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| Sunday, 7 August 2005 |
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Junior Observer |  |
| News Business Features Editorial
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DNA evidence to catch criminals If you watch the Discovery Channel, you may know about the popular detective programme 'The New Detectives'. This programme highlights the techniques used by detectives to capture criminals. One of the popular methods used by such detectives is DNA evidence. Well.. if you read and listen to news, you might remember 'Baby 81'. For those who missed the news, the story goes like this, a few days after the tsunami, an infant without his parents was found and handed over to the Police. Nearly ten women came forward to say the baby belonged to them. Finally the Court ordered a DNA test to find out who the real parents were. Following the DNA test, the Police handed over the baby to his real parents. Advance technology saved the day! Thanks to DNA evidence, now there is a way to prevent the wrong person from being locked up in jail. In this article, we'll look at how DNA "fingerprinting" works and find out what DNA evidence can be used for. Proving that a suspect's DNA matches a sample left at the scene of a crime requires two things. First, creating a DNA profile using basic molecular biology protocols. The second is crunching numbers and applying the principles of population genetics to prove a match mathematically. We all like to think that we are unique. Unless you are an identical twin, at the nuclear level, you are! Humans have 23 pairs of chromosomes containing the DNA blueprint that encodes all the materials needed to make up your body as well as the instructions how to run it. One member of each chromosomal pair comes from your mother, and the other is contributed by your father. Every cell in your body contains a copy of this DNA. While the gene majority of DNA doesn't differ from human to human, some three million base pairs of DNA (about 0.10 per cent of your entire genome - gene collection) vary from person to person. The key to DNA evidence lies in comparing the DNA left at the scene of a crime with a suspect's DNA in these chromosomal regions that do differ. There are two kinds of polymorphic regions (areas where there is a lot of diversity) in the genome: Sequence polymorphisms and Length polymorphisms. Sequence polymorphisms are usually simple substitutions of one or two bases in the genes themselves. Genes are the pieces of the chromosome that actually serve as templates for the production of proteins. Amazingly, despite our complexity, genes make up only five per cent of the human genome. Individual variations within genes aren't very useful for DNA fingerprinting in criminal cases. The other 95 per cent of your genetic make-up doesn't code for any protein. Because of this, these non-coding sequences used to be called "junk DNA", but it turns out that these regions do actually have important functions such as: regulation of gene expression during development, aiding or impeding cellular machinery from reading nearby genes and making protein and finally serving as the bricks and mortar of chromosomal structure. Non-coding DNA is full of length polymorphisms. Length polymorphisms are simply variations in the physical length of the DNA molecule. DNA evidence uses a special kind of length polymorphism found in non-coding regions. These special variations come from stretches of short, identical repeat sequences of DNA. A particular sequence can be repeated anywhere from one to 30 times in a row, and so these regions are called variable number tandem repeats (VNTRs). The size of a DNA fragment will be longer or shorter, depending on how many copies of a VNTR there are. In the case of DNA evidence, the great thing is that the number of tandem repeats at specific places (called loci) on your chromosomes varies between individuals. For any given VNTR loci in your DNA, you will have a certain number of repeats. The basic procedure used to isolate an individual's DNA fingerprint is called Restriction Fragment Length Polymorphism (RFLP) analysis. This is a complicated way of saying that investigators determine the number of VNTR repeats at a number of distinctive loci to come up with an individual's DNA profile.In 1985, DNA entered the courtroom for the first time as evidence in a trial, but it wasn't until 1988 that DNA evidence actually sent someone to jail. This is a complex area of forensic science that relies heavily on statistical predictions; in early cases where jurors were hit with reams of evidence heavily laden with mathematical formulas, it was easy for defence attorneys to create doubt in jurors' minds. Since then, a number of advances have allowed criminal investigators to perfect the techniques involved and face down legal challenges to DNA fingerprinting. **** For what it's used Most people know DNA profiles are used by criminal investigators to: * Prove guilt - Matching DNA profiles can link a suspect to a crime or crime scene. The British police have an online database of more than 360,000 profiles that they compare to crime scene samples; more than 500 positive matches come up a week. * Exonerate an innocent person - At least 10 innocent people have been freed from death row in the United States after DNA evidence from their cases was studied. So far, DNA evidence has been almost as useful in excluding suspects as in fingering and convicting them; about 30 per cent of DNA profile comparisons done by the FBI result in excluding someone as a suspect. DNA evidence is also useful beyond the criminal courtroom in: * Paternity testing and other cases where authorities need to prove whether or not individuals are related * Identification - Police investigators often face the unpleasant task of trying to identify a body or skeletal remains. DNA is a fairly resilient molecule, and samples can be easily extracted from hair or bone tissue; once a DNA profile has been created, it can be compared to samples from families of missing persons to see if a match can be made. The military even uses DNA profiles in place of the old dog tag. Each new recruit must provide blood and saliva samples, and the stored samples can subsequently be used as a positive ID for soldiers killed in the line of duty. Even without a DNA match to conclusively identify a body, a profile is useful because it can provide important clues about the victim, such as his or her sex and race. * Studying the evolution of human populations - Scientists are trying to use samples extracted from skeletons and from living people around the world to show how early human populations might have migrated across the globe and diversified into so many different races. * Studying inherited disorders - Scientists also study the DNA fingerprints of families with members who have inherited diseases like Alzheimer's Disease to try and ferret out chromosomal differences between those without the disease and those who have it, in the hope that these changes might be linked to getting the disease. |
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