The use of DNA evidence has revolutionised forensic science in the last 30 years. It now provides very powerful evidence in a range of criminal investigations, often identifies possible suspects at an early stage in an inquiry and enables old or so-called ‘cold’ cases to be re-investigated.
DNA is deoxyribonucleic acid. It is found in the nucleus of cells in the human body. The structure of DNA is a double helix, similar to twisted ladder, with phosphate groups and deoxyribose sugar units forming the backbone of the helix and base pairs joining the two backbone strands together, sometimes referred to as the ‘rungs of the ladder’. There are four of these bases: adenine always pairs with thymine; guanine always pairs with cytosine.
The type of DNA profiling known as Short Tandem Repeats (STR) works by measuring the lengths of sections of the DNA which are present in the DNA of everyone. The size of these sections varies between different people because there are different numbers of small repeating sub-units, consisting of only a few base pairs, within them. The isolated DNA sections are analysed by electrophoresis – which in effect measures their size. For many years in the United Kingdom, STR DNA profiles were produced by measuring the length of ten of these DNA sections plus a sex-dependent section. From July 2014, a new method of DNA profiling has been introduced, termed DNA-17, which provides greater sensitivity to small DNA amounts and to degraded DNA. It analyses 16 sections of DNA plus a sex-dependent section. Profiles can be obtained from blood, saliva, hair roots and other tissue and body fluids.
The STR DNA sections can be produced and multiplied using the polymerase chain reaction (PCR) which uses primer units to bind onto specific positions of separated DNA strands and then builds up new STR sections as a polymerase enzyme adds on individual DNA units (nucleotides) from the primer position. The fact that PCR repeatedly multiplies the number of the STR sections allows minute amounts of DNA to be analysed.
The first time that DNA evidence was used to secure a conviction was in 1988 in the United Kingdom when Colin Pitchfork was found guilty of a double murder. A ‘mass screen’ of the blood groups and DNA of about 5000 men local to the crimes had been carried out.
In the United Kingdom there is a National DNA database on which are stored the DNA profiles of people convicted of and arrested for certain criminal offences together with DNA profiles from the scenes of unsolved crimes. The criteria for inclusion of profiles on the database have been extensively debated. DNA evidence against some people has arisen through ‘familial searching’ which means that there is a very similar profile (but not the same profile) on the database to the unknown profile from the crime – suggesting that a relative of the person having the very similar profile might be involved. The use of DNA databases raises many ethical issues which are still under active discussion.
There are now DNA tests that can determine hair colour, eye colour and skin colour with a high degree of probability. Their genetic signatures are characteristic (but don’t ask, as someone once did, if dyeing your hair changes your DNA). Devices that can analyse crime and reference samples within two hours are now being tested by the police. And research is taking place to produce tests which are even more discriminating and which will provide information about physical characteristics and ancestry alongside conventional profiles.
In Sci-High events students carry out a DNA interpretation exercise to help solve the crime under investigation. With a little bit of deepish thought there is an exciting and surprising outcome which shows just how powerful science can be!