Mitochondrial DNA expression

Abstract In vertebrates, the mitochondrial genome consists of double-stranded, covalently-closed DNA molecules that are present as multiple copies per mitochondrion. The two strands of DNA can be distinguished on the basis of G + T base composition, which results in different buoyant densities of each strand (‘heavy’ and ‘light’) in an isopycnic gradient. In metabolically active vertebrate cells, a ∼1-kb non-coding region of a large fraction of the mitochondrial DNA (mtDNA) molecules contains a triplex structure, in which a short (7S) DNA strand complementary to the light (L) strand displaces the heavy (H) strand. This region, which is flanked by the genes for the transfer RNAs (tRNAs) tRNAPhe and tRNAPro, is called the dis- placement loop or D-loop region and is the least conserved part of mitochondrial genomes.