History of DNA structure discovery
DNA discovery led to a battle of scientists competing to discover the correct structure of deoxyribonucleic acid (DNA.)
Phoebus Levene in 1930 demonstrated that the building blocks of DNA are called nucleotides. Nucleotides in DNA are a phosphate group attached to a deoxyribose sugar, and the sugar is attached to a nitrogenous base. He also identified that DNA is a linear polymer of nucleotides. Levene suggested the tetranucleotide model, which we now know was incorrect. The model suggested that the nucelotides would join together in a block with bases facing outwards. To picture this, imagine the nucleotides joining to form a square.
Watson and Crick proposed a structural model for DNA famously known as Watson-Crick base pairing. There are four types of bases in DNA, adenine, guanine, thymine and cytosine. Adenine and guanine are purines (bases with two carbon-nitrogen rings attached to one another) and thymine and cytosine are pyrimidines (one carbon-nitrogen ring.) Watson and Crick identified flaws in previous DNA models and suggested a model which we now use today. One model was produced where bases would face outwards, this means that phosphate groups would face inwards. This was a detrimental mistake as phosphate groups are negatively charged and would repel one another. Thus, bases must be in the inner area of the double helix and phosphate groups must face outwards. Watson and Crick realised that two pyrimidines joined together would be too small, and two purines together would be too large, so a purine must pair with a pyrimidine. Therefore, adenine pairs with thymine and guanine pairs with cytosine.
But how do we know that the bases pair in this way? If a purine joins to a pyrimidine, then surely adenine can bind to cytosine and guanine to thymine? How do we know this is not the case, this is where Chargaff’s rules of base pairing come into place. Erwin Chargaff in 1950 looked at the tetranucleotide model and thought, if it were correct, then the number of bases should all be equal. So, to test this out, he carried out an experiment, isolating DNA from different organisms. The prediction was % thymine= % adenine = % guanine = % cytosine. As you may guess, this was not the result. The real results were % adenine= % thymine and % guanine = % cytosine. Therefore, for those percentages to occur, adenine can only bind to thymine, and guanine can only bind to cytosine.
Linus Pauling was the one to propose a triple helix 3- strand model where the nitrogenous bases pointed inwards. He used x-ray crystallography to discover the alpha-helix and won a Nobel prize. X-ray crystallography is a process that uses an x-ray source and detects the diffracted rays on photographic film. Maurice Wilkins and Rosalind Franklin both used this method to observe DNA structure. Both stretched the DNA but Wilkins decided to air dry it, whilst Franklin left it hydrated. Wilkins had a set of unclear results that were undistinguishable whilst Franklin produced an image with a cross on it, identifying that DNA must be helical.
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