Uses of DNA Technology
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DNA typing was first used in Great Britain for law enforcement purposes in the mid- 1980s and has revolutionized forensic science and the ability of law enforcement to match perpetrators with crime scenes. It wasn't employed in the United States until 1987. DNA profiling has changed forensic science. DNA technology has given police and the courts a means of identifying the suspects of rapes and murders. Thousands of cases have been closed and innocent suspects freed with guilty ones punished because of the power of a silent biological witness at the crime scene. Today, the Federal Bureau of Investigation performs the bulk of the forensic DNA typing for local and state law enforcement agencies. In criminal investigations, DNA from samples of hair, bodily fluids or skin at a crime scene is compared with those obtained from suspected suspects. (http://faculty.ncwc.edu) ‘DNA fingerprinting,' or DNA typing (profiling) as it is now known, was first described in 1985 by an English geneticist named Alec Jeffreys. Dr Jeffreys found that certain regions of DNA contained DNA sequences that were repeated over and over again next to each other. He also discovered that the number of repeated sections present in a sample could differ from individual to individual. By developing a technique to examine the length variation of these DNA repeat sequences, Dr Jeffreys created the ability to perform human identity tests. (John Butler, 2005) Sir Alec John Jeffreys, was born 9 January 1950 at Oxford in Oxfordshire. He is a professor of genetics at theUniversity of Leicester, and he became an honorary freeman of the City of Leicester on 26 November 1992. (Leicester City Council, 1992) In 1994, he was knighted by her Majesty Queen Elizabeth II of the United Kingdom, for Services to Science and Technology.
Jeffreys had a "eureka moment" in his lab in Leicester after looking at the X-ray film image of a DNA experiment at 9:05 am on Monday 10 September 1984, which unexpectedly showed both similarities and differences between the DNA of different members of his technician's family. (BBC Radio, December 9, 2007) Within about half an hour, he realized the possible scope of DNA fingerprinting, which uses variations in the genetic code to identify individuals. The method has become important in forensic science to assist police detective work, and it has also proved useful in resolving paternity and immigration disputes. (BBC Radio, December 9, 2007) The method can also be applied to non-human species, for example in wildlife population genetics studies. Before his methods were commercialized in 1987 his laboratory was the only center carrying out DNA fingerprinting in the world, and during this period of about two or three years it was very busy, receiving inquiries from all over the globe. (Neston, Giles February 2, 2004) The technique used by Dr Jeffreys to examine the VNTRs was called restriction fragment length polymorphism (RFLP) because it involved the use of a restriction enzyme to cut the regions of DNA surrounding the VNTRs. This RFLP method was first used to help in an English immigration case and shortly thereafter to solve a double homicide case. Since that time, human identity testing using DNA typing methods has been widespread. The past 25 years have seen tremendous growth in the use of DNA evidence in crime scene investigations as well as paternity testing. Today over 100 public forensic laboratories and several dozen private paternity testing laboratories conduct hundreds of thousands of DNA tests annually in the United States. In addition, most countries in Europe and Asia have forensic DNA programs. The number of laboratories around the world conducting DNA testing will continue to grow as the technique gains in popularity within the law enforcement community. (John Butler, 2005)
How DNA Typing Is Done
Only one-tenth of a single percent of DNA (about 3 million bases) differs from one person to the next. (Internet Source www.ornl.org, 2009) Scientists can use these variable regions to generate a DNA profile of an individual, using samples from blood, bone, hair, and other body tissues and products. In criminal cases, this generally involves obtaining samples from crime-scene evidence and a suspect, extracting the DNA, and analyzing it for the presence of a set of specific DNA regions (markers). Scientists find the markers in a DNA sample by designing small pieces of DNA (probes) that will each seek out and bind to a complementary DNA sequence in the sample. A series of probes bound to a DNA sample creates a distinctive pattern for an individual. Forensic scientists compare these DNA profiles to determine whether the suspect's sample matches the evidence sample. A marker by itself usually is not unique to an individual; if, however, two DNA samples are alike at four or five regions, odds are great that the samples are from the same person. If the sample profiles don't match, the person did not contribute the DNA at the crime scene. If the patterns match, the suspect may have contributed the evidence sample. While there is a chance that someone else has the same DNA profile for a particular probe set, the odds are exceedingly slim. Many judges consider this a matter for a jury to take into consideration along with other evidence in the case. (Internet Source www.nfstc.org, 2009) Experts point out that using DNA forensic technology is far superior to eyewitness accounts, where the odds for correct identification are about 50:50. The more probes used in DNA analysis, the greater the odds for a unique pattern and against a coincidental match, but each additional probe adds greatly to the time and expense of testing. Four to six probes are recommended. Testing with several more probes will become routine, observed John Hicks (Internet Source www.alabany.edu/nerfi, 2009). He predicted that DNA chip technology will enable much more rapid, inexpensive analyses using many more probes and raising the odds against coincidental matches.
