DNA

DeoxyriboNucleic Acid or DNA is the nucleic acid which carries the genetic information that determines how our bodies are constructed and regulates our development. It consists of two linked molecules which spiral around each other, in the famous double-helix shape.

The two molecules consist of a series of four bases called adenine, cytosine, guanine and thymine. Cytosine always pairs with thymine, guanine always pairs with adenine and the two strands zip together whilst spiraling about each other.

Because of the huge number of possible combinations of these bases, every human being (with the exception of an identical twin) carries a unique genetic profile within every living cell of their bodies.

It is this uniqueness which allows us to use genetic profiling to identify the person who has deposited DNA (in the form of a body fluid or cells) at a crime scene.

When a CSI attends the scene of a crime they look, amongst other things, for any body fluid (or object which may carry body fluids) from the offender.

Most frequently this will be in the form of drops of blood, where the offender has cut themselves, or through saliva on cigarette butts or on drink containers.

Blood spots and drink containers will be photographed to show their position and swabs taken using a combination of moist and dry swabs. Using both moist and dry swabs maximises the amount of cellular material collected.

Cigarette butts are collected whole, and submitted intact to the forensic science laboratory. This is because the paper wrapping and the filter (if present) provide a ready media for absorbing large amounts of saliva, and hence cellular material containing DNA.

Another obvious source of DNA is semen. In cases of rape, swabs can potentially recover seminal fluid which, if sperm is present, can conclusively prove a link between a suspect and a victim.

All of these samples are examined at a forensic science laboratory and a DNA profile obtained. This profile is searched against the national DNA database and any match reported to the police force dealing with the offence.

If a full match is obtained (SGM+ crime scene stain to SGM+ offender’s reference sample) the statistical probability that this person is responsible is in excess of one in a billion.

Sometimes, the DNA has degraded for various reasons and a partial DNA match is obtained. In these cases the forensic science service will provide a reduced statistical likelihood that the person is responsible and the courts will place an appropriate weight to this evidence. In all cases, the emphasis has to be to ensure fairness in the judicial process and our court system is extremely good in providing robust checks and balances.

The national DNA database contains the profiles of anyone who has been convicted of a recordable offence.

The DNA profile is generated by a process called electrophoresis, in which an electric charge is used to separate molecules in a solution or gel according to electrical charge and size. This creates a series of bars, which looks similar to the chromatography experiments we all performed with blotting paper and ink in science lessons at school. These bars are coded and the resultant series of numbers entered into the national DNA database.

Once this is done it is a simple task to ask the computer to search DNA profiles from a crime scene against the DNA profiles on the national DNA database.