Size Distribution of Circular DNA from Petite‐Mutant Yeast Lacking ρDNA

The size distribution of circular DNA, of buoyant density 1.701 g/cm3, from a petite mutant yeast lacking DNA of buoyant density 1.684 g/cm3 (ϱDNA) has been studied by sucrose gradient analysis and electron microscopy. The DNA prepared from subcellular particles fractionated on a sucrose gradient, is found to be heterodisperse but with a major component having a length of 1.9 μm, a molecular weight of 4.02 × 106 and sedimenting as a supercoil at 22S and in the relaxed form at 15S. Minor components having lengths of 2.8, 3.9 and 5.8 μm and corresponding molecular weights of 5.71 × 106, 8.09 × 106 and 12.4 × 106 have also been observed. The significance of circular · DNA in relation to current ideas on the export of nuclear DNA to the cytoplasm is considered.