Examples of scientific method
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Here is an annotated example of this scientific method example titled Microbial Genes in the Human Genome: Lateral Transfer or Gene Loss?.
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[edit] DNA example
- Each element of scientific method is illustrated below by an example from the discovery of the structure of DNA:
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- DNA-characterizations: in this case, although the significance of the gene had been established, the mechanism was unclear to anyone, as of 1950.
- DNA-hypotheses: Crick and Watson hypothesized that the gene had a physical basis - it was helical.
- DNA-predictions: from earlier work on tobacco mosaic virus, Watson was aware of the significance of Crick's formulation of the transform of a helix.[1] Thus he was primed for the significance of the X-shape in photo 51.
- DNA-experiments: Watson sees photo 51.
- The examples are continued in "Evaluations and iterations" with DNA-iterations.
[edit] DNA-characterizations
- The history of the discovery of the structure of DNA is a classic example of the elements of scientific method: in 1950 it was known that genetic inheritance had a mathematical description, starting with the studies of Gregor Mendel. But the mechanism of the gene was unclear. Researchers in Bragg's laboratory at Cambridge University made X-ray diffraction pictures of various molecules, starting with crystals of salt, and proceeding to more complicated substances. Using clues which were painstakingly assembled over the course of decades, beginning with its chemical composition, it was determined that it should be possible to characterize the physical structure of DNA, and the X-ray images would be the vehicle. ..2. DNA-hypotheses
[edit] DNA-hypotheses
- Linus Pauling proposed that DNA was a triple helix. Francis Crick and James Watson learned of Pauling's hypothesis, understood from existing data that Pauling was wrong and realized that Pauling would soon realize his mistake. So the race was on to figure out the correct structure. Except that Pauling did not realize at the time that he was in a race! ..3. DNA-predictions
[edit] DNA-predictions
- When Watson and Crick hypothesized that DNA was a double helix, Francis Crick predicted that an X-ray diffraction image of DNA would show an X-shape. Also in their first paper they predicted that the double helix structure that they discovered would prove important in biology, writing "It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material". ..4. DNA-experiments
[edit] DNA-experiments
- Before proposing their model Watson and Crick had previously seen x-ray diffraction images by Rosalind Franklin, Maurice Wilkins, and Raymond Gosling. However, they later reported that Franklin initially rebuffed their suggestion that DNA might be a double helix. Franklin had immediately spotted flaws in the initial hypotheses about the structure of DNA by Watson and Crick. The X-shape in X-ray images helped confirm the helical structure of DNA[2]. ..1. DNA-characterizations
[edit] DNA-iterations
- After considerable fruitless experimentation, being discouraged by their superior from continuing, and numerous false starts, Watson and Crick were able to infer the essential structure of DNA by concrete modeling of the physical shapes of the nucleotides which comprise it. They were guided by the bond lengths which had been deduced by Linus Pauling and Rosalind Franklin's X-ray diffraction images. ..DNA Example
[edit] References
- ^ Cochran W, Crick FHC and Vand V. (1952) "The Structure of Synthetic Polypeptides. I. The Transform of Atoms on a Helix", Acta Cryst., 5, 581-586.
- ^ "The instant I saw the picture my mouth fell open and my pulse began to race." -- James D. Watson (1968), The Double Helix, page 167. New York: Atheneum, Library of Congress card number 68-16217. Page 168 shows the X-shaped pattern of the B-form of DNA, clearly indicating crucial details of its helical structure to Watson and Crick.
