Hair shafts

Hair shafts that have been pulled out often possess a root that is rich in cellular material and DNA can be extracted using any of the commonly used techniques - plucked roots have been shown to contain as much as 0.5 |g of DNA [14]. Hair that has been shed when it is in the resting telogen phase often contains no cellular material 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 successfully to profile mitochondrial DNA [14], although nuclear DNA can, in some cases, be recovered [15].

The hair shaft, like the spermatozoa acrosome, is rich in disulphide bridges and requires either mechanical grinding [16] or the addition of a reducing agent such as dithiothreitol [14, 15] that will break the disulphide 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 [17] or organic phenol-chloroform based extraction [14-16]. Alternative methods include digestion in a buffer containing proteinase K followed by direct PCR [18,19] or dissolving the hair shaft in sodium hydroxide and, after neutralization, the released DNA is concentrated using filter cen-trifugation [20].

Because the hair shaft contains very low levels of DNA it is prone to contamination 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 [21].

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