M6 Critical Reading Form for Watson & Crick- Franklin Papers.docx

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Questions for Watson &Crick In writing “Molecular Structures of Nucleic Acids”, Watson and Crick addressed two previously proposed structures of DNA, pointing out the faults within each proposed model. The first structure mentioned was Pauling and Corey’s proposal of a triple stranded DNA model. Within this triple stranded helix, Pauling and Corey hypothesized that the phosphates were on the inside (facing the central axis) and the bases were pointing outwards. According to Watson and Crick, the triple helix model doesn’t agree with the X-ray diffracting images of DNA developed by Franklin. Furthermore, they stressed that the negatively charged nature of the phosphates would not allow for bonding to occur between the helical strands and would instead drive the structure apart due to the repelling forces, literally driving the structure apart. They further argued that the hydrogens apart of the nucleic acids are what hold the structure together, indicating the necessity of the nucleic bases needing to be on the inside. Another critique regarding Pauling and Corey’s model was that the van der Waals distances (the intermolecular forces that allow for the hydrogens of the nucleic acids to bind together) were too short in distance which would alter the structure of the DNA. The second structure was proposed by Fraser, who also hypothesized a triple stranded helix. Yet in his model, the phosphates were on the outside of the structure while the bases were on the inside; forming hydrogen bonds to link them together. Watson and Crick do not critique Fraser’s model due to the structure being “ill-defined”. Whether Fraser did not include a detailed analysis supporting his version of the DNA model in his publication for which Watson and Crick could accurately critique, or the structure of the proposed model itself was too ambiguous to critique; the assumption dismissing Fraser’s DNA model is probably similar to the critique of Pauling and Corey’s DNA structure.

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