DNA Day: Discovery of the Double Helix
The discoverers of the structure of DNA
Source: http://www.ba-education.com/for/science/dnadiscovery.html
DNA is a molecule present in all of the cells in your body, and it’s what makes you, you! DNA is short for “deoxyribonucleic acid” and it not only determines your hair color and how tall you are, but also has the blueprints for your body to make proteins it needs to function. Although DNA has one of the most iconic structures that we think about when it comes to biology, we didn’t know how it was shaped until the 1950s.
DNA was first discovered in 1869 by a scientist named Friedrich Miescher. Miescher was originally trying to isolate a type of protein from white blood cells, but accidentally came across a molecule that had a very different makeup from that of other known proteins. He realized that he had discovered something new, and originally called this molecule “nuclein”.
(left) Nucleotides and their bases. (right) How nucleotides fit together in a chain.
Many scientists worked to figure out what this “nuclein”, now renamed DNA, was made out of. Together their findings described a component of DNA called nucleotides. Nucleotides are made up of a sugar, a phosphate group, and one of four bases: Adenine (A), Thymine (T), Guanine (G), or cytosine (C). These bases were then organized into two groups: pyrimidines (C and T) which have one ring, and purines (A and G) which are made up of two rings. Chains of nucleotides are formed by the sugar of one base linking to the phosphate group of the next base, creating the “sugar-phosphate backbone”.
X-ray diffraction image of DNA taken by Rosalind Franklin
Source: https://www.nature.com/articles/nature01399
Despite knowing the components of DNA, scientists still weren’t sure how they fit together in a three-dimensional way. Rosalind Franklin was an English chemist working at King's College whose research was focused on the molecular structure of DNA. She used a technique called x-ray crystallography in which x-rays are directed at a crystal form of a molecule and the crystalline atoms deflect the x-rays, forming a diffraction pattern. This pattern can give clues about the structure of molecules to experts who know how to interpret them. Franklin’s crystallography images of DNA were given to two other biologists, James Watson and Francis Crick, without her knowledge by her supervisor Maurice Wilkins.
The three-dimensional structure of DNA. The two strands twist around each other in a double helix with the sugar-phosphate backbone on the outside, and the nucleotides pairing together on the inside.
Source: https://www.nature.com/scitable/topicpage/discovery-of-dna-structure-and-function-watson-397/
Watson and Crick were also working on the molecular structure of DNA. Although they didn’t perform experiments themselves, they performed research using their scientific knowledge and experimental findings of other researchers. They weren’t the only ones racing to create this model. Scientists Paulint and Cory proposed a triple helix with the phosphate groups on the inside and the bases radiating outwards. With Franklin’s crystallography images, Watson and Crick were able to determine that DNA had a helical structure. Using a new way of model building of three-dimensional structures using known molecular distances and bond angles, Watson and Crick discovered that complementary bases fit together perfectly (A to T and C to G), joined together by a hydrogen bond. With these findings together they were able to propose a model of DNA that we still use today with some minor modifications. The sugar-phosphate backbone forms the outside of the right-handed double helix structure, and complementary bases are paired together on the inside.
In 1962, Watson, Crick, and Wilkins were awarded the Nobel Prize in Medicine "for their discoveries concerning the molecular structure of nucleic acids and its significance for information transfer in living material." Although many think that Rosalind Franklin should have been awarded the Nobel Prize, she had passed away in 1958 from ovarian cancer, and it cannot be awarded after death. Franklin did not get much acknowledgement for her contribution to the discovery at the time, but current historians and scientists are working to give her the recognition she deserves.
The discovery of the structure of DNA paved the way for the study of genetics, and has led to our current understanding of biology and medicine. In fact, knowledge of DNA and it’s complementary molecule RNA, led to the creation of the COVID-19 mRNA vaccine.
Other Resources
Here are some links to other resources that may help you better understand the structure of DNA and its discovery:
https://www.youtube.com/watch?v=0E4p34mqJbg
https://www.nature.com/scitable/topicpage/rosalind-franklin-a-crucial-contribution-6538012/
https://www.nature.com/scitable/definition/double-helix-277/
References:
Discovery of the structure of DNA. (2016). Khan Academy. https://www.khanacademy.org/science/high-school-biology/hs-molecular-genetics/hs-discovery-and-structure-of-dna/a/discovery-of-the-structure-of-dna.
Pray, L. (2008). Discovery of DNA structure and function: Watson and Crick. Nature Education, 1(1), 100. https://www.nature.com/scitable/topicpage/discovery-of-dna-structure-and-function-watson-397/
Maddox, B. (2003). The double helix and the “wronged heroine.” Nature, 421(6921), 407–408. https://doi.org/10.1038/nature01399
NOBEL PRIZE FOR MEDICINE. (1962). The Lancet, 280(7261), 868. https://doi.org/10.1016/s0140-6736(62)90639-6
