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Part 1: Crick and Watson

About the Program

Although deoxyribonucleic acid, or DNA, was discovered in the late 1860s, the substance was largely ignored for nearly a century because it seemed too simple to serve any significant purpose. This view changed dramatically in the 1940s. At this time scientists discovered that chromosomes, which were known to carry hereditary information, consisted of DNA and proteins. Experiments conducted throughout the 1940s showed that, contrary to the prevailing opinion that proteins carry the genetic information, DNA actually seemed to be the genetic material. However, it was still not known what the structure of DNA was, and how such a simple molecule could contain all the information needed to produce a human being or other living organisms.

This program examines the celebrated partnership between James Watson and Francis Crick, a postdoctoral fellow and a graduate student, who together managed to solve the mystery of DNA structure. The partnership began when Watson, an American, took a research position at Cambridge University in England in 1951. Crick was also at Cambridge, studying protein structure with a technique called Xray crystallography. By their own admission, both were more interested in the prevailing scientific problems of that day than in their own work, and the structure of DNA was definitely an interesting problem. Over the next few years, Watson and Crick would collect, by coincidence, hard work, and a little luck, key pieces of information that they would use to solve the DNA puzzle.

Crick and Watson already knew the main components of DNA, phosphates, sugars, and four nitrogenous bases: adenine, thymine, cytosine, and guanine. But how did the elements fit together? One clue came from Xray photographs of DNA taken by Rosalind Franklin. These photographs suggested that the structure of DNA was a helix. Another impetus came from Linus Pauling, who had already built a model of a helical protein. Watson and Crick were inspired by the fact that Pauling had used his imagination along with molecular models to deduce the structure of this protein. And they thought that if an eminent scientist such as Pauling could model a structure with little experimentation, then they might be able to do the same with DNA.

Using wire and pieces of metal, Crick and Watson began building scale models of DNA. After several embarrassing failures, the two men recognized another clue to the puzzle from biochemist Erwin Chargaff. Chargaff had found that the amounts of adenine and thymine were approximately equal and the amounts of guanine and cytosine were also approximately equal. This information helped solidify the idea that the bases might be paired in a specific way. What if DNA consisted of two strands of phosphates on the outside, and paired bases on the inside? At first, Watson imagined that the bases paired like with like, for example adenine with adenine, and cytosine with cytosine. But the resulting model did not fit the Xray data. Then Watson and Crick discovered that thymine and guanine could adopt a slightly different chemical shape, and that they had been trying to make the models from one version of the bases. Using the new forms, Watson discovered that he could make two base pairs, one consisting of adenine and thymine, and the other consisting of guanine and cytosine, that had exactly the same size. This discovery was the final key to the DNA structure.

Viewing Objectives:

  • To learn that collaboration as well as competition are important factors in a scientific endeavor.
  • To understand what compelled Watson and Crick to study DNA structure.
  • To recognize that Watson and Crick relied on information from several different sources to determine the structure of DNA.
  • To reinforce an understanding of the general structure of a DNA molecule.

Discussion Questions

  1. In the 1940s and early 1950s, identifying the chemical basis of heredity was a major problem in biology. Many scientists believed that proteins, not DNA, were the carriers of genetic information. What led them to this conclusion?
  2. When Watson and Crick first began to look for the structure of DNA, many other, more established scientists were trying to solve the same mystery. Why did Watson and Crick succeed in finding the structure ahead of these other scientists?
  3. How does the Watson and Crick model of DNA provide an explanation for genetic variability, Chargaff's rules, and DNA replication?
  4. What role did Rosalind Franklin play in the discovery of the double helix? Why was her role important? Why did she not receive a Nobel Prize with Watson, Crick, and Wilkins?

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