The use of DNA evidence has revolutionised forensic science in the last 25 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 stands for 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 within them which consist of only a few base pairs. Enzymes are used to isolate the DNA sections allowing them to be analysed by electrophoresis - which in effect measures their size. At present in the United Kingdom, STR DNA profiles are produced by measuring the length of ten of these DNA sections. Profiles can be obtained from blood, saliva, hair roots and other tissue and body fluids.

The first time that DNA evidence was used to secure a conviction was in 1986 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 500 men local to the crimes had been carried out.

Minute amounts of DNA can now give rise to profiles because forensic scientists use a technique called the polymerase chain reaction to copy DNA – multiplying the amount many thousands of times. If there is a particularly small amount of DNA present originally, extra multiplication can be carried out using a 'low copy number' methodology. Techniques have also been developed to separate out DNA in mixed body fluids from different people.

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 are under active review. 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 ethical issues which are still under discussion.

There is now a DNA test that can determine with a high degree of probabilty whether a person has ginger hair. The genetic signature for this is very characteristic (but don't ask, as someone once did, if dyeing your hair ginger changes your DNA). It may well be that methods will be developed in the future to identify many more human physical charactersitics from DNA.

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!