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Name: jcowan
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Date: Around 1993

We have been studying DNA. If no one has ever seen a sequence of nucleations, how do scientists really know they are there?

What it boils down to is that it is the best explanation for the evidence. That evidence includes electron micrographs of DNA. Even with this, DNA only looks like a bumpy line. There is more though. Remember that sequences of nucleations include both DNA and RNA. When Watson and Crick came up with their model they had to explain the evidence that was available. This included knowing what chromosomes looked like, that there are only 4 types of nucleations in DNA/RNA, that there must be a way for the material to reproduce itself. With balls and sticks they could build models of what the nucleation look like and the position of the most likely areas for bonding between them. The sequential model makes sense of the data. Once the hypothesis was formed they were able to predict that there is cellular machinery which does the work of reproduction. Much of that protein machinery has been discovered and described, this also provides more evidence that the theory is correct.


After scientists knew for sure that DNA played a central role in heredity, some set out to determine the exact structure of DNA. In 1953, James Watson and Francis Crick succeeded in finding a reasonable DNA structure. They worked from two kinds of clues:

1) X-Ray diffraction data, supplied by Maurice Wilkins. Wilkins fired X-rays at DNA fibers, and recorded the way the fibers scattered (or diffracted) the X-rays onto photographic film. The resulting photos looked like a wild scattering of spots. The angles and positions of the spots gives information about the position of atoms or group of atoms in the DNA. This is a very difficult experiment and the interpretation of the spot scatters requires extensive calculations and analysis. The data suggested that DNA is long, skinny, has two similar parts running parallel to one another, and helical, as well as other clues. But it was Watson and Crick who pieced all the information together like a puzzle to suggest a three dimensional model.

2) Chargaff's observations that the total amount of pyrimidine nucleations (T & C) always equalled the amount of purine nucleations (A & G) and that the amount of T = A, and G = C, but that the amount of T + A does not necessarily equal G + C. So, like good detectives, Watson and Crick put all the pieces together. In a way, we really have "seen" nucleations!


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