DNA typing
DNA typing is a forensic technique used to identify individuals by analyzing specific regions of their DNA. This process involves separating amplified DNA fragments to create profiles that can be compared with known samples, helping to establish similarities or differences between individuals. Its significance lies in its ability to provide compelling evidence in criminal investigations, supporting the conviction of guilty parties or the exoneration of innocent individuals.
The human genome is composed of approximately three billion nitrogenous bases, with only about one percent contributing to an individual’s unique DNA profile, except for identical twins who share the same DNA. Various DNA typing techniques, such as short tandem repeats (STRs) and single nucleotide polymorphisms (SNPs), are employed depending on the sample type and required accuracy. The analysis often utilizes genetic analyzers that employ capillary electrophoresis technology to separate DNA fragments based on their size and charge.
Advancements in DNA technologies, like massively parallel sequencing, are enhancing the capabilities of forensic science by enabling quicker and more comprehensive sequencing of DNA, which could lead to a deeper understanding of genetic diversity and improve forensic outcomes. Overall, DNA typing remains a crucial tool in the field of forensic investigation, providing a scientific basis for identifying individuals in legal contexts.
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DNA typing
DEFINITION: Process of separating amplified DNA fragments in order to obtain profiles that can be compared with known persons’ profiles to establish similarities or differences that will include or exclude those persons as possible sample donors.
SIGNIFICANCE: In forensics, the collection and processing of evidence are steps in crime scene investigations that are expected ultimately to aid in the process of conviction or exoneration of potential suspects. DNA typing can provide a unique “picture” that can identify an individual. If the DNA found in a crime scene sample is a perfect match for that of a known sample, this constitutes a powerful piece of evidence that can often help in the conviction of the guilty; lack of an exact match can potentially exonerate an individual.
The human genome comprises approximately three billion nitrogenous bases, of which 99 percent are identical across the human population, leaving only 1 percent that makes each person’s (deoxyribonucleic acid) unique; the only exception is identical twins, who have identical DNA. When DNA is amplified, an analyst uses a set of primers to target locations in the molecule that are known to vary between individuals. Within these varying regions, however, possible similarities still exist; for example, at locus A, person X and person Y are both heterozygous with alleles named 13, 15. In locus C, these same individuals are homozygous for allele 12, but the fact that they share the same alleles for these two markers does not indicate that they are the same person. The power of DNA discrimination is evident when a combination of markers (usually thirteen) gives at least one nonmatching allele between the samples being typed, indicating that the samples did not come from the same individual.
Among the different human DNA typing techniques are restriction fragment length polymorphisms (RFLPs; often referred to as variable number of tandem repeats, or VNTRs), single nucleotide polymorphisms (SNPs), short tandem repeats (STRs), mitochondrial (mtDNA), and Y chromosome. The technique selected depends on the source of the sample and the degree of separation needed. Some of the products of the techniques noted above are often visualized on agarose or polyacrylamide slab gels, whereas others can be loaded into genetic analyzers. Although gels separate DNA, the resolution obtained is much less exact than that required for DNA typing in criminal cases and often more complicated to analyze. Genetic analyzers are instruments based on capillary electrophoresis technology. They use a capillary loaded with a gel polymer that acts like a sieve and is able to separate amplified DNA fragments based on size and charge. The DNA is electrokinetically injected into the capillary and kept at a constant heat to keep the DNA traveling through it in a denatured (single-stranded) form. The shorter fragments travel faster and elute first out of the capillary; the longer fragments move more slowly and are retained longer in the capillary. As the fragments are eluted, the fluorescent tag associated with each fragment is detected and recorded by a camera, and the data are transferred to a computer. When all the fragments present have traveled through the capillary, a unique DNA fingerprint is obtained that can be profiled.
DNA technologies being developed in the 2020s have the potential to aid forensic science. Massively parallel sequencing (MPS) enabled the rapid, simultaneous sequencing of millions of DNA fragments. Third-generation sequencing (TGS) would resolve complex genomic regions such as repetitive sequences and structural variations. TGS could improve genome composition and enable a better understanding of genomic diversity.
Bibliography
Butler, John M. Forensic DNA Typing: Biology, Technology, and Genetics of STR Markers. 2d ed. Burlington, Mass.: Elsevier Academic Press, 2005
Carracedo, Angel. Forensic DNA Typing Protocols. Totowa, N.J.: Humana Press, 2005.
"DNA Typing by RFLP Analysis." Crime Scene and DNA Basics for Forensic Analysis, National Institute of Justice, 16 June 2023, nij.ojp.gov/nij-hosted-online-training-courses/crime-scene-and-dna-basics-forensic-analysts/history-and-types-forensic-dna-testing/dna-typing-rflp-analysis. Accessed 14 Aug. 2024.
Halimureti, Simayijiang and Jiangwei Yan. "Recent Developments in Forensic DNA Typing." Journal of Forensic DNA Typing, vol. 9. no. 4, Oct.-Dec. 2023, pp. 353-359, DOI: 10.4103/jfsm.jfsm‗127‗23. Accessed 14 Aug. 2024.
Moreno, Lilliana I., and Bruce McCord. “Separation of DNA for forensic Applications Using Capillary Electrophoresis.” In Handbook of Capillary and Microchip Electrophoresis and Associated Microtechniques, edited by James P. Landers. 3d ed. Boca Raton, Fla.: CRC Press, 2008.