Ribonucleotides in closed circular mitochondrial DNA from HeLa cells

Closed circular mitochondrial DNA from HeLa cells is sensitive to both alkali and ribonucleases. The kinetics of ring opening in alkali suggest at least two classes of molecules. One class undergoes rapid breakdown, ultimately to fragments smaller than unit length, in contrast to the second class, which is more resistant to alkaline cleavage and is converted in large part to unit length single strands. Ribonucleases A, T₁ and H relax the supercoiled molecules, indicating that the alkali susceptibility is due to the presence of ribonucleotides in the DNA. By comparison with the rate of hydrolysis of RNA, the alkali-resistant class of mitochondrial DNA molecules is estimated to contain approximately 3 ribonucleotides and the alkalisensitive class 10–18.