1. X-ray Diffraction Images from Rosalind Franklin:
This was crucial. Franklin's X-ray diffraction images, particularly the famous "Photo 51," provided key insights:
* Double Helix: The image showed a clear pattern of a helix, indicating the DNA molecule had a helical shape.
* Dimensions: The images revealed the spacing between the repeating units of the helix and the overall diameter of the molecule.
2. Chemical Data from Erwin Chargaff:
Chargaff's experiments on the composition of DNA bases provided the following important rules:
* Equal Amounts of Purines and Pyrimidines: Chargaff observed that the amount of adenine (A) always equaled the amount of thymine (T), and the amount of guanine (G) always equaled the amount of cytosine (C).
* Species-Specific Variation: Although the A=T and G=C ratios held true, the overall proportions of these bases varied between different species.
3. Prior Knowledge of DNA Structure:
Watson and Crick were not starting from scratch. They were aware of the following:
* Phosphate-Sugar Backbone: Previous work had established that DNA was composed of nucleotides containing a phosphate group, a sugar (deoxyribose), and a nitrogenous base.
* Four Bases: The four nitrogenous bases (A, T, G, C) were known and their chemical structures were well-understood.
4. Model Building and Deductions:
Watson and Crick used these pieces of information to build physical models of DNA, trying different arrangements of the components until they found one that fit the data. They were guided by the following principles:
* Complementary Base Pairing: The A=T and G=C ratios led them to the concept of complementary base pairing, where A always paired with T, and G always paired with C. This ensured that the two strands of the helix had complementary sequences.
* Antiparallel Strands: They realized that the two strands of the helix ran in opposite directions (antiparallel), which was crucial for the base pairing and stability of the molecule.
In summary, Watson and Crick's work was built on a foundation of experimental data from Franklin and Chargaff, coupled with existing knowledge of DNA structure and a lot of clever model building and deduction. This combination allowed them to unlock the secrets of the DNA double helix.
