This section focuses on the forensic and research-based uses of polymerase chain reaction (PCR). PCR is a fast and cost-efficient method used extensively in forensic labs and in scientific research worldwide.
How is PCR useful in forensics?
PCR involves amplifying specific regions of DNA which can then be analysed.
In forensic labs, this becomes vital when a sample of DNA recovered from a crime scene is too small to be used in DNA fingerprinting.
DNA fingerprinting involves identifying individuals based on the differences in their genomic make up. Samples can include blood, saliva, semen, skin cells, hair, etc. In this situation, forensic scientists are able to take a sample and amplify it millions of times so it can be easily used to produce a DNA profile. This can then be used to compare the DNA found at the crime scene against that of the victim and other prospective suspects. It can also be used to prove the innocence of suspects and to clear the name of previously indicted convicts. The animation below explains the process of DNA fingerprinting used in labs.
The first recorded use of PCR, invented by Kary Mullis in 1983, in a criminal trial was in 1993, when a woman was sexually assaulted and murdered in her Maryland home. A microscopic speck of saliva found in a soft drink bottle was used to incriminate handyman, Albert Givens, who was consequently sentenced to life in prison. This was a huge breakthrough in the field of forensic science, as it offered a solution to the problem of lack of viable DNA at crime scenes.
Other forensic applications of PCR
– PCR presents us with another possible solution to an ever-increasing issue – the imposing threat of terrorism. PCR can be used in both the identification of terrorists and of dangerous biological agents that can be easily weaponised in the form of bioterrorism and biowarfare. Biothreat agents include ebola, smallpox and yellow fever, all of which can be deadly to humans. They can spread easily from person to person, and many have the advantage of not showing symptoms for a prolonged period of time after the infection sets in. These agents can also be genetically modified to be immune to previous vaccination/treatments.
– PCR can also be used in the paternity tests, to identify the father of a child using both the father and the child in question’s DNA samples. Swabs from the inside of the mouth are generally used for analysis, although hair and blood can also be tested.
A paternity test where the father in question is the biological parent of the child.
How is PCR useful in research?
The use of PCR has become an indispensible tool, commonplace in scientific research worldwide,. Some examples of this are included below.
Archaeology
PCR can be used in the phylogenic analysis of ancient sources of DNA, known as aDNA. It can be used to track human migration patterns and to decipher relationships between species. Mostly mitochondrial DNA is used, as mitochondrial DNA appears several times in a cell, meaning it is easier to obtain and copy from degraded DNA samples. It also has a much smaller mutation rate, making lineages over thousands of years much easier to see.
However, despite its effectiveness, PCR has its limitations. PCR will multiply any undamaged DNA present, which could include modern human DNA or microbrial DNA which have accidentally been included during the amplification process, which may lead to contamination, and thus produce inaccurate results.
Cloning
In recent times, PCR cloning has become the preferred method of cloning instead of the traditional method of gene cloning. It differs from the traditional method as it is both a much faster process, and is more amenable to automation. PCR cloning takes place in vitro (i.e. in a tube) as opposed to in vivo like traditional cloning (i.e. in a vector). Its advantage is it allows cloning when there is not a large amount of DNA available, and it is less error-prone. However, traditional cloning is still the adopted method when the DNA template of the sequence of interest cannot be easily isolated.
Genetics
The Human Genome Project is an ongoing international research study, started in 1990, that focuses on identifying and mapping all of the genes in the human body, and is the largest collaborative biological study to date. Most of the mapping and sequencing techniques used in the project relied heavily on the PCR technique. Mapping can be used to locate disease-causing genes and track inheritance in family lineage.
References:
Biolabs, N.E. (2016) PCR cloning method. Available at: https://www.neb.com/applications/cloning-and-synthetic-biology/pcr-cloning (Accessed: 5 November 2016).
Advameg (2016) Polymerase chain reaction (PCR) -. Available at: http://www.faqs.org/espionage/Pa-Po/Polymerase-Chain-Reaction-PCR.html (Accessed: 3 November 2016).
Ancient DNA (2016) in Wikipedia. Available at: https://en.wikipedia.org/wiki/Ancient_DNA#History_of_ancient_DNA_studies (Accessed: 29 October 2016)
my VMC (2008) PCR (polymerase chain reaction). Available at: http://www.myvmc.com/investigations/pcr-polymerase-chain-reaction/ (Accessed: 20 October 2016).
Hedman, J., Knutsson, R., Ansell, R., Rådström, P. and Rasmusson, B. (2013) ‘Pre-PCR processing in bioterrorism preparedness: Improved diagnostic capabilities for laboratory response networks’, Biosecurity and bioterrorism : biodefense strategy, practice, and science., 11.
Use of DNA test cited in Williams murder trial increasing (1995) Available at: http://articles.baltimoresun.com/1995-03-12/news/1995071066_1_pcr-dna-appellate-courts (Accessed: 13 October 2016).
Newman, M.E., Parboosingh, J.S., Bridge, P.J. and Ceri, H. (2002) ‘Identification of archaeological animal bone by PCR/DNA analysis’, Journal of Archaeological Science, 29(1), pp. 77–84. doi: 10.1006/jasc.2001.0688.