ps3s_05

ps3s_05 - MIT Department of Biology 7.014 Introductory...

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MIT Department of Biology 7.014 Introductory Biology, Spring 2005 7.014 Problem Set 3 Solutions Question 1 After acing the 7.014 Quiz 1, you take a well-deserved break and go “looking for Baker House.” Somewhere in the tunnels you stumble on a device you have never seen before, and start playing with its dials. It turns out to be a time- and reality-transporting device. It lands you in the office of the editor of the journal Nature in February of 1953. This is a reality much like our own, except that in this reality no one has yet seen Rosalind Franklin’s data. The editor is happy to see you, since he believes the knowledge you acquired in 7.014 will help him with the decision he must make. In front of him are four papers proposing various models for the structure of DNA. For each model below, indicate whether the editor should accept or reject the paper (i.e. whether the model is plausible or not) and explain your advice. If your advice is based on data the editor has not yet seen, explain what the data is and how it will affect the plausibility of the model. If your advice is based on data the editor is familiar with, explain how that data is either consistent or inconsistent with the model. i. Model A: DNA is a double-stranded helix with sugar-phosphate backbones in the center, and bases sticking out into solution. In this model, the strands are running anti-parallel to each other. This model should be rejected. DNA backbone is an alternating sugar-phosphate polymer. Phosphates are negatively charged. This model (originally favored by James Watson) would put these negative charges in close proximity. Putting negative charges that close together would be very energetically unfavorable. ii. Model B: DNA is a four-stranded helix with bases looking inwards. The model claims to be based on the current crystallographic data. This model should be rejected. While this model might be consistent with all data available to the editor, it is inconsistent with Rosalind Franklin’s data. In particular, she was able to figure out that DNA adapts two forms in solution and to isolate and crystallize molecules of a unique form. It is that data that indicated to Crick that DNA was a double helix with matched bases on the inside of the molecule. iii. Model C: DNA is a double-stranded helix with sugar-phosphate backbones on the outside, and bases in the middle, where purines (A and G) pair with purines and pyrimidines (T and C) pair with pyrimidines. The strands are running anti-parallel to each other. This model should be rejected. This design does not assure compliance with Chargaff rules. In addition,
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This note was uploaded on 11/11/2011 for the course BIO 7.012 taught by Professor Lander during the Fall '10 term at MIT.

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ps3s_05 - MIT Department of Biology 7.014 Introductory...

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