Forensic Investigation and DNA Profiling

Over recent years, great progress has been made in the area of DNA analysis and this has allowed for positive identification of criminals. In his book, Invisible Evidence, Bill O'Brien describes how DNA profiling works and how samples are gathered from crime scenes as well as individuals.

What is DNA

DNA stands for deoxyribonucleic acid. It is made up of two long strands that form a twisted ladder-like structure called a double helix. DNA contains genetic information that determines a person's appearance and abilities. The chances of two people sharing an identical genetic profile are virtually nil apart from identical twins.

How are DNA Samples Collected

The human body is comprised of around one billion cells which are constantly dying and being replaced. As a result of this process, people are always shedding cells wherever they are and whatever they are doing.

Investigators can perform DNA analysis using as little as twenty cells. These often come from blood, saliva or semen left at the crime scene but can come from any part of the body.

When searching for DNA samples, investigators will examine the crime scene thoroughly. DNA can be recovered from any surfaces including:

• Blood-stained clothing
• Semen stains on clothing
• Saliva in discarded chewing gum
• Hair brushes
• Cigarette stubs
• Surfaces such as concrete, metal, wood, fabric, plastic and glass

People who are suspected of being involved in a crime are asked to give a DNA sample. The simplest way of procuring this is by swabbing the inside of the cheek.

How is DNA Tested

When genetic material arrives in the laboratory it has to be separated from contaminating material such as clothing dye or paint. Scientists then use chemicals and enzymes to break open the cells and release the DNA into a solution. Ninety-nine percent of human DNA is the same and it is the other one percent that is analyzed.

In forensic testing, polymerase chain reaction selectively copies the unique parts of DNA. The sample is increased greatly in size and is then placed into a glass tube with a special polymer. The sample is labelled with fluorescent dyes and the results are read by a computer that uses lasers and color-sensitive detectors.

DNA testing only came into use into the 1980's and has progressively transformed the way a crime scene is handled. People continually shed cells and it is almost impossible not to leave evidence at a crime scene even when protective clothing and gloves are used.

Reference:

O'Brien, Bill, Invisible Evidence, David Bateman Ltd, 2007