Because mitochondria have their own DNA, separate from nuclear DNA, one can speak of a “mitochondrial genome,” all the DNA contained in a mitochondrion. Cells have many mitochondria, so there are multiple copies of mtDNA in every cell. The MtDNA genome is significantly smaller than the nuclear genome. The nucleus has about 3.5 billion base pairs in its DNA; the mitochondrion has about 16,500 base pairs in its DNA.
All the variability between people that is forensically useful can essentially be seen by looking through a couple of short sequences in the hypervariable regions. In practice, forensic scientists copy the sequences of interest using the polymerase chain reaction and then analyze the copies. Unlike nuclear DNA typing, which consists of determining the sizes of tandem-repeat regions, mtDNA analysis is sequencing. The variability from person to person in mtDNA consists of a few variations in the base sequence. Thus, the PCR products (the copies) from the hypervariable regions have to be sequenced (the order of the bases in the DNA strand has to be determined).
