Rearrangement of mitochondrial DNA molecules during the differentiation of mitochondria in yeast. I.-Electron microscopic studies of size and shape.

Size and shape of purified mitochondrial DNA was analyzed by electron microscopy as a function of mitochondrial differentiation. The mitochondrial DNA was extracted at fourth growth stages corresponding to different steps of mitochondria repression and depression. It was heterogeneous both in form and length. The size of linear molecules ranged from 1 mu to 25 mu but most of the molecules could be assigned into four Gaussian subpopulations with mean lengths of 2.2 mu to 4.0 mu, 6.0 mu and 10.0 mu. The circular molecules were all open and sized varied from 0.5 mu to 10 mu. Their length repartition was congruent with a logarithmic Gaussian distribution. The relative proportion of the different classes of molecules changed according to the stage of the growth cycle: during the repression most of the mitochondrial DNA molecules were short: the population of 2.2 mu was predominant. The longest linear molecules were observed during derepression where the populations of 4.0 mu and 10.0 mu were only found as well as the highest proportion of circular molecules. At the stationary phase the mitochondrial DNA became short again and the circles disappeared completely. The mitochondrial DNA extracted from a cytoplasmic "petite" was composed of linear and circular molecules. The linear molecules ranged from 0.1 mu to 32 mu and most of them could be assigned to two subpopulations of 1.3 mu and 4.2 mu. The circular molecules which accounted for 11 percent had contour lengths of 0.7 mu and 1.5 mu. The physiological meaning of the change in the relative proportion of different classes of mitochondrial DNA is discussed.