Mitochondrial DNA
Mitochondrial DNA was discovered in the 1960s by Margit M. K. Nass and Sylvan Nass by electron microscopy as DNase-sensitive threads inside mitochondria, and by Ellen Haslbrunner, Hans Tuppy and Gottfried Schatz by biochemical assays on highly purified mitochondrial fractions.
Mitochondrial DNA
In human mitochondrial DNA (and probably in that of metazoans in general), 100–10,000 separate copies of mtDNA are usually present per somatic cell. (Egg and sperm cells are exceptions.) In mammals, each double-stranded circular mtDNA molecule consists of 15,000–17,000 base pairs. The two strands of mtDNA are differentiated by their nucleotide content, with a guanine-rich strand referred to as the heavy strand (or H-strand) and a cytosine-rich strand referred to as the light strand (or L-strand). However, confusion in the labeling of these strands is widespread; the confusion appears to have originated from the identification of the majority coding strand as the heavy in one influential article in 1999. The light strand encodes 28 genes and the heavy strand encodes 9 genes, for a total of 37 genes. Of these 37 genes, 13 are for proteins (polypeptides), 22 are for transfer RNA (tRNA), and two are for the small and large subunits of ribosomal RNA (rRNA). The human mitogenome contains overlapping genes (ATP8 and ATP6 as well as ND4L and ND4: see the human mitochondrial genome map), a feature that is rare in animals' nuclear genomes. The 37-gene pattern is also seen in most metazoans, although the mtDNA size range is greater and in some cases one or more of these genes is absent.