Watson and Crick (1953)

Awaiting details from Porter…

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We’ve seen Pauling and Corey’s competing view for the structure of DNA and now here’s Watson and Crick’s. The double helix model is vastly different in terms of bonding and overall structure from the model presented on Wednesday. While both models incorporate a repeating base/phosphate/sugar unit, they differ in fundamental ways. Why is this? Why is it the case that these two models are so different when they were published around the same time and even used the same technique (x-ray chrystallography)? On Friday we’ll be discussing what makes the double helix model so unique and what data lead Watson and Crick to this groundbreaking conclusion.

http://www.nature.com/scitable/content/ne0000/ne0000/ne0000/ne0000/13997975/Watson_171737a0.pdf#toolbar=0
Here’s an online version if you prefer that to your packet

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10 Responses to “Watson and Crick (1953)”

  1. Hunter Burgin Says:

    So we all know now that Watson and Crick were the victors in the great race to discover the chemical structure of nucleic acids, and specifically deoxyribonucleic acid. They correctly concluded that DNA was double stranded (and not triple stranded as some predicted) and that the two strands ran antiparallel to one another. My question is how exactly did they determine that the strands ran antiparallel? How could they ascertain this from an X-ray diffraction? What other experimental evidence/techniques did they use to correctly determine what they did regarding DNA?

  2. Daniel Begay Says:

    I enjoyed how they seem to put Pauling and Corey in the spotlight for being incorrect in there structure. A bold move.

    I’m curious to see if Pauling and Corey ever postulated that the purine groups and the pyrimidine group could come together by hydrogen bonding. It’s a pretty big deal alone that Watson and Crick were able to postulate this type of arrangement. I know that they were able to determine the ratio of purine and pyrimidine to be 1:1. What other techniques were used to solidify these findings?

    They even suggested that could be used in copying these ‘genetic material? Could this be leading into DNA repair?

  3. Kevin Greenwood Says:

    Working with the data available to them, all researchers at this point in time agree that the direction of twist in DNA is right-handed. The chirality of the sugars dictate this shape, giving it a smooth backbone. However, consideration of the left-handed version is noticeably absent. Is it possible to tell if DNA is left or right-handed with the x-ray diffraction techniques of the 1950s? In looking at Franklin’s B-DNA diffraction spectra, I don’t see how one could say one way or the other which direction the strands are twisting. We know now that in very salty conditions, B-DNA can flip to Z-DNA, and the first single crystal x-ray diffraction of DNA did show the Z-form (10.1038/nrg1115). It just seems strange to me that it isn’t mentioned, even to provide justification to ignore the possibility.

  4. Antony Says:

    An interesting thing to note was that they mentioned artificial neucleoprotiens. Were they creating artificial DNA at that time? If so how did they go about doing it without understanding the structure.

    Second on figure 5 they show only 2 H-bonds between guanine and cytosine. Now most textbooks show 3 H-bonds. Do you think they excluded the third for simplicity, out of symmetrical bias (cause adenine and thymine have 2), or is there data that they are going of on that suggest too far a distance for a third H-bond?

  5. Josh Ellsworth Says:

    I found the conjecture about the enol tautomers to be a little odd. Are they proposing tautomerization while the strands are in the double helix, or when they are replicating? It seems that in the absence of external influences neither scenario is likely, as the adenine and cytosine are not capable of undergoing a complementary tautomerization to maintain the Hydrogen bonding between their enol counterpart.

    I was also struck by Franklin’s mention of Gulland’s failed attempts to titrate the NH2 groups of the nucleotides. It highlights both the rush that Pauling must have been in to be first to publish and his hubris in ignoring published results that were over four years old and directly contradicted his assumptions about the activity of nucleic acid.

  6. Adam Settimo Says:

    Alright, this is more like it. Reading the Pauling paper i got the feeling that things were really rushed. When reading the background about him doing all the research and rushing to write the paper within such a short time, I felt like it showed in the writing; then again, maybe I’m just not able to interpret the genius that is Pauling. This paper however trickles through to the remnants of my previous biology classes. My question is in the sample preparation. I notice that there is no mention of uracil, and therefore no discussion of RNA. They obviously have a very clear sense of how DNA is replicated. Did they at the time know what the RNA was? Did they think it was just a broken off fragment? They do talk extensively about density, so did they separate by mass?

  7. Antony Says:

    It is interesting that immediately after they discover the shape of DNA the mechanism of copying is elucidated to them. They must have notice that one strand is the negative to the other. Another thing the noted is that they tried to create a double helix RNA but it didn’t work out since the oxygen in the ribose would cause “too close van der Waals contact”. What is exactly meant by that? Is it steric hindrance they are referring too. Does the fact that they cant form a double helix with RNA suggest that it is monomeric to them immediately?

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