Electron microscopy of DNA: determination of absolute molecular weights and linear density

The molecular lengths of several phage DNA's and one plasmid DNA have been measured and the molecular linear density of double stranded DNA under standard conditions has been determined. This determination was possible since the absolute molecular weight for phi X 174 DNA has become known from sequence work. RF DNA of phi X 174 phage consists of 5375 +/- 20 basepairs equivalent to a molecular weight of (3.558 +/- 0.013) x 10(6) dalton and since the length of this DNA has been determined to be 1.71 +/- 0.02 micrometer the molecular linear density of double stranded DNA prepared for electron microscopy under the conditions described is (2.08 +/- 0.03) x 10(6) dalton micrometer-1. These data have been used to determine the molecular weights of several phage DNA's and of the plasmid DNA PML 21. The latter DNA exhibits a remarkable heterogeneity with respect to its size.