Genetics : genetic information and the control of protein structure and function transactions of the first conference October 19, 20, 21, and 22, 1959, Princeton, N.J / edited by H. Eldon Sutton.

  • Macy Conference on Genetics 1959 : Princeton, N.J.)
Date:
[1960]
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Licence: Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)

Credit: Genetics : genetic information and the control of protein structure and function transactions of the first conference October 19, 20, 21, and 22, 1959, Princeton, N.J / edited by H. Eldon Sutton. Source: Wellcome Collection.

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    Рте Structure of Chrofnosornes, and Coding 39 about the nature of the chromosome by accepting as a tentative hypoth¬ esis the notion that the DNA of a chromosome is all one piece. Atwood: Do you mean by a chromosome, that which is in a phage particle ? Levinthal: No, by chromosome, I mean the same thing that you do. Atwood: Then, in that case, your view is quite untenable. Levinthal: I mean these little stainable objects in cells. I realize that this is taking an extreme position, and I would like to take this extreme position to see how easy it is to eliminate. Atwood: It is an impossible position. Levinthal: There are of course several ways in which one might hope to rule out such a hypothesis. One is by considering the fact that when¬ ever DNA is prepared free of contaminating material, it consists of pieces which are very much smaller than the amount in a chromosome. This observation seemed quite meaningful since pieces which were more or less of the same size were obtained. The estimates of the molecular weight of DNA have come from several sources. The DNA of the phages T2 and T4 have been studied by means of sedimentation ve¬ locity, sedimentation equilibrium, and a nuclear emulsion autoradio¬ graphic technique. The latter method, as used by Thomas and me (23), gave a molecular weight for one component which was of the order of 45 million, and in addition a number of smaller pieces were found. Mesel- son, Stahl, and Vinograd (28), using the sedimentation equilibrium method, found that all of the DNA was of the same size and of the order of 12 million molecular weight, and the sedimentation velocity measurement gave a sedimentation constant of approximately 30 S units ; all of the phage DNA seem to have this sedimentation constant. Using the formula which was discussed earlier in the Conference, we find this is also equivalent to a molecular weight of about 12 million. Thomas has suggested for some time that the lower values obtained in the ultracentrifuge might be a result of degradation of the DNA caused by velocity gradients in the fluid in which the material is sus¬ pended. One of the difficulties however was in trying to understand why this shear degradation would be introduced in some experiments and not in others. A possible answer to this question has been provided by Davison (6), who has shown that the velocity gradients produced when the material is introduced into the cell of the analytical centrifuge in the usual ways could be very high. Experimentally he has shown that the shears in the hypodermic needle used to load the cell of the analytical centrifuge were in fact high enough to degrade DNA. Because of the way in which the cells of the analytical centrifuge are