Franklin’s Response to the Crick-Watson 3-Chain Model (1952):
Franklin objected to the proposed 3-chain model. She stated that its phosphate groups held together by Mg2+ ions was unlikely considering the Mg2+ ions would be surrounded by tight shells of water molecules.
Pauling’s 3-helix model:
Pauling formulated a structure of 3 intertwined helical polynucleotide chains. Each chain is formed by phosphate di-ester groups, linking D-ribose or D-deoxyribose residues with 3’, 5’ linkages. They have approximately 24 nucleotide residues in 7 turns of the helix, which has the sense of a right handed skew. The phosphate groups are closely packed about the axis of the molecule with the pentose residues surrounding them and the purine and pyrimadine groups projecting radially.
Watson’s response to Pauling’s Ideas (1953):
Watson realised that the phosphate groups in Linus’ model were not ionised as each group contained a bound hydrogen atom and so had no net charge. Therefore the molecule was not an acid at all.
Watson also realised that the molecule must contain hydrogen atoms for hydrogen bonding to occur. Without this bonding the molecule would fall apart.
Pauling’s Later Comments:
Pauling realised that a 3-chain helical structure was an extraordinarily tight one with little opportunity for change in positions of the atoms.
He realised that a double helix was a much more appropriate and realistic model.
Crick and Watson’s critical paper on the structure of DNA:
Crick and Watson decided that the bases were on the inside of the helix and the phosphates were on the outside, as cations would have easy access to them. They suggested that the backbone (sugar and phosphates) were perpendicular to the bases. They assumed that there was an angle of 36° between adjacent residues in the same chain so that the structure repeats after 10 residues on each chain. The structure was seen as an open one and having a very high water content.
The two bases joined perpendicular to the backbone were held together by hydrogen bonding (one base a purine and one a primadine for this bonding to occur). The hydrogen bonds are made as follows: purine position 1 to pyrimadine position 1 and purine position 6 to pyrimadime position 6. These pairs are adenine with thymine and guanine with cytosine. The sequence of bases on a single chain does not appear to be restricted in any way, however, if only specific pairs of bases can be formed, and the sequence of bases in one chain is given, then the sequence on the other chain is automatically determined.
Summary of the history and development of DNA:
- Franklin (1951) suggested the helical structure of DNA
- Crick and Watson discovered the sugar-phosphate backbone in 1951/1952
- Watson (1953) discovered that hydrogen bonding took place in DNA
- Pauling suggested DNA was a 2-chain rather than a 3-chain molecule as suggested earlier.
- Crick and Watson wrote a paper on the final structure of DNA
