DNA Profiling
DNA profiling is the analysis of STR or short tandem repeats, present in DNA on a specific locus. Led by a combination of molecular biology, genetics and information technology; analysis of DNA makes it possible to identify individuals using their own genetic sequence. DNA profiling has greatly contributed towards molecular science in changing conventional methods of analysis in areas such as; bio-technology and forensic science (Harris, 2008). 26 years since its discovery, DNA analysis has helped scientists interpret many answers to questions which in fact have change the very basis of life.
DNA profiling was first discovered by British geneticist Sir Alec Jeffreys in the year 1984. It was formally known as DNA fingerprinting and was later modified to become DNA profiling. His research at the University of Leicester found that certain regions of DNA present in chromosomes contained DNA sequences that repeated over and over again in a particular loci. He also discovered the number of short tandem repeats or STR’s presents in a single locus differed from individual to individual, this key observation led Dr. Jeffreys to develop a technique to analyse the sequence present in DNA in order to distinguish individuals. Together with his colleagues he developed a method to scan and interpret the lengths of the short tandem repeats present in DNA which helped perform human identity tests. Scientifically the procedure was known as mini satellite hybridization; however Dr. Jeffreys named the process DNA profiling in order to create a much simplified understanding that every human posses a unique DNA as their fingerprints. This scientific breakthrough made geneticist research deeper in the study of DNA molecule itself.
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| Sir Alec Jeffreys |
Just as every other molecular biological technique DNA profiling also origin from the human cell, which is the basic unit of life. An average human being is composed of approximately 100 trillion cells, with each cell containing the exact genetic sequence. Deoxyribonucleic acid, or DNA, is sometimes referred to as the genetic blueprint of life because it stores the information necessary for passing down genetic attributes to future generations. Although a vast majority of human DNA molecules (over 99.7%) is the same between people, only a small fraction of DNA (around 0.3%) differs between individuals (John Butler, 2004). There are certain regions of DNA along chromosomes that code for proteins; however, 95% of the DNA is non-coding, these non-coding proteins do not perform any important functions hence they are also known as junk DNA. Though non-coding DNA seems useless their presence is greatly valued in DNA profiling. About 30-40% of this DNA consists of short tandem repeats which are repeated many times within a specific location of DNA double helical chain (Green, 2003). In fact it is the number of times that these segments of STR’s, or mini-satellite DNA, are repeated that produces the variation in individuals. Dr. Jeffrey’s analysis of these mini-satellites led him to a remarkable discovery of DNA profiling. Unless an individual have an identical twin, his or her genetic fingerprint will remain unique and different from everyone else. This is what makes DNA so valuable in making a genetic fingerprint.
There are many different ways and techniques in creating a DNA profile which depends on a number of factors such as; cost, time available for analysis and the quality and quantity of the DNA sample available. How different the procedure may be they all follow the fundamental principles of DNA profiling which Sir Alec Jeffreys described. An individual’s genetic makeup can be directly determined from very small samples of DNA present in blood stains, saliva, skin cells, bone, hair, semen, or other biological material. Since all body cells of an individual contains the same DNA, virtually any living or non-living sample can be processed to create a DNA fingerprint, thus helping forensic scientist and researches to solve crimes with very little evidence.
One such technique is Restriction fragment length polymorphism (RFLP) analysis. This was the
method used by Sir Alec Jeffreys in order to create his first ever DNA fingerprint. In RFLP analysis the target DNA sample is broken down in to small parts by restriction enzymes. The enzyme chosen cut’s either sides of the DNA strand, leaving them intact so their variable lengths are unaltered. These fragments are run through gel electrophoresis which will separate them according to their length. Shorter DNA segments moved further away from their original location forming characteristic patterns of bands. Once the electrophoresis is complete it’s transferred to a nylon membrane by the use of a blotting technique. The sheet is stained so the different lengths of DNA bands are visible to the naked eye. After staining, the paper it’s exposed to a solution which contains radioactively labelled probes. The probes hybridize with the complimentary DNA segment forming an autoradiograph. Lastly the autoradiograph is printed out with the band patterns formed by electrophoresis creating the final DNA profile. However, there are major drawbacks in RFLP analysis, mainly due to the high quality of DNA sample needed and the time taken for the process to complete. RFLP analysis require DNA samples with fairly large quantities without any contamination and can take up to 2 or 3 weeks to obtain final results (Freeman, 2008). Although this technique is not often used today in large scale forensic analysis, all other new techniques which succeeded follow the same principle.
