Thirteen of the ~80 different proteins present in the respiratory chain of human mitochondria are encoded by the mitochondrial genome (mtDNA). The circular mtDNA, which is present in 1000 to 10000 copies in the human cell, also encodes for 2 ribosomal RNAs, and 22 transfer RNAs. The double-stranded mitochondrial genome lacks introns and the longer non-coding region contains the control elements for transcription and replication of mtDNA (Shadel and Clayton, 1997). The two mtDNA strands are referred to as the heavy (H-strand) and the light (L-strand) due to their differing G+T content. In human cells, each strand contains one single promoter for transcriptional initiation, the light-strand promoter (LSP) or the heavy-strand promoter (HSP). Transcription from the mitochondrial promoters produce polycistronic precursor RNA encompassing all the genetic information encoded in each of the specific strands. The primary transcripts are processed to produce the individual tRNA and mRNA molecules (Clayton, 1991; Ojala et al., 1981). There is likely a second initiation site for heavy strand transcription, which produces RNAs spanning the rDNA region. The resulting transcript including the genes for the two mitochondrial rRNAs and ends at the boundary between the 16 S rRNA and the tRNALeu(UUR) genes (Montoya et al., 1982; Montoya et al.,1983; Christianson and Clayton 1986). The existence of such a separate transcription unit may explain why the steady-state levels of rRNAs are much higher than the steady state levels of mRNAs.