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, James Watson and Francis Crick were two scientists working at the Cavendish Laboratory in Cambridge, UK. Their primary goal was to understand the structure of DNA using evidence from various sources.

One critical piece of evidence available to Watson and Crick was the X-ray crystallography data of DNA fibers, particularly the images produced by Rosalind Franklin and Maurice Wilkins. The most famous of these images, Photo 51, showed a clear X-shape pattern. This pattern suggested that DNA had a helical structure.

Erwin Chargaff's work on DNA base composition was another crucial piece of evidence that Watson and Crick used. Chargaff found that the amount of adenine (A) was equal to the amount of thymine (T), and the amount of guanine (G) was equal to cytosine (C). These findings became known as Chargaff's rules and hinted at the possible base-pairing within DNA.

Although Linus Pauling's proposed model of DNA was incorrect, the success of Pauling's alpha-helix model for proteins inspired Watson and Crick in their search for the DNA structure. Pauling's work showed that applying the principles of chemistry and understanding molecular shapes could lead to accurate models of complex molecules.

Using these pieces of evidence, Watson and Crick began to explore possible DNA structures. Their approach can be distilled into three main activities: 1. Building physical models: Watson and Crick used models made of metal rods and wires to represent atoms and chemical bonds. This hands-on process allowed them to quickly test different configurations and evaluate structural compatibility with existing experimental data. 2. Determining the helical structure: Using X-ray crystallography data, Watson and Crick were able to establish that DNA had a helical structure. They deduced that the helix was a double helix with two intertwined strands running in opposite directions, also known as antiparallel strands. 3. Base pairing and pairing rules: Watson and Crick recognized that the Chargaff's rules suggested specific base-pairing within DNA. Eventually, they concluded that adenine (A) forms hydrogen bonds with thymine (T), and guanine (G) forms hydrogen bonds with cytosine (C). This complementary base pairing provided a possible mechanism for DNA replication and encoding genetic information. By synthesizing existing evidence from various sources and experimenting with physical models, Watson and Crick successfully proposed the double-helix model of DNA structure that is now widely accepted.