Short Tandem Repeat Markers for Human Identification - Essay Example

STR Markers for Human Identification INTRODUCTION: Forensic science is a specialism where scientific investigations was done to help judges to solve legal issues, It has wide spectrum of investigation methods based on different area of science like biology, chemistry, physics mathematics and combination of all. The classical examples are the analysis of finger prints; Blood stain investigations, identification of explosives, recorded sound and handwriting etc. But in recent past biological technique have taken center stage in identification of criminals over all other techniques and that is based on DNA composition of individual. As all living things contain DNA, and all DNA exhibits variability both among and within species, any biological material associated with a legal case carries in it information about its source (Mark A. Jobling e'tal).The major advantage of DNA based technique is requirement of very small amount of sample like drop of body fluid, cigarette butts , or hair is enough to determined the identity of criminal or person related to crime. There is almost century old history of forensic genetics started with human ABO blood group polymorphisms. It was used for quite long time but having few drawbacks it was not used quite frequently in present investigations. The major back trough occurred in 1984 when Alec Jeffreys identified the hyper variable loci known as MINISATELLITE. The banding pattern obtained from this micro satellite known as DNA fingerprinting and was widely used since it discovery. The only drawback of this system is complicated data and difficulty to interpret. Than Development of 'single-locus probes' (SLPs) here it revealed only a single, highly polymorphic, restriction fragment length polymorphism. And hence simplifying interpretation. In Typical SLP based investigation four SLPs were used successively to probe a Southern blot, yielding eight hyper variable fragments per individual.( Mark A. Jobling e'tal). SLP is widely used technique and still in used but limitation of this system realized in many instance where sample is mixture of many origins and at this juncture PCR based techniques came in to lime light particularly STR (Short tandem repeat).Development of STR based technique along with Automated DNA sequencing technology makes it most powerful technique for crime investigations. Being sensitive, high discriminating power and reliable in case of mix sample qualifies STR as perfect forensic tool. Fig 1 shows chronologic evolution of forensic genetics (Nat Rev Genet. 2004 Oct; 5 (10):739-51.) STR (Short tandem repeat): STR, short tandem repeat are short sequences of DNA normally of length 2-5 base pairs. and one can fine numerous time repetition of these sequences. There are almost 10,000 published STR sequence available in different literature. most widely used STRs are tetra or penta tendon repeat as there are certain advantage of smaller repeat over larger one, like it can withstand DNA degradation and avoids preferential amplification during PCR. Similarly short repeat is getting attention in terms of genetic diseases like tri-nucleotide repeat associated Huntington disease. STR markers are mainly classified under two category 1) Autosomal STR and 2) Y-chromosomal STR. 1) Autosomal STR: Initially there were four simple STR were used but there were high probability of similar matching hence later on it raise two more Complex STR along with PCR assay targeted at the XY-homologous amelogenin genes. As Forensic experts realized the capability of STR based identification system more attention was provided toward development of standard technique. In this line four more loci were added in to the over all process and name as SGM plus. Based on this development effort were made to set up STR database to mention record of individual having crime history. There are some differences between the overall techniques employed for STR typing in different countries and legal systems. For example in USA FDI CODI Shaving 16 STR loci along with the amelogenin sex test, while in Germany there are eight loci. Now efforts are underway to make this process uniform through out the systems. 2) Y-Chromosome STR: Variability in Autosomal STR arises due to Recombination, Chromosomal ressortment and Mutation while in case of Y chromosome STRs it is mainly due to mutation, hence it have higher probability of match compared to autosomal STR, But the biggest advantage for Y chromosome STR based technique is, it is male specific and hence highly useful in case of sample having both male and female DNA. And autosomal STR analysis is difficult to perform. Now there are around 219 known useful STRs present on Y chromosome out of which only 9 to 11 loci are used for typing. Table 1 FBI Core STR Loci with chromosomal position (Source: http://www.cstl.nist.gov/biotech/strbase/fbicore.htm) Advantages of STR over other DNA based technique: There are large number of STR (more than 2000 markers are mapped and published) Smaller size makes it more resistance against lost of sample due to DNA degradation. and it is possible to differentiate single base pare variation. Optimum size 4-5 repeat compared to VNTR(50-80bp) which is non reliable in case of degraded DNA and microsatellite 2-3 repeat which have less fidelity in terms of PCR amplification. Probability of chance matching is >10-3 makes it more reliable over others. Variability is mainly due to number of repeat and not due to insertion or deletion. Use of only 9-13 loci marker makes banding pattern and further analysis simpler than other. Highly sensitive and requires small quantities of sample which subsequently amplified with PCR. Technological advancement like automated DNA sequencing and florescence based detections makes STR faster and sensitive over other conventional techniques like RFLP and VNTR. Availability of exhaustive database having thousands of entry and constant up gradation makes crime interlink and easy accessibility of criminal's record. Table 2. Comparison of STR and other DNA based techniques Reference: 1) Mark A. Jobling and Peter Gill Nature Genetic Review.2004 (5), 739-751 2) James W. Schumm Notes Magazine Numbern1996 (58)12-18. 3) Cotton, E. A. et al. Validation of the AMPFlSTR SGM plus system for use in forensic casework. Forensic Science International. 2000(112)151-161. 4) Butler, J. M. Forensic DNA Typing: Biology and Technology Behind STR Markers (Academic Press, 2001). 5) http://www.cstl.nist.gov/biotech/strbase/fbicore.htm Read More