AbstractTetrahymena pyriformisST (3 × 104cells/ml) was treated with 0.1 mg/ml chloramphenicol (CAP). Cell division ceased after 1.5 divisions with no decreased viability. Total mitochondrial volume and succinic dehydrogenase (SDH) activity/liter increased 1.7‐fold and 3‐fold, respectively. SDH activity/cell decreased whereas malate dehydrogenase activity/cell and respiratory control ratios and P: O ratios of isolated mitochondria were unchanged in treated cells. During 12 hours of growth in CAP the total surface area of mitochondrial inner and outer membrane was essentially unchanged or increased 4‐fold, respectively. Mitochondria from cells treated with chloramphenicol had decreased size, buoyant density and protein: lipid ratio in the membranes. The membrane ubiquinone: protein ratio was unchanged.Tetrahymenacells contained 3.6 × 10−12g of mitochondrial DNA and 6,800 mitochondria in a volume of 41,000 μ3. A 4‐hour treatment with CAP caused a 4‐fold increase in the number of mitochondria/cell and a 10‐fold increase in mitochondria/liter in contrast to a 4‐fold increase in number of mitochondria/liter in control cells. Thus CAP stimulated division of mitochondria. Individual mitochondria of treated cells had one‐tenth the volume of control mitochondria. The rate of increase of mitochondrial DNA/liter was the same in control and CAP‐treated cultures. The amount of DNA/mitochondrion decreased 75 in CAP‐treated cells due to the rapid division of mitochondria. The cell volume, cell protein content and mitochondrial DNA content/cell decreased with gr
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