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DNA: Collection and Interpretation

Mar 12 2014 Read 5926 Times

Author: Tracy Alexander on behalf of LGC Forensics

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In this second article of a mini-series* focusing on forensics, author Tracy Alexander looks at how the use of DNA profiling has been developed and refined, where DNA samples are collected, and how they are used.
The use of DNA evidence in criminal trials has increased beyond imagination since its first use in the mid-1980’s. Deoxyribonucleic acid is a macromolecule generally found within the nucleus of cells. The genome is the entirety of this cellular DNA and encodes all the genetic information that governs an organism’s structure and function, and is unique to that organism, except in genetically identical individuals. Analysis of the variable sections of DNA is frequently employed to determine paternity and to resolve immigration disputes, but public perception is focussed on its application as an aid to police investigative processes, to identify victims of crime and, by the transfer, exchange and persistence of traces of DNA, the offenders responsible for crime.

The nucleus is not the only place that has DNA; cells contain tiny sausage-shaped organelles known as mitochondria. These are the powerhouses of the cell specifically designed to make energy. Each mitochondria has between 1 and 10 copies of circular DNA that code for proteins required for energy production and each cell may have hundreds of mitochondria depending on its function (for example, mitochondria are found in large numbers in muscle cells). Hence in adverse conditions when the genomic DNA is too severely degraded to be useful it is sometimes still possible to get some useful genetic information from the mitochondria. This information is not as statistically powerful as genomic DNA but is very useful in identifying family members. The mitochondria are inherited through the maternal line as at fertilisation it is only the sperm’s nucleus that is injected into the egg, not the mitochondria.    
Whilst all nucleated cells contain DNA, not all nuclear DNA is contained within cells. Like many of the elements that go to make up an organism the DNA is recycled. Dead cells are harvested and the DNA broken down. In the process of this long stretches of DNA can be found in the plasma and, a recent discovery, in sweat. This discovery has far-reaching implications for the evidential value of “touch” DNA, particularly in terms of secondary and tertiary transfer between individuals and touched items.
Sampling
The National DNA Database (NDNAD) holds samples from three sources: personal samples from those arrested or charged, crime scene samples and voluntary personal samples.  Before 2004 these were known as Criminal Justice (CJ) samples, and were not admissible as evidence without confirmatory samples being taken on the re-arrest of a suspect. The taking of this second sample could entail a delay of up to two weeks during which time the suspect had the opportunity to abscond or to undertake multiple crimes, knowing their arrest was imminent. However, since 2004 samples are taken under the provision of the Police and Criminal Evidence Act (PACE) and can be used for evidential purposes.
There are three possible samples that can be taken from arrested persons for search against and inclusion on the database: blood, buccal scrapes (from the epithelial lining on the inside of the cheek against the buccal muscles) and pulled head hairs.  Improvements in technology have meant that blood samples which used to be taken from major crime suspects due to the likelihood of obtaining a usable profile, have been largely replaced by the use of buccal scrapes. Pulled head hairs are an option if a suspect refuses to give a buccal sample voluntarily.
From a crime scene there are many sources from which to obtain a DNA profile, with varying degrees of evidential value depending on the circumstances.
Often blood is found, particularly at major crime scenes. The red blood cells, of which there are about 4.5-5 million red blood cells per micro-litre (one thousandth of a millilitre) of blood, have no nuclear or mitochondrial DNA. It is therefore only the white blood cells, of which there are only 5-10 thousand per micro-litre of blood, which can be analysed for a DNA profile.
Saliva does not contain DNA at point of production but epithelial cells from the inside of the cheek regularly slough off and are deposited in the saliva in sputum and on items coming into contact with the mouth, such as cigarette butts, drinking vessels, masks, gags and on licked stamps and envelopes.

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