SHORT TANDEM REPEATS
(STR)
The short tandem repeat (STR) methodology for extracting DNA is the system most widely used form of DNA fingerprinting. This system is based on the features of PCR, as it utilizes specific areas that have short sequential repeat DNA.
The STR analyzes how many times base pairs repeat themselves on a particular location on a strand of DNA. The big advantage in this method is that the DNA comparisons can match the possibilities into an almost endless range.
These repeated sequences come in various sizes and are classified as on the basis -
The STR analyzes how many times base pairs repeat themselves on a particular location on a strand of DNA. The big advantage in this method is that the DNA comparisons can match the possibilities into an almost endless range.
These repeated sequences come in various sizes and are classified as on the basis -
- According to the length of the core repeat units.
- The number of contiguous repeat units.
- The overall length of the repeat region. [27], [28] & [33]
CHARACTERISTICS OF "STR"
- DNA regions with short repeat units (usually 2-6 bp in length) are called Short Tandem Repeats (STR). STRs are found surrounding the chromosomal centromere (the structural center of the chromosomes).
- STRs have become popular DNA markers because they are easily amplified by Polymerase Chain Reaction (PCR) without the problem of differential amplification; that is, the PCR products for STRs are generally similar in amount, making analysis easier.
- An individual inherits one copy of an STR from each parent, which may or may not have similar repeat sizes. [27]
In the figure alongside, Schematic of short tandem repeat (STR) markers. In this heterozygous example, differences in the number of CA repeats (6 vs 9) between the two alleles result in two distinct polymerase chain reaction peaks by capillary electrophoresis. These peaks can be used as genetic markers of individual identity.
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1. For human identification purposes, it is important to have DNA markers that exhibit the highest possible variation in order to discriminate between samples. It is often challenging to obtain PCR amplification products from forensic samples because either the DNA in those samples is degraded, or mixed, such as in a sexual assault case.
2. Because of their smaller size, STR alleles can also be separated from other chromosomