DNA Analysis

DNA

What is DNA?

The human body is composed of billions of cells of many different types i.e. blood cells, hair, skin, bone, muscle cells, heart, lung, spleen. Almost all cells contain a nucleus (except the red blood cell) and within the nucleus of each cell is an identical copy of a person’s DNA. Because of this DNA from a person’s blood will be the same as that found in their saliva or hair roots.  Half of our DNA is inherited from our mother and half from our father. Except for identical twins, each person’s DNA is unique.

What is DNA profiling?

Even though each person’s DNA is unique, with current technology it is not practical to look at each difference.  Currently in the laboratory we look at 10 different areas of DNA, which are known to vary widely between people.  These 10 areas contain short repeating sequences known as Short Tandem Repeats (STR). The number of these repeating sequences varies between individuals.  An additional area is also provided which indicates whether the person is male or female.  The technique of DNA profiling is centred on analysing and measuring these differences in length.

Typical example of a DNA profile

How is DNA profiling done?

Essentially the technical process for DNA profiling involves the following main steps

	Dissolve the crime stain Separate, clean and measure the quantity and quality of DNA Target the specific pieces of interest within the DNA molecule
	Produce multiple copies of these (PCR) Sort the pieces of DNA according to size Measure the size of the pieces

DNA Use in Forensic Cases

Biological fluids such as blood, semen, hair and saliva can play a role in shedding light on various types of criminal cases.  In the case of sexual assault cases, semen found on vaginal swabs and /or clothing of a victim can be compared to the DNA profile of a nominated suspect.  Similarly in murder/assault cases blood found on the clothing of a nominated suspect can be compared to the DNA profile of the victim.

Contrary to a number of years ago the smallest of traces left behind in criminal offences can now suffice for a DNA analysis.  With the currently applied DNA techniques, a profile can be constructed, even of old and (partially) decayed biological traces.

Forensic DNA analysis can also establish the origin of a biological sample with an extremely high degree of probability.