Because DNA is the same in all cells of the body, DNA profiles extracted from various types of samples, collected at different times and in several locations, can be compared to determine whether they may have come from the same person.
Deoxyribonucleic Acid (pronounced Dee-ox-ee-rye-bo-new-klee-ic Acid and abbreviated as DNA) is the blueprint for life. It is a very long molecule that carries genetic information that governs a person's physical characteristics. Each individual inherits half their DNA from their mother and the other half from their father. With the exception of identical twins, no two individuals share the same DNA sequence. DNA is found in the nucleus of every cell in the body except red blood cells and is the same throughout the body.
There are many regions of DNA molecules that do not hold any known genetic information, but that vary enormously from person to person. These are called non-coding or 'junk' DNA, and are used by forensic scientists to distinguish between individuals. Junk DNA cannot be used to build up a physical picture of an individual, or identify race or age.
Because forensic scientists do not look at the whole of a person's DNA sequence, but rather a set of characteristics in non-coding DNA, the results are called a DNA profile. DNA profiles are a very powerful means of determining whether two samples may or may not have come from the same person. If two DNA profiles do not match, they must have come from two separate individuals. However, if they do match, there is only a small chance that they come from two different people.
Because DNA is the same in all cells of the body, DNA profiles extracted from different types of samples at different times and in different places can be compared to determine whether they may have come from the same person. Therefore, if human biological samples are found at a crime scene, DNA profiling can be used to determine whether a suspect could be a possible source of a sample.
Possible examples are:
Sufficient quantities of DNA for profiling can be extracted from minute traces of human biological material such as blood, semen, sweat, hair and saliva.
Before a DNA profile can be prepared from a human sample the DNA must be isolated from other organic and non-organic components of the sample. The type of sample will determine the particular isolation technique used. Isolation techniques may involve the use of enzymes to break down proteins and other cellular material; chelating agents to ensure that non-organic material does not degrade the DNA; and organic solvents to separate the DNA from the other organic and non-organic material where necessary.
Once the DNA has been extracted from the sample, a number of different techniques can be used to develop a DNA profile. Techniques for developing profiles are constantly improving or being superseded.
It is more than two decades since the first DNA profiling technique was discovered. However, since then there have been significant advances in DNA profiling systems, enabling matching of DNA profiles from different samples with increasing accuracy.
The last decade and a half has clearly demonstrated the power of forensic DNA profiling. This has resulted in demand for cheaper, faster and more sensitive DNA profiling techniques.
Mitochondrial DNA (mtDNA) is found outside the nucleus of cells and occurs in cells that do not have a nucleus (eg mature red blood cells and hair shaft cells). The mtDNA molecule is considerably smaller than nucleic DNA and has only 16,569 base pairs. However, each cell may contain many hundreds or thousands of copies of the mtDNA molecule. In cases where biological samples have been severely degraded by environmental factors (eg skeletal material) there is a greater chance of recovering mtDNA than nucleic DNA as there is only one copy of nucleic DNA per cell. mtDNA profiling may be used where a nucleic DNA profile cannot be recovered from a sample either because it is severely degraded or the sample type does not contain nucleic DNA.
mtDNA profiling is carried out using PCR and sequence variation analysis at two hypervariable locations of non-coding areas of the molecule. mtDNA profiling is particularly susceptible to contamination and extreme care must be taken during processing. mtDNA profiling is the most time-consuming and rigorous of DNA profiling techniques.
mtDNA profiling cannot provide the same level of discrimination as nucleic DNA profiling, as there is a very limited number of locations that can be tested. Additionally, a person inherits mtDNA from only their mother, rather than both parents as in the case of nucleic DNA. Therefore, any maternally related individuals will share the same mtDNA profile. Apparently unrelated individuals may share the same mtDNA profile because they share a common female ancestor at some distant point in the past.
Most analysing equipment currently used for capillary gel electrophoresis has only one capillary column and can generally only process around three samples per hour. However, some Australian forensic laboratories have purchased analysers with sixteen capillary columns to increase their processing capacity. Prototype analysers with forty-eight capillary columns operating in parallel have been developed, resulting in significant increases in the number of samples that can be processed.
Mass spectrometry provides a speedy mechanism for measuring the size of alleles in PCR STR analysis. Following PCR amplification, the DNA samples are mixed with an organic matrix and allowed to crystallise. A laser pulse is applied to the sample, producing DNA ions. The ions are accelerated by an electric field and pass through a flight tube to a detector. Smaller DNA ions travel faster than larger ones. The weight of the DNA ions can be determined by measuring the time between the laser pulse and the DNA ion reaching the detector, or the 'time of flight'. Multiple laser pulses can be processed in a few seconds to improve the accuracy and precision of the size measurement of the allele. A robotic workstation used in combination with a time of flight mass spectrometer has been able to process over 2,000 samples in a day. However, given the costs of the equipment, this technology is unlikely to be widely adopted by forensic laboratories unless there is a need to be able to process such sample volumes.
The US Department of Justice is currently funding a number of research projects to develop portable microchip DNA profiling devices to be used in the field. Reservoirs for PCR amplification primers, reaction chambers and capillary electrophoresis channels have been manufactured onto the surface of microchips. PCR amplification and STR analysis of a small DNA sample can be achieved on the microchips in a matter of minutes as opposed to the hours that current PCR STR analysis systems take.