DNA Fingerprinting

DNA has been used since 1986 to identify persons involved in crimes, confirm the identity of human remains from tragedies, and vindicate the innocent for crimes they didn’t commit. The sensitivity of DNA fingerprinting has increased dramatically over the past two decades. This has increased its importance in forensic analysis and even enabled past crimes to be solved by re-examining evidence.

DNA is the inherited genetic information which determines the characteristics of each individual. Everybody’s DNA is unique (except in the case of identical twins who have identical DNA). Your body has 60 trillion cells, and each one contains a copy of your DNA. The DNA is coiled into structures called chromosomes inside the nucleus of each cell.

The  structure of DNA is illustrated below as a double helix molecule. Each spiral strand is composed of a  sugar phosphate backbone and attached bases. One strand is connected to a complementary strand by a type of bonding known as hydrogen bonding. The bases which join the two strands together make up the DNA code. These bases are called adenine (A) with thymine (T) and guanine (G) with cytosine (C). In simple terms, it is the sequence of these four bases which determines every characteristic in the human body (and in fact any other living organism).

Analysis of DNA (often referred to as DNA fingerprinting)  left behind at the scene of a crime or accident is becoming more and more important in the identification of victims and people involved in the crime.

When a person enters a room, touches or picks up objects, cells from their skin are shed. Often samples of DNA can be collected and analysed from these samples. If blood or other body fluids (such as saliva or semen) are left behind at the scene of a crime there is even more chance of obtaining a sample of DNA for forensic analysis. Only dead cells or cells which do not contain a nucleus, such as red blood cells, do not contain DNA.

Since the development of DNA fingerprinting, a technique for duplicting samples of DNA has been developed called Polymerase Chain reaction. This allows a tiny sample of DNA to be replicated many times, so that only a tiny sample of DNA is needed as a starting point to produce a large enough sample for analysis. The main problem with such a sensitive technique, is that any contaminating DNA which gets into a sample could also be replicated.  For this reason, when a crime has been committed, a crime scene analyst will restrict access to the area and wear protective gloves and clothing to reduce the chance of contaminating the area. Swabs are used to collect any possible DNA samples from areas which may have come into contact with the perpetrator of a crime, eg. Door handles, steering wheels, the clothing of a victim, etc.

When a sample of DNA is obtained from a cell sample, only a small part analysed. This means that even though as individuals we have a unique set of DNA, it is possible that the sections which are examined could be the same for two individuals. A balance is reached so that enough of the DNA is examined to make it approximately 1 in 100,000,000 chance or less  that two individual would come up with exactly the same DNA sequence. In a court of law this is acceptable odds to provide a conviction, but it does make it hard for Juries to understand as the process of DNA fingerprinting is complex and the  chances of someone having the same ’analysed’ sequence is in itself difficult to understand.