What evidence did Watson and Crick have at their disposal in 1953? What was their approach in arriving at the structure of DNA?

In 1953, when Watson and Crick were working on the structure of DNA, they had access to crucial pieces of evidence that helped them in their research: 1. Chargaff's Rule: The purine-pyrimidine base-pairing rules, discovered by Erwin Chargaff, which stated that the amount of adenine (A) in an organism always equalled the amount of thymine (T), and the amount of guanine (G) always equalled the amount of cytosine (C). 2. Rosalind Franklin's X-ray diffraction images of DNA: These images, particularly Photo 51, provided crucial insights into the structure of DNA. Franklin's research showed, among other things, that DNA has a helical structure and that the bases were stacked perpendicularly to the main axis. 3. Linus Pauling's alpha-helical model of protein: Pauling's model of proteins established the concept of a helical structure, which influenced Watson and Crick's work on DNA. 4. Maurice Wilkins's DNA fiber X-ray diffraction work and knowledge on wet-fixed fibers, which contributed to understanding the structure and periodicity of DNA.

Watson and Crick used a combination of experimental results, collaboration, and model building to arrive at the structure of DNA: 1. Integration of existing data: They first collected all the available evidence (Chargaff's rules, Franklin's X-ray, and Wilkins' fiber diffraction work) to form a basis for their model. 2. Trial and error model building: Watson and Crick used physical models to represent the molecular structures of DNA components (bases, sugar, and phosphate). They would then arrange and rearrange these components to find the most plausible structure that explained the experimental results. 3. Collaborative approach: Watson and Crick discussed intensely with each other and consulted with colleagues (such as Rosalind Franklin and Maurice Wilkins) to gain insights and assess the validity of their models. 4. Double helix structure: After several iterations of trial and error, Watson and Crick eventually arrived at the double helix structure of DNA, in which two strands of the molecule run antiparallel to each other and are held together by hydrogen bonds between complementary base pairs (A with T, and C with G). The discovery of the double helix structure provided a key insight into how genetic information is stored within DNA and how it is duplicated during cell replication.