Biology Reference
In-Depth Information
roots have been shown to contain as much as 0.5
g of DNA [22]. Hair that has
been shed when it is in the resting telogen phase often contains no cellular mate-
rial around the root. The hair shafts are composed of keratin, trace metals, air and
pigment - cell fragments, including DNA can get trapped in the matrix of the hair
and provide enough DNA to produce a profile. However, hair is notoriously difficult
to analyse, and in many cases it is only possible to successfully profile mitochondrial
DNA [22], although nuclear DNA can, in some cases, be recovered [23].
The hair shaft, like the spermatozoa acrosome, is rich in disulfide bridges and
requires either mechanical grinding [24] or the addition of a reducing agent such as
DTT [22, 23] that will break the disulfide bonds and allow proteinase K to digest the
hair protein and release any trapped nucleic acids. Once released, the DNA can be
extracted using the salting-out procedure [25] or organic phenol - chloroform-based
extraction [22 - 24]. Alternative methods include digestion in a buffer containing
proteinase K followed by direct PCR [26, 27] or dissolving the hair shaft in sodium
hydroxide and, after neutralization, the released DNA is concentrated using filter
centrifugation [28].
Because the hair shaft contains very low levels of DNA it is prone to contami-
nation, but unlike many other types of biological evidence with low levels of DNA
it is possible to clean the hair shaft prior to DNA extraction. Several methods have
been used to clean hair including washing in mild detergents, water and ethanol and
also using a mild lysis step in the same way as is used in the differential extraction
of semen [29].
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Hard tissues
Following murders, terrorist attacks, wars and fatal accidents it is desirable to group
together body parts from individuals when fragmentation has occurred and ultimately
to identify the deceased. If the time between death and recovery of the body is short
then muscle tissues provide a rich source of DNA [30], which can be extracted using,
for example, any of the Chelex
, salting-out and organic extraction methods. If,
however, the soft tissues are displaying an advanced state of decomposition they will
not provide any DNA suitable for analysis. When the cellular structure breaks down
during decomposition, enzymes that degrade DNA are released and the DNA within
the cell is rapidly digested. This process is accelerated by the action of colonizing
bacteria, fungi and insects.
Osteocytes are the most common nucleated cells in the bone matrix (Figure 4.6a).
In the teeth, odontoblasts within the dentine and fibroblasts in the cell rich zone
of the pulp cavity provide a source of nucleated cells [31] (Figure 4.6b). The hard
tissues of the body, bone and teeth provide a refuge for DNA. In addition to the
physical barriers, the hydroxyapatite/apatite mineral that is a major component of
the hard tissues, stabilizes the DNA which becomes closely bound to the positively
charged mineral; this interaction limits the action of degrading enzymes [32].
Hard tissues provide an advantage over other forms of biological material because
they have a surface that can be cleaned to remove any contaminating DNA using
