Evidence for semi-conservative replication of mitochondrial DNA from Paramecium aurelia

The mode of replication of mitochondrial DNA in Paramecium aurelia was studied using 5-bromouracil incorporation. Density gradient analysis showed that 5-BrUra-substituted mitochondrial DNA had a density of 1.702 g/cm³, corresponding to 4% substitution in the duplex. Analysis of monomer length molecules revealed that these consisted of a mixture of normal density and 5-BrUra-substituted DNA, whereas dimer length molecules consisted of only 5-BrUra-substituted DNA. Analysis of denatured, 5-BrUra-substituted DNA indicated that the 5-BrUra was contained in just one strand, while the other strand had the density expected of normal mitochondrial DNA. It was concluded that the linear molecules from mitochondrial DNA of P. aurelia replicate via a semi-conservative mode of replication.