. Biophysics: concepts and mechanisms. Biophysics. PRIMARY EFFECTS (ON CHEMICAL COMPOSITION) 243 beings without prior knowledge of the genetic result. Butler et al (1959) have partially clarified a rather confused picture, made not the least bit simple by the fact that the molecule is huge: as obtained from leucocytes it has a molecular weight of about five million. Two standard methods of determin- ing molecular weight (also outlined in Chapter 6) were used. One, by meas- urement of the viscosity of DNA solutions and measurement of the speed with which the molecules settle out in a high-speed
. Biophysics: concepts and mechanisms. Biophysics. PRIMARY EFFECTS (ON CHEMICAL COMPOSITION) 243 beings without prior knowledge of the genetic result. Butler et al (1959) have partially clarified a rather confused picture, made not the least bit simple by the fact that the molecule is huge: as obtained from leucocytes it has a molecular weight of about five million. Two standard methods of determin- ing molecular weight (also outlined in Chapter 6) were used. One, by meas- urement of the viscosity of DNA solutions and measurement of the speed with which the molecules settle out in a high-speed centrifuge, showed that the molecular weight falls during irradiation, as though the big molecule were being split into pieces. The other, however, by light-scattering tech- niques, gave a constant molecular weight during irradiation. The implica- tion is that the molecule is broken all right, but the pieces do not completely uncoil. With such a loosened structure, easier degradation by heat should result, and that is just what has been found, not only for DNA but also for several enzymes as well. loss of NH3 and loss of H2 rupture of hydrogen bonds rupture of sugar-base linkage and oxidation of the sugar iberation of purine bases breakdown of pyrimidine* bases rupture of nucleotide chain liberation of organic phosphate. Nucleic acid Figure 9-6. Things That Can Happen if a Macromolecule Such as DNA is Irradiated by Ionizing Radiation. * Order of radioresistance: adenine > guanine » cytosine > uracil > thymine (on iso- lated components). Enzymes are known to have various sensitivities to radiations, at least in dilute solutions. The data on these are somewhat suspect because of the marked effect of impurities. However, one of the most sensitive enzymes seems to be carboxypeptidase; ribonuclease is ten times more resistant, and catalase ten times more resistant yet. Some enzymes are inactivated even when in the dry, crystalline state (this supports the target theorists
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