Elongation of displacement-loop strands in human and mouse mitochondrial DNA is arrested near specific template sequences.

Animal mitochondrial DNA (mtDNA) maintains a displacement loop (D loop) at the heavy strand origin of replication. These D loops represent sharply limited synthesis of heavy strands and provide a unique opportunity to examine the termination of DNA synthesis. Direct sizing at the nucleotide level indicates that the 3' ends of D-loop strands of human and mouse mtDNA are discrete and map within three to five nucleotides on the complementary template strand.

Microheterogeneity detected in circular dimer mitochondrial DNA.

Exhaustive EcoRI digests of circular dimer mitochondrial DNA (mtDNA) from mouse cell lines LD and LDTK- yield two major fragments whose average lengths are slightly smaller than the corresponding fragments of circular monomer mtDNA from mouse LA9 and LMTK- cells. A third fragment approximately 400 nucleotide pairs in length is frequently produced in less than molar yield. Exhaustive EcoRI digests of circular dimer mtDNA from human acute myelogenous leukemic leucocytes yield three major fragments.

Absence of a pyrimidine dimer repair mechanism for mitochondrial DNA in mouse and human cells.

We have assayed the ability of mammalian cells to remove pyrimidine dimers from their mitochondrial DNA. The results indicate that pyrimidine dimers are not repaired for as long as 48 hr after UV irradiation. Furthermore, molecules containing pyrimidine dimers are unable to replicate and are simply diluted out in subsequent cell divisions.

The absence of a pyrimidine dimer repair mechanism in mammalian mitochondria.

We have investigated whether mammalian cells can repair pyrimidine dimers in their mitochondrial DNA which have been induced by ultraviolet light. The assay system is based upon the ability of the phage T4 UV endonuclease to nick covalently closed circular mitochondrial DNA that contain pyrimidine dimers. Our results show that dimers are not removed from the mitochondrial DNA of mouse L cells or human KB and HeLa cells.