Like most bacterial genomes, the human mitochondrial genome consists of a single type of circular, double-stranded DNA. Its sequence of 16,569 nucleotides was first reported by Fred Sanger and colleagues at Cambridge, UK, in 1981, almost a decade before the Human Genome Project began. A revision of the sequence, published in 1999, provided the revised Cambridge reference sequence (GenBank: NC_012920). This tiny genome— less than 0.4% of the size of the E. coli genome (and even less than 10% of the size of the human Epstein–Barr virus genome)—resembles bacterial genomes in being packed with genes. There are no introns, and significantly more than 90% of the genome directly specifies a protein or functional RNA.
Whereas the nuclear DNA is tightly complexed with proteins that dictate its con formation, mitochondrial DNA is, like bacterial DNA, comparatively protein-free. (It is, however, complexed with certain proteins to form nucleoids, nucleoprotein structures that have a diameter of around 100 nm). The two strands of mitochondrial DNA are transcribed to give long RNA transcripts that, like bacterial polycistronic RNAs, are cleaved to generate individual functional mRNAs and noncoding RNAs. And, as described above, the mitochondrial mRNAs are translated on mitochondrial ribosomes using mitochondrial tRNAs.
These and other observations support the endosymbiont hypothesis to explain the origin of eukaryote cell lineages by a cell-fusion event. In its modern form, it envisages that an aerobic α-proteobacterium was engulfed by a larger eukaryotic precursor cell, almost certainly a type of archaeon (see the cell evolution description in Section 2.1 for further details). The resulting single cell had, therefore, two genomes and two sets of protein-synthesis machinery. The genome of the engulfed α-proteobacterium progressively shed DNA sequences, becoming much smaller in size, eventually giving rise to the mitochondrial genomes of eukaryotic cells. Many of the shed DNA sequences integrated into the genome of the engulfing cell. The latter genome also increased in size during evolution by undergoing various types of DNA duplication, and gave rise to the large nuclear genomes of eukaryotic cells.
Replication and transmission of mtDNA 16S The replication of both the heavy (H) and light (L) strands of mtDNA is unidirectional and starts at specific origins (Figure1). Although mtDNA is principally double stranded, repeat synthesis of a small segment of the H-strand DNA produces a short third DNA strand called 7S DNA. The 7S DNA strand can base-pair with the L-strand, displacing the H-strand, which forms a loop, the displacement loop (D-loop). This region contains many of the mtDNA control sequences (including the major promoter regions) and so it is referred to as the CR/D-loop region (CR = control region). The origin of replication for the H-strand lies in the CR/D-loop region, and that of the L-strand is sandwiched between two tRNA genes some distance from the control region.
Fig1. Organization of the human mitochondrial genome. The circular 16,569 bp genome has a base composition of 44% GC, but has a heavy (H) strand rich in guanines, and a light (L) strand rich in cytosines. Twenty-four RNA genes make 12S and 16S rRNAs and 22 tRNAs (tRNA genes are shown as thin red bars with a letter indicating the amino acid; note there are two tRNALeu genes, L1 and L2 , and two tRNASer genes, S1 and S2 ). Thirteen protein-coding genes make components of the oxidative phosphorylation system: seven NADH dehydrogenase subunits (ND1–ND6 and ND4L), two ATP synthase subunits (ATP6 and ATP8), three cytochrome c oxidase subunits (CO1–CO3), and cytochrome b (CYB). Two promoters, green boxes labeled PH (in two segments) and PL , transcribe respectively the H- and L-strands in opposite directions, generating large multigenic transcripts from each strand that are subsequently cleaved. OH and OL (purple boxes) signify replication origins (dashed arrows mark the direction of DNA synthesis). A roughly 500 bp control region (CR), commencing at position 16024, has multiple regulatory sequences. It has an internal displacement loop (D-loop), a triple-stranded structure formed by repeat synthesis of a short piece of heavy-strand DNA, 7S DNA, that base pairs with the L-strand, causing local looping of the H-strand. Gene symbols for protein coding genes are abbreviated by omitting the prefix MT-. For further information, see the MITOMAP database at http:// www. mitomap.org.
Unlike chromosomal DNA molecules that are normally subject to strict control of copy number, mtDNA replication is more flexible and varies between cells. According to the cell type, between 1000 and 10,000 mtDNA copies are found within the inner mitochondrial compartment (matrix), but oocytes are exceptional in having about 100,000 mtDNA copies. During mitotic cell division, the multiple mtDNA molecules in a dividing cell segregate in a purely random way to the two daughter cells.
During zygote formation, a sperm cell contributes its nuclear genome, but not its mitochondrial genome, to the egg cell. Originating exclusively from the unfertilized egg, the mitochondrial DNA of the zygote is maternally inherited: males and females both inherit their mitochondria from their mother, but males do not transmit mitochondrial DNA to subsequent generations.
