SFR Guidance > Mitochondrial DNA Analysis to determine species of origin

Guidance Notes: Mitochondrial DNA Analysis to determine species of origin

Mitochondria are small organelles present inside many types of cells in all multi-cellular organisms. They are the site of energy production and each cell can contain several thousand mitochondria. Each mitochondrion contains numerous copies of its own genetic material consisting of a small circular strand of DNA.

The high number of mitochondria present in cells and the small size of the mitochondrial genome mean that it is often possible to detect mitochondrial DNA from very small, old or degraded samples. It is also possible to obtain sequence data from samples such as hair shafts and feathers as well as burnt bone when there may not be sufficient nuclear DNA present to obtain a conventional STR DNA profile.

The gene for 12S ribosomal RNA is located in the mitochondrial genome. The DNA sequence of this gene is highly conserved within a species but shows variability between most species. Approximately 100 bases of this region are sequenced for species determination. Due to the region of the 12S ribosomal RNA gene used for sequencing, this assay can be used to identify vertebrate species (although there are exceptions) but is less useful for invertebrates.

12S sequences obtained from samples of interest are compared against a database of 12S DNA sequences held by the National Center for Biotechnology Information (BLAST database) in the USA, in an attempt to identify the species of origin. This database contains a wide range of DNA sequences. If a match to the 12S sequence of particular species is obtained then this provides evidence that the tested sample has also originated from the same species. For species/groups of species with low representation in the database, the level of sample identification is determined on a case by case basis.