Types of DNA Technologies
Restriction Fragment Length Polymorphism - RFLP is a method used by molecular biologists to follow a particular sequence of DNA as it is passed on to other cells. RFLPs can be used in many different settings to accomplish different objectives. RFLPs can be used in paternity cases or criminal cases to determine the source of a DNA sample. RFLPs can be used determine the disease status of an individual. RFLPs can be used to measure recombination rates which can lead to a genetic map with the distance between RFLP loci measured in centiMorgans. (Internet Source www.bio.davidson.edu, 2009) Total DNA is first extracted from the microbial community and the16S rRNA gene is amplified from samples using fluorescently-labeled forward and reverse primers. Next, the PCR product is purified and subjected to restriction enzyme digestion with enzymes that have 4 base pair recognition sites. This step generates fluorescently-labeled terminal restriction fragments. The digested products are then separated and detected on an appropriate electrophoresis platform. For a given sample the terminal fragments will contain a fluorescent label at the 5' end and will therefore be detected. The output will be a series of peaks (fragments) of various sizes and heights that represents the profile of that sample. (Osborn, A. M., Moore, R.B. and Timmis, K.N., 2000)
Polymerase chain reaction - PCR is used to make millions of exact copies of DNA from a biological sample. DNA amplification with PCR allows DNA analysis on biological samples as small as a few skin cells. A polymerase is a naturally occurring enzyme, a biological macromolecule that catalyzes the formation and repair of DNA (and RNA). The technique was made possible by the discovery of Taq polymerase, the DNA polymerase that is used by the bacterium Thermus auquaticus that was discovered in hot springs. This DNA polymerase is stable at the high temperatures need to perform the amplification, whereas other DNA polymerases become denatured. Since this technique involves amplification of DNA, the most obvious application of the method is in the detection of minuscule amounts of specific DNAs. This is important in the detection of low level bacterial infections or rapid changes in transcription at the single cell level, as well as the detection of a specific individual's DNA in forensic science. It can also be used in DNA sequencing, screening for genetic disorders, site specific mutation of DNA, or cloning or subcloning of cDNAs. (Internet Source www.plattsburgh.edu, 2009)
Short tandem repeat - STR technology is used to evaluate specific regions (loci) within nuclear DNA. Variability in STR regions can be used to distinguish one DNA profile from another. The Federal Bureau of Investigation (FBI) uses a standard set of 13 specific STR regions for CODIS. CODIS is a software program that operates local, state, and national databases of DNA profiles from convicted offenders, unsolved crime scene evidence, and missing persons. The odds that two individuals will have the same 13-loci DNA profile is about one in a billion. (Internet Source www.ornl.org, 2009) The Federal Bureau of Investigation (FBI) has chosen 13 specific STR loci to serve as the standard for CODIS. The purpose of establishing a core set of STR loci is to ensure that all forensic laboratories can establish uniform DNA databases and, more importantly, share valuable forensic information. If the forensic or convicted offender CODIS index is to be used in the investigative stages of unsolved cases, DNA profiles must be generated by using STR technology and the specific 13 core STR loci selected by the FBI. (Internet Source www.dna.gov, 2009)
