DNA Typing - Polymerase chain reaction
When Dr, Kary Mullis discovered that DNA could be copied in the laboratory much as it is in the natural world, the evolution of DNA testing significantly advanced.
The copying process (PCR), uses an enzyme (Polymerase) to replicate DNA regions in the test tube. By repeating the copying process, a small number of DNA molecules can be reliably increased to billions within several hours.
The initial technique used RFLP (Restriction Fragment Length Poylmorphism), requires a biological sample about the size of a quarter, but PCR can be used to reproduce millions of copies of the DNA contained in a few skin cells. Since PCR analysis only takes a small quantity of DNA, it can enable the laboratory to analyze highly degraded evidence for DNA. However, because the sensitive PCR technique replicates any and all of the DNA contained in an evidence sample, greater attention must be payed to contamination issues when identifying, collecting, and preserving DNA evidence.
The copying process (PCR), uses an enzyme (Polymerase) to replicate DNA regions in the test tube. By repeating the copying process, a small number of DNA molecules can be reliably increased to billions within several hours.
The initial technique used RFLP (Restriction Fragment Length Poylmorphism), requires a biological sample about the size of a quarter, but PCR can be used to reproduce millions of copies of the DNA contained in a few skin cells. Since PCR analysis only takes a small quantity of DNA, it can enable the laboratory to analyze highly degraded evidence for DNA. However, because the sensitive PCR technique replicates any and all of the DNA contained in an evidence sample, greater attention must be payed to contamination issues when identifying, collecting, and preserving DNA evidence.
DNA Typing - STR Analysis
A forensic analysis that evaluates specific regions (loci) that are found on nuclear DNA. The variable (polymorhpic) nature of the Regions in the DNA that are analyzed for forensic testing intensifies the discrimination between one DNA profile and another. For example, the likelihood that two individuals (except identical twins) will have the same 13-Loci* DNA profile can be as high as 1 in 1 billion or greater
*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.
*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.
DNA Typing - Y-Chromosome Analysis
- Several genetic markers have been identified on the Y chromosome that can be used in forensic applications
- Y-Chromosome markers target only the male fraction of a biological sample, this technique can be very valuable if the laboratory detects complex mixtures (multiple male contributors) within a biological evidence sample
- The Y chromosome is transmitted directly from a father to all of his sons, it can be a trace of family relationships among males
- Advancement in Y-chromosome testing may eventually eliminate the need for laboratories to extract and separate semen and vaginal cells (for example, from a vaginal swab of a rape kit) prior to analysis
- Y-Chromosome markers target only the male fraction of a biological sample, this technique can be very valuable if the laboratory detects complex mixtures (multiple male contributors) within a biological evidence sample
- The Y chromosome is transmitted directly from a father to all of his sons, it can be a trace of family relationships among males
- Advancement in Y-chromosome testing may eventually eliminate the need for laboratories to extract and separate semen and vaginal cells (for example, from a vaginal swab of a rape kit) prior to analysis
DNA Typing- Mitochondrial Analysis
- allows forensic labs to develop DNA profiles from evidence that may not be suitable for RFLP or STR.
- While RFLP and STR techniques analyse DNA extracted from the nucleus of a cell, Mitochondrial Analysis analyzes DNA found in a different part of the cell, such as hair shafts,bones and teeth
- It cab be very valuable to the investigation of an unsolved case, for example, a case log may show that biological evidence in the form of blood, semen, and hair was collected in a particular case, but that all were improperly stored for a long period of time
- PCR analysis sometimes enables the crime labs to generate a DNA profile from very degraded evidence, however it is possible that the blood and semen would be so highly degraded that nuclear DNA analysis would not yield a DNA profile. Luckily, the hair shaft could be subjected to MTDNA analysis and thus be the key to solving the case
- While RFLP and STR techniques analyse DNA extracted from the nucleus of a cell, Mitochondrial Analysis analyzes DNA found in a different part of the cell, such as hair shafts,bones and teeth
- It cab be very valuable to the investigation of an unsolved case, for example, a case log may show that biological evidence in the form of blood, semen, and hair was collected in a particular case, but that all were improperly stored for a long period of time
- PCR analysis sometimes enables the crime labs to generate a DNA profile from very degraded evidence, however it is possible that the blood and semen would be so highly degraded that nuclear DNA analysis would not yield a DNA profile. Luckily, the hair shaft could be subjected to MTDNA analysis and thus be the key to solving the case
Short Tandem Repeats Analysis
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Mitochondrial DNA
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PCR Cycle
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