A Study of DNA Denaturation in the U1 t racent rif uge

The melting transition of DNA in alkaline CsCl can be followed in the analytical ultra-centrifuge. Equilibrium partially denatured states can be observed. These partially dena-tured DNA bands have bandwidths of up to several times those of native DNA. Less stable molecules melt early and are found a t heavier densities in the melting region. An idealized ultracentrifuge melting transition is described. The melting transition of singly nicked PM-2 DNA resembles the idealized curve. The DNA profile is a Gaussian band a t all points in the melt. DNA's from mouse, D. melanogaster, M. lysodeikticus, T4, and T7 also show equilibrium bands a t partially denatured densities, some of which are highly asymmetric. Simple sequence satellite DNA shows an all-or-none transition with no equi-librium bands a t partially denatured densities. The temperature a t which a DNA denatures is an increasing function of the (G + C) content of the DNA. The T, does not show a molecular-weight dependence in the range 1.2 X 106-1.5 X loi daltons (single strand) for mouse, M. lysodeikticus, or T4 DNA. The mouse DNA partially denatured bands do not change shape as a function of molecular weight. The T4 DNA intermediate band develops a late-melting tail a t low molecular weight. M. lysodeikticu