The Cytoplasmic DNAs of Cultured Human Cells

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Abstract
Ethidium bromide at a concentration of 1 μg × ml−1 in the medium completely inhibits the synthesis of mitochondrial DNA (ϱ= 1.706 g×ml−1) in exponentially growing Chang liver cells (ATCC; CCL 13), as indicated by the fact that labeled thymidine is not incorporated into any of the three forms of mitochondrial DNA (closed circular, open circular and linear DNA). Existing mitochondrial DNA undergoes a structural change under these growth conditions. At first closed circular DNA with a higher super‐helix density is generated. After prolonged exposure of the cells to the drug (48 h) no remaining closed circular DNA is detectable in the mitochondria. The bulk of the remaining mitochondrial DNA behaves during sedimentation like a linear DNA molecule with the length of the circumference of the circular mitochondrial DNA. Removal of the drug from the growth medium reverses this process of interconversion of the different forms of mitochondrial DNA.No such drastic effects of ethidium bromide on the light cytoplasmic DNA (ϱ= 1.688 g × ml−1) have been observed.