The Discovery of DNA’s Molecular Structure

Theoretical models for the molecular structure of DNA can be likened to scientific theories. DNA’s structure was determined largely because scientists scrutinized the relationship between theory (a particular theoretical model of DNA) and observation (x-ray crystallographic patterns, or bonding patterns between bases and sugar-phosphate groups, for example). Inductivists, falsificationists, Kuhn, and Feyerabend all have different accounts of how scientists have related theory to observation. These accounts are important because, not only do they delineate frameworks scientists use to develop their theories, but because these frameworks subsequently became important in developing a theory for the molecular structure of DNA.

The inductivist account of science recognizes five steps that are essential to scientific progress, and consequently, the discovery of the molecular structure of DNA. First, scientists compile a large body of facts from observation and experiment. Using the principle of induction, these facts can, often with severe logical difficulties, be generalized to form the basis for a theory or law. Then, once a theory has been developed, scientists can use the theory as part of a valid logical argument to make new predictions or explanations of phenomena. According to Chalmers, the inductivist account has “a certain appeal” to it, namely, that all of scientific progress can be seen as the result of a linear, highly structured inductive scientific method (54). “Its attraction lies in the fact that it does seem to capture in a formal way some of the commonly held intuitions about the special characteristics of scientific knowledge, namely its objectivity, its reliability, and its usefulness” (57).

An inductivist account of the discovery of DNA’s molecular structure might proceed in the following way. First, early molecular biologists compiled a large body of facts from observation and experiment, such as Rosalind Franklin’s findings on the structure of DNA based on her x-ray crystallography work. From these facts, a theory of DNA structure was developed. Watson demonstrates, in The Double Helix how one aspect of DNA structure was determined from factual experimental observations. In the b-model of DNA,

…the meridional reflection at 3.4 A was much stronger than any other reflection. This could only mean that the 3.4 A-thick purine and pyrimidine bases were stacked on top of each other in a direction perpendicular to the helical axis. In addition we could feel sure from both electron-microscope and X-ray evidence that the helix diameter was about 20 A (110).