DNA Fingerprinting in Forensic Science

Gel Electrophoresis Fingerprint - Mnolf

Comparing the DNA from a crime scene with that of suspects and victims is a powerful tool in criminal investigations. Forensics relies on several methods.

First developed in 1984 by Sir Alec Jeffreys, DNA fingerprinting (or profiling), involves comparing specific non-coding regions (introns) of the DNA molecule from various samples (saliva, skin, blood or semen) found at crime scenes or taken from suspects. These non coding regions make up about 30% of the DNA molecule, and consist of numerous repeated base sequences (variable number tandem repeats, or VNTRs). These intron sequences can be highly variable from one person to the next.

RFLP Fingerprinting

Restriction Fragment Length Polymorphism (RFLP) fingerprinting, where the variable fragments are relatively long (up to 35000 bases in length) was the original technique used until more efficient methods were developed around a decade ago. This procedure involves using specific restriction enzymes to cut DNA at points representing the targeted tandem repeat sequences.

Because a particular VNTR can vary with respect to the number of repeats from person to person, this results in DNA fragments of different lengths. The DNA strands are separated on electrophoresis gels, which consist of an agarose or polyacrylamide gel subjected to an electric potential difference (see fig. 1). Because DNA is negatively charged, it will migrate towards the positive end of the gel. Longer fragments, however, will not move as fast as shorter ones, resulting in the effective separation of any variable tandem repeats present.

DNA samples from several sources are run along separate lanes on the same gel. After electrophoretic separation has been achieved, the DNA is denatured into single strands by heating or applying chemicals. The resulting fragments are then transferred to a nitrocellulose or nylon membrane using the Southern Blot technique.

Radioactively labelled DNA strands of different lengths that are complementary to the different VNTR fragments are then introduced. These 'probes' will bind to complementary strands on the membrane and thus indicate their presence on X- ray film when the membrane is exposed to autoradiography (see fig. 2).

Typically, DNA fingerprinting compares several fragments from each sample, each containing a different tandem repeat sequence. During Bill Clinton's impeachment trial, for instance, seven different variable repeat sequences were used from Clinton's DNA and the DNA from the semen stains on Monica Lewinsky's dress. The bands on the gel matched exactly, representing a one in eight trillion probability that they were not from the same person.

PCR/STR Fingerprinting

The more traditional RFLP profiling procedure is now being replaced by the polymerase chain reaction(PCR) method, which often involves the use of shorter DNA segments known as short tandem repeats (STRs). This method is faster and requires less DNA. Shorter DNA strands are also less vulnerable to degradation.

Moreover, STR fragments are more suited to being amplified by the polymerase chain reaction, which can produce millions of copies of one initial STR fragment within the space of around two hours. The process involves repeated heating and cooling amid a mix of free nitrogen bases, primers and the enzyme DNA polymerase. As a result, even the smallest DNA samples - as little as one billionth of a gram - can be utilised.

Although STR separation can be carried out on electrophoretic gels, capillary electrophoresis, in which a fused silica capillary is used instead of a gel slab, is now being used more frequently . While applying the same principles of separation as the more traditional gel slab electrophoresis, it is more rapid and has a higher resolution.

Uses of DNA Fingerprinting in Forensic Science

The more STRs that are compared from each DNA sample, the less likely the chance will be that they have arisen from the same source. The likelihood of any two specimens being identical is calculated using the 'product rule'. Here, the probability of the frequency of occurrence of each STR in a population is multipled by that of the other STRs separated from the DNA sample.

When all thirteen STRs used in the U.S. national database, CODIS, are combined, the likelihood that the DNA has arisen from another source can be as little as 1 in 575 trillion for Caucasian Americans and 1 in 900 trillion for African Americans.

During the O.J. Simpson trial in 1995, for instance, O.J.’s guilt was suggested by STR tests carried out on his blood and blood from the crime scene, which revealed the probability of his being innocent was1 in 240 000. Further RFLP tests narrowed this down to 1 in 57 billion. The main reason he wasn’t convicted was due to the seed of doubt the defence sowed in the minds of the jurors about possible interference with the evidence by the FBI.

In addition to its use in providing evidence to incriminate suspects, DNA profiling can also be of use in exonerating suspects accused of crimes. About 25% of violent crime cases in the U.S. since 1989 have resulted in the exoneration of suspects because of DNA profiling procedures. By 1996, over 108 post-conviction exonerations had in fact occurred in the USA using DNA profiling.

Because STRs and VNTRs are inherited from each of our parents, DNA profiling can also be used to establish paternity in cases involving custody and child support. Other uses in forensics include the identification of victims of catastrophes such as the September 11 attacks.

It must be remembered that these tests are not always foolproof and should be used in conjunction with other evidence where possible. DNA fingerprinting has nevetheless affected the outcome of criminal investigations in a revolutionary way.

References

Australian Government, 2001, 'DNA Profiling', Biotechnology Online, biotechnologyonline.gov.au, accessed 24/5/2010

Brinton, K. and Liebermann, K,1994 "Basics of DNA Fingerprinting-Southern Blot', washington.edu, accessed 22/5/2010

Saferstein, R., 2004, 'Criminalistics, an Introduction to Forensic Science', Pearson