DNA Technology in Cold Cases

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New advances in science and technology have turned cold cases into closed cases, due to the incredible changes technology has made to policing in recent years.

One of the most well known forensic techniques is DNA fingerprinting. But what is a person’s DNA fingerprint and how do we know if they match? An individual’s DNA consists of exons (which code for proteins) and introns (which are non-coding). The majority of the sequence within these introns is made of the same sequence of nucleotides. These short repeats are termed Short Tandem Repeats (STRs) and are what we look for in a DNA fingerprint, as everyone has a unique composition of different lengths of STRs. Using nuclease enzymes, we can cut the DNA at specific points to generate shorter strands, which will vary in length depending on the individual. This mixture of lengths can then be separated using a process known as Gel Electrophoresis. This exploits the naturally negative charge of DNA, by running samples of the cut DNA strands through a gel, which acts as a net, slowing larger pieces of DNA down. This results in a pattern of bands, where the smaller lengths of DNA have run further than the longer lengths. As everyone has different DNA fingerprints, a suspect’s DNA can be run against a sample acquired from the crime scene, and the banding patterns compared. If they match, then well done – you’ve caught the perp!

Credit: Pixabay (CC0 Creative Commons).

However, in many cases, the amount of DNA found at a crime scene will be minimal, and likely in insufficient quantities to run a DNA fingerprinting investigation. A Polymerase Chain Reaction (PCR) is then useful: it amplifies DNA exponentially. PCR works by exploiting another enzyme, Taq polymerase, which acts as a tiny molecular builder, extending a sample of DNA from a small starting block. As DNA is double stranded, high temperatures first break the bonds holding the two strands together. The DNA is then cooled and primers attached at specific sites complimentary to short base sequences. The Taq polymerase enzyme then builds a new DNA strand from the primer using free bases in the reaction mixture. This process doubles the number of DNA double strands, and is carried out multiple times to generate huge amounts of double stranded DNA.

However,what happens if no DNA is on file? Well, luckily, STRs are inherited between generations with very minor mutations. Familial DNA fingerprinting has been used in recent years to solve cold cases. Take the case of Melanie Road, 17,  raped and murdered in 1984. Police took DNA samples from the crime scene and froze them until the technology was around to analyse them. The DNA fingerprint of the killer was uploaded to the national DNA database in 1990, but received no matches. The trail had gone cold, until 24 years later. In 2014, a woman was arrested for domestic abuse and had her DNA fingerprints taken. This fingerprint matched very closely to that of the DNA taken from the crime scene in 1984. The case was reopened, and all the DNA fingerprints taken from male relatives. A match was found belonging to the woman’s father Christopher Hampton. Hampton was convicted for a crime that seemed unsolvable, and the evidence both from blood and semen analysis was concrete.

How can DNA analysis aid solve other crimes? Well, via analysis of either the mitochondrial DNA or the Y-chromosome, for instance when confirming a dead body’s identity. A mitochondrial DNA analysis can be performed. This works in a similar way to STR fingerprinting. As the mitochondrial DNA is passed via the egg, it requires a female sample to be compared to their biological mother. So if the mother has reported the body missing, a sample of DNA from the body can be amplified and compared to that of the mother. If there are few discrepancies between the two, it can be confirmed that the body is that of the daughter. Limitation to this is it requires a maternal relationship to a daughter. So what if the body is that of a male? A similar process with the same logic behind it can be used by looking at relationship between the Y chromosomes of a father and his son, as there will be very little variation between the two samples. Both techniques can be used to confirm the identity of a missing person.

Forensic scientists also utilise entomology (the study of insects), to be able to tell how long a body has been there. This works by tracking the mini ecosystem of insects and their life cycles on a decomposing cadaver. Different species of insect feast on the flesh at different stages of the body decomposing. Thus, analysis of the age of the maggots at each stage of the organisms’ growth and population sizes can tell you how many generations of fly have been there. The younger the maggot and the fewer the maggots, the shorter the time the body has been there, and this can give some idea as to when the murder was committed.

Over the years, many advances in science have enhanced the way that the police can fight crime, as forensics answer questions that would have previously been unanswerable. Forensics helps solve thousands of crimes every day, and that’s down to the unique properties of the human genetic code.

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