Polymerase Chain Reaction, or PCR analysis, is another mechanism which is used to create a DNA fingerprint. This method is basically a greater modification of the RFLP technique. In this process PCR generates thousands and millions of copies of a particular DNA sequence and replicates the sample DNA to desired amount within minute’s time using thermal cycling and enzyme activity. Identical copies of the sample DNA is made much like DNA copies itself in a cell. This technique brought light a new era in DNA analysis. In fact it replaced the contemporary method of RFLP analysis saving both time & cost due to the high accuracy and delivery of amplified DNA within a short period of time. Today with the aid of robotics and computer-aided testing, PCR systems have been made both fully automated and reliable (Harris, 2008). The benefit of using very little sample of contaminated DNA and minimal time to complete analysis, made PCR a sort after technique in the field of forensic science where there is less evidence present and high accuracy needed. Applications of DNA profiling have become very familiar at present because of its particular relevance in forensic science and the system of justice. DNA profiling has been used to solve a range of crimes, including 83 killings and 184 rapes in Britain alone (BBC, 2009). The Federal Bureau of Investigation has chosen 13 specific STR loci to serve as the standard for DNA Profiling worldwide because of the probability of two individuals having the same genetic profile being high as 1 in 1 billion or greater (John Butler, 2005). Many countries have initiated programs to maintain National DNA Index Systems or NDIS databases which stores DNA profiles of individuals and automatically searches and matches the DNA evidence in linking suspects to unsolved crimes. The technique of biological profiling became well known to the general public through the introduction of DNA evidence in solving the controversial murder committed by American football star O.J. Simpson in 1995. DNA evidence plays a vital role in the modern justice system; however, it’s not only limited to crime, paternity testing and human identifications is also conducted by the use of genetic profiling. DNA profiling, just like solving crimes, can help predict an individual’s susceptibility to a certain disease like Alzheimer’s disease, breast cancer and diabetes. Historians are also turning to sophisticated methods of DNA profiling to learn more about the past, especially evolution of humans by using mitochondrial DNA analysis in order to help unwind the diversity of different races and its migration across the world. In 1998, PCR based analysis was used in the identification of the bones of Tsar Nicholas II and his family. The Bolsheviks executed Tsar Nicholas II and his family after the Russian revolution in 1917; however, after 80 years their remains were identified with the use of forensic techniques, including DNA profiling (Williams, 2003). Although the applications of DNA profiling sound simple, there are number of other researches being carried out in order to apply it in the fields of medical science and pharmaceuticals.
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| DNA Profile |
It is hard to believe the advancement in molecular science, specially the progress made in DNA
Profiling. Technology has been able to penetrate into DNA, the biological molecule of life itself. Sir Alec Jeffreys made one the remarkable discoveries since Watson and Crick deducing the structure of DNA in 1953. As each day progress new techniques gain light in making DNA profiles much faster and accurate. Wide range of applications brings in greater demand for improvements thus creating a better technique to solve tomorrow’s questions. President Bill Clinton made his remark on DNA profiling at the launch of the first human genome project stating; “Today, we are learning the language in which God created life” (President Bill Clinton, 26 June 2000 announcing the first draft sequence of the human genome).
BBC NEWS | UK | DNA profiling 25 years old. (2009). BBC NEWS | News Front Page. Retrieved June 18, 2010, from http://news.bbc.co.uk/2/hi/uk_news/8247641.stm
Butler, J. M. (2005). Forensic DNA Typing, Second Edition: Biology, Technology, and Genetics of STR Markers (2 ed.). Toronto: Academic Press.
Freeman, S. (2008). HowStuffWorks "How DNA Profiling Works". Howstuffworks "Science".
Retrieved June 16, 2010, from http://science.howstuffworks.com/dna-profiling.htm
Green, N., Soper, R., Stout, W., & Taylor, D. (2003). Biological Science 1 and 2. New York: Cambridge University Press.
Harris, W. (2008). HowStuffWorks "How DNA Evidence Works". Howstuffworks "Science". Retrieved June 16, 2010, from http://science.howstuffworks.com/genetic-science/dna-evidence.htm
Williams, G. (2003). Advanced Biology for You. Cheltenham: Nelson Thornes.
