Complex Mitochondrial DNA

Mitochondrial DNA differs from nuclear and bacterial DNA in that it can be isolated as whole molecules with a uniform molecular weight and buoyant density. It presumably represents a unique genome and in this respect is similar to viral DNA. Mitochondrial DNA also has the same structure as some viral and episomal DNA, namely the closed circular structure. This structure results in certain physical properties of the DNA in solution and places certain restrictions on the biochemistry of the DNA in the cell. The special physical properties of closed circular DNA arise from the topological restriction that any change in the average pitch of the duplex helix must result in a change in the tertiary structure. Biochemical restrictions result from the fact that the two strands of the molecule are topologically bonded. Semiconservative replication of closed circular DNA requires particular DNA species and enzymatic activities, such as nicking enzymes and...