1. Rosalind Franklin's X-ray diffraction images: Franklin's work at King's College London produced extremely clear and detailed X-ray diffraction images of DNA. These images revealed the helical nature of DNA and provided critical information about its dimensions, such as the diameter and spacing of the repeating units. The famous "Photo 51," though obtained without Franklin's knowledge or consent, played a crucial role in Watson and Crick's model building.
2. Erwin Chargaff's base pairing rules: Chargaff, through his chemical analysis of DNA, had discovered 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). These rules, known as Chargaff's rules, provided a crucial constraint on the possible configurations of DNA. Watson and Crick realized that A and T must pair with each other, and G and C must pair with each other, to satisfy these rules. This pairing scheme also explained the constant diameter of the DNA molecule.
These two pieces of information, along with their own extensive model building and analysis, allowed Watson and Crick to correctly propose the double-helical structure of DNA in 1953.
