When people refer to the human genome, they are likely using the narrow sense of the word to describe the collection of human chromosomes, i.e. the nuclear genome. It can also be used more broadly to encompass all DNA found within an individual, including the "other" human genome contained within mitochondria.
The mitochondrial genome sequence has been around since the early days of sequencing. Fred Sanger and colleagues sequenced the mitochondrial genome in the late 70s and published the sequence, referred to as the "Cambridge Reference Sequence," in 1981.
A recent review article summarized several of the major differences between the nuclear genome and the mitochondrial genome.
|Characteristic||Nuclear Genome||Mitochondrial Genome|
|Size||~3.3 x 10^9 bp||16,569 bp|
|# DNA molecules per cell||23 in haploid cells; 46 in diploid cells||Several thousand copies per cell|
|# Genes Encoded||~20,000-30,000||37|
|Gene Density||~1 per 40,000 bp||1 per 450 bp|
|Introns||Frequent in most genes||Absent|
|%age of coding DNA||~3%||~93%|
|Mode of Inheritance||Mendelian inheritance for autosomes & X chromosome; Paternal for Y chromosome||Exclusively maternal|
Anderson, S., Bankier, A.T., Barrell, B.G., deBrujin, M.H.L., Coulson, A.R., Drouin, J., Eperon, I.C., Nierlich, D.P., Roe, B.A., Sanger, F., Schreier, P.H., Smith, A.J.H., Staden, R., Young, I.G. 1981. Sequence and Organization of the Human Mitochondrial Genome. Nature 290, 457-465.
R. Taylor and D. Turnbull. "Mitochondrial DNA Mutations in Human Disease." Nature Reviews Genetics. May 2005, p.389-402.