106 Forensic dentistry
double helix of DNA is located in organelles called mitochondria distributed
throughout the cytoplasm, not the nucleus. Furthermore, this genome is
inherited from the mother only, and thus is not unique to the individual. All
offspring from the same mother will have the same mitochondrial genome.
Maternal relatives, including siblings and maternal cousins, cannot be differ-
entiated on the basis of their mitochondrial DNA. However, mtDNA analysis
can be instrumenta l in resolv ing identit y in cer tain contexts, including ana ly-
sis of desiccated skeletal remains, hair, and other severely degraded evidence.
Because of the generational constancy of the mtDNA genome, a reference
from a living individual today can be used to help identify a maternal relative
missing from decades earlier.
No matter whether the DNA is from the nucleus or from mitochondria,
or whether the scientist is analyzing autosomes or sex chromosomes, all
human DNA contains the same four nucleotide bases paired in the same
faithful arrangement described earlier and arrayed in a double helix.^3
7.3 Laboratory Processing
Upon its arrival at an analysis laboratory, DNA evidence is formally accessioned
into the Laboratory Information Management System (LIMS). The evidence
will be unpackaged in a biosafety hood, inventoried, reconciled with the
accompanying chain-of-custody documents, and assigned a laboratory track-
ing number. This tracking number is usually an alphanumeric composition
that may include a case designator, as well as a sample or specimen designator.
Regardless of how the tracking number is derived, it will be the unique iden-
tifier used for that sample throughout laboratory processing and reporting.
After accessioning, the evidence is documented photographically and repack-
aged with the submitter’s identifying information and the laboratory tracking
number. Evidence that does not undergo immediate analysis is sealed with
the date and analyst’s initials and placed in secure cold storage at –20°C.
7.3.1 Extraction (Isolation of DNA)
The first step in laboratory processing of the evidence is DNA extraction.
This is the chemical process by which DNA molecules are removed from the
biological source, such as blood, tissue, tooth, hair, etc., and isolated from
cellular and protein debris. Bones and teeth are usually pulverized or ground
in preparation for this procedure to render the DNA molecules closer to
the surface and available for extraction (see Figures 7.1 to 7.3). Hairs may
be ground and tissue is usually finely minced. Traditionally, extraction and
isolation of DNA is achieved by mixing the prepared sample with stabilizing
salts along with detergents that break up cellular membranes and enzymes