Application:
• Crick and Watson’s elucidation of the structure of DNA using model making
The structural organisation of the DNA molecule was correctly proposed in 1953 by James Watson and Francis Crick
- These British scientists constructed models to quickly visualise and assess the viability of potential structures
Their efforts were guided by an understanding of molecular distances and bond angles developed by Linus Pauling, and were based upon some key experimental discoveries:
- DNA is composed of nucleotides made up of a sugar, phosphate and base – Phoebus Levene, 1919
- DNA is composed of an equal number of purines (A + G) and pyrimidines (C + T) – Erwin Chargaff, 1950
- DNA is organised into a helical structure – Rosalind Franklin, 1953 (data shared without permission)
Making DNA Models
Using trial and error, Watson and Crick were able to assemble a DNA model that demonstrated the following:
- DNA strands are antiparallel and form a double helix
- DNA strands pair via complementary base pairing (A = T ; C Ξ G)
- Outer edges of bases remain exposed (allows access to replicative and transcriptional proteins)
As Watson and Crick’s model building was based on trial and error, a number of early models possessed faults:
- The first model generated was a triple helix
- Early models had bases on the outside and sugar-phosphate residues in the centre
- Nitrogenous bases were not initially configured correctly and hence did not demonstrate complementarity
The Rosalind Franklin Controversy
The final construction of a correct DNA molecule owed heavily to the X-ray crystallography data generated by Franklin
- This data confirmed the arrangement of the DNA strands into a helical structure
- The data was shared without Franklin’s knowledge or permission and contributed profoundly to the final design
- Hence, Franklin is now recognised as a key contributor to the elucidation of DNA structure
Watson & Crick DNA Model
(Image courtesy of Cold Spring Harbor Archives)