Forensic Dentistry, Second Edition

108 Forensic dentistry

that degrade the proteins. As a result, the DNA molecules are released from

their natural location within the cells and float out into solution. DNA is

water soluble, whereas most of the other components in the mixture are

soluble in organic solvents. Thus, when phenol and chloroform are added, the

aqueous layer containing the DNA separates out and remains on top of the

organic layer.9–11 The aqueous solution is removed and centrifuged through a

series of filters to collect the DNA.

Newer extraction techniques employ special beads to isolate the DNA.^14

One particular bead extraction technique is called Chelex. Following an ini-

tial boiling step in which the cells are broken apart and DNA is released,

the chelating resin beads bind the non-DNA debris. Once the beads have

been removed by centrifugation carrying the unwanted material along, a

single-stranded DNA layer is left behind for analysis. Another technique uses

a lysis buffer to expose the DNA. Silica beads absorb the DNA molecules ,

thereby taking them out of the sample solution. Several wash steps remove

the unwanted cellular debris from the beads. Subsequently, a second buffer

elutes the DNA from the beads, which is then available for amplification. This

particular approach is amenable to automation and can provide increased

efficiency when large numbers of samples require high-throughput robotic

processing9–13 (Figure 7.4).

7.3.2 Amplification (Quantitation and PCR)

Regardless of the outward appearance of evidence that is submitted for
analysis , at the molecular level biological materials will exhibit extreme
ranges of human DNA content and condition. Depending on the environ-
ment in which the evidence was recovered, there may also be nonprimate
animal DNA or microbial overgrowth contributing to the total volume of

Figure 7.3 preparing to pulverize a whole tooth in a freezer mill with liquid
nitrogen by encasing the tooth in a sterile polycarbonate tube sealed with two
metal anvils, including a ferromagnetic plunger. (Courtesy of the Bold Forensic
laboratory at uBC.)