Konfiguration und längenverteilung von DNA-molekülen in lösung

Abstract

A diffusion method is described for visualization of DNA molecules which is equivalent to the spreading of a mixed DNA-protein solution. Macromolecules diffusing in a solution of about 5×10⁻⁸ g DNA per ml are adsorbed by a cytochrome c-film at the air-water interface. Subsequently they are transferred to electron microscopic supports. In the enlarged two-dimensional image the end-to-end distances and the length distribution are measured for different DNA preparations (from cod fish, trout, holothuria and T₂ phage). Following recalculation in the three-dimensional state a relation is given for the mean square of the end-to-end distances, $\text{h}{}^2$, and the length, L, as $\text{(}\text{h}{}^2\text{)}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{= 1.3 × 10}{}^{\mathrm{-1}}\text{L(1 + √}\text{1 + 2.3 × 10}{}^{\mathrm{-3}}\text{L}$ at ionic strengths from 0.09 to 0.35, according to a theory by Katchalsky and Lifson for diluted linear polyelectrolytes. The molecular shape is not a random coil or a rod but intermediate on account of electrostatic interaction. An application of the results on the theory of Kuhn, Kuhn and Buchner yields equations for the diffusion coefficient or other characteristics of DNA as a function of length or molecular weight. The length distribution of DNA, after disintegration by preparation and storage, usually has the form of a descending exponential function. Within the accuracy of the length distribution it is allowed to deduce random scissions along the native DNA molecule.