin’s famous ‘Photo 51’: an X-ray diffraction image of DNA taken in May 1952. The central cross-formation represents a helical structure
During the 1920s and 1930s, it was suspected that proteins were the main hereditary molecules. By the mid-1940s, studies indicated the quantity of DNA in any somatic cell (non sex- or stem-cell) of a given species was the same, while sperm cells contained half the amount. Further analysis revealed DNA contained various amounts of adenine, cytosine, guanine and thymine molecules (known as bases). Although the quantities of these four bases varied in different species, whatever the amount of adenine, there was always the equivalent amount of thymine, and likewise for guanine and cytosine. It was also apparent that these four bases alternated and were attached to a sugar-phosphate backbone at a 90o angle, forming a long thread-like molecule.
When Franklin and Wilkins studied purified DNA (possibly derived from squid sperm heads) via x-ray diffraction techniques, it was revealed the molecular structure was helical. Watson and Crick observed adenine and thymine joining at almost exactly the same length that guanine joined to cytosine (with each base-pairing attached by hydrogen-bonds). It was suggested the four bases could be distributed in any order, with each base-pairing connected to both sugar-phosphate backbones (resulting in a spiral ladder-like form). This elegant chemical arrangement inspired Watson and Crick to create their famous model.
Such a structure is able to split along the hydrogen-bonds, which join the base-pairs, enabling either helix to separate. Each unpaired base on the now single strands acquires a new corresponding base that is already joined to a sugar-phosphate unit. Enzymes unite the sugar-phosphate groups, which form a new backbone and completes molecular duplication. Genetic information, inherited from both parents in the form of genes (sections of DNA), is stored as a sequence of base-pair arrangements. This information enables the assembly of proteins: the main structural forms of all cells and which also regulate metabolic processes. Changes or mutations in DNA base-pair configurations can lead to alterations during protein synthesis, and therefore variation within species.
The DNA helix: Figure b. clearly shows the adenine-thymine (A-T) and cytosine-guanine (C-G) base-pairings, joined by two helical sugar-phosphate backbones
Genetics Transforms Society
Unravelling the structure of DNA has changed our outlook on all aspects of life. In biology, genetics has furthered our understanding of cellular metabolism, reproduction, hereditary, evolutionary development and genetic engineering. Genetics explains inheritance and why traits in organisms are expressed in subsequent generations. In medicine, the processes which dictate how DNA is structured and behaves has assisted the diagnosis and treatment of many disorders, including birth defects, cancer, cystic fibrosis and muscular sclerosis, and led to genetic screening techniques and drugs that work on protein synthesis systems. Advancements in medical genetics are predicted to increase over this century, leading to personalized medicines, xeno-nucleic acids, and gene therapies. The developments of genetically modified plants, such as maize, soybean and cotton, which can produce greater yields in adverse conditions, have furthered agriculture. DNA analysis in crime investigation also provides evidence for judicial systems.
There are controversies: if this science is not regulated, genetically modified organisms, animal cloning, designer babies, and other ethical issues are a cause for concern. As with all major scientific breakthroughs, there are benefits and detriments; it is our responsibility to ensure we adapt appropriately, utilize the advantages that genetics can deliver, and enforce policy to prevent adverse developments that can endanger biological systems, including our own.
It is important we reflect on the genetic advancements that have arisen over the past sixty years, and also remember the scientists – both the renowned and the lesser known – who unraveled the 20th century’s most important scientific discovery.