摘要：

In a recent paper from this Laboratory dealing with structural models for deoxyribonucleic acid (DNA) with particular emphasis on the two strand model proposed recently by Watson and Crick it was pointed out that a detailed study of the kinetics of the enzymatic digestion of DNA might yield information about different models. For example a one strand structure would be split at each attack by the enzyme. A two strand structure would require at least two attacks and therefore the efficiency of splitting would increase with the square of the number of attacks. A two strand structure with some "preformed gaps," a model proposed in the above mentioned paper, would degrade in a manner intermediate between a single strand model and a continuous two strand model. This communication presents a detailed theory for the degradation of such long chain macromolecules. From the distribution of fragments formed upon random hydrolysis the decrease in the viscosity of such a solution is calculated. The calculation is generalized to include solutions of polymers each molecule of which contains a definite number of chains bound side by side. Certain parts of this treatment are general for molecules polydisperse with respect to both molecular weight and shape. This communication also presents new data on the viscosity and sedimentation changes as a function of the number of enzymatic attacks on the DNA macromolecule. The number of enzymatic attacks is determined in a "pH-stat" by measuring the hydrogen ion liberation resulting from the cleavage of the phosphodiester bonds. The theory developed may be applied to the experimental data, and thus the single strand model for the DNA molecule is ruled out. The best agreement is obtained with a doubly stranded model. Moreover the number of preformed gaps in the structure as determined by this study is less than one in about 3000 nucleotides.

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