For many years only nuclear DNA was studied, until a researcher, Anderson, published in 1981 the complete sequence of the mitochondrial DNA. This DNA, coming from the organelle responsible for cellular respiration, possesses no link with nuclear DNA. While less discriminating in the analysis of STRs than nuclear DNA, the characterization of mitochondrial DNA represents important progress in identification for legal medicine.
Specific characteristics of mitochondrial DNA
Listed below are the specific characteristics of mitochondrial DNA.
1. The mitochondrial genome is completely different from nuclear DNA. Compared with two times three billion (3 x 109) nucleotides assembled into 23 pairs of linear molecules contained in the nucleus, a mitochondrion has a single circular DNA molecule with just 16 569 nucleotides.
2. In the cell where the nuclear DNA is represented by a unique copy (or two for diploid cells), there exist several tens (or several hundreds) of copies of mitochondria. An enrichment is naturally performed and supports later amplifications.
3. Mitochondria are extremely solid organelles, which can resist extreme conditions that would destroy the nucleus.
4. Mitochondria have variable zones, which arc points of mutations at one or several nucleotides. The variation is, however, less informative than observations on nuclear DNA.
5. Mitochondria being transmitted by females, it is simple to identify a cadaver using any relative, provided there is a maternal link (mother, sister, etc.). A contrario, unknown DNA found at a crime scene will implicate all individuals in the same maternal lineage (brothers, maternal cousins, etc.).
Utility of mitochondrial DNA
The enormous advantage of nuclear DNA is its large power for discrimination. With current technology, the frequency of a genetic profile in the general population is so low as to render all discussion useless. However, there exist a certain number of cases where nuclear DNA is not accessible, or in such low quantities that it is very difficult to characterize (identification of a body, characterization of excrement or hair).