Mitochondrial DNA analysis - mtDNA can be used to examine the DNA from samples that cannot be analyzed by RFLP or STR. Nuclear DNA must be extracted from samples for use in RFLP, PCR, and STR; however, mtDNA analysis uses DNA extracted from another cellular organelle called a mitochondrion. (Internet Source www.fbi.gov, 2009) While older biological samples that lack nucleated cellular material, such as hair, bones, and teeth, cannot be analyzed with STR and RFLP, they can be analyzed with mtDNA. In the investigation of cases that have gone unsolved for many years, mtDNA is extremely valuable. (Internet Source www.dna.com, 2009) All mothers have the same mitochondrial DNA as their offspring. This is because the mitochondria of each new embryo come from the mother's egg cell. The father's sperm contributes only nuclear DNA. Comparing the mtDNA profile of unidentified remains with the profile of a potential maternal relative can be an important technique in missing-person investigations. (Melton, T. et. al., 2001)
Y-Chromosome Analysis - The Y chromosome is passed directly from father to son, so analysis of genetic markers on the Y chromosome is especially useful for tracing relationships among males or for analyzing biological evidence involving multiple male contributors. Y chromosome analysis is a useful technique for analyzing DNA that can be likened in one sense to studying male surnames. Think about the way that male surnames are passed down from one generation to another and continue on through sons. This mechanism is a simplistic representation of Y chromosomes. A son inherits a Y chromosome from his biological father and he also inherits an X chromosome from his biological mother. Conversely, a female would inherit an X chromosome from her biological mother and an X chromosome from her biological father. (Internet Source www.esploredna.co.uk, 2009) As such, when scientists study Y chromosomes, they are studying these chromosomes as they are inherited over time through males in a familial line. This type of DNA analysis has important ramifications for scientists wishing to investigate the familial ties between male members. (Internet Source www.ncbi.nlm.nih.gov, 2009)
No field has benefited more from the tools of molecular biology than forensic science. DNA technology affords the forensic scientist the ability to eliminate individuals who have been falsely associated with a biological sample and to reduce the number of potential contributors to a few (if not one) individuals. Inculpations are strong evidence regarding the source of the biological sample. Today, some wrongly convicted people have been exonerated because of DNA evidence. Moreover, in casework, individuals are excluded routinely. Since the inception of forensic DNA profiling, there has been a debate in the legal setting regarding admissibility on the methods and the practices of computing DNA profile frequencies. While the scientific basis of DNA typing were sound, both the methodology and the statistical interpretations were aggressively challenged in court. The methods challenge focused on reliability and validity testing. The statistics debate focused on the reliability of the assumption of independence for applying the product rule to derive estimates of DNA profile frequencies.
Butler, John “Forensic DNA Typing: Biology, Technology, and Genetics of STR Markers” 2nd Edition 2005 Elsevier Science
Desert Island Discs, “Desert Island Discs with Alec Jeffreys” BBC Radio 4 December 9, 2007
Leicester City Council “List of persons upon whom the honorary freedom of the city has been conferred” http://www.leicester.gov.uk/aboutleicester/lordmayorcivic/freeman/honorary-freemen/list-of-freemen Retrieved December 10, 2009
Newton Giles, “Discovering DNA fingerprinting: Sir Alec Jeffreys describes its development”. Wellcome Trust. http://genome.wellcome.ac.uk.doc Retrieved December 10, 2009
Osborn, A.M., Moore, R.B. and Timmis, K.N. (2000). An evaluation of terminal-restriction fragment lengty polymorphism (T-RFLP) analysis for the study of microbial community structure and dynamics. Environmental Microbiology 2(1): 39-50.
Journal of Forensic Science “Diversity and Heterogeneity in Mitochondrial DNA of North American Populations.” January 2001; 46 (1):46-52. Melton T. et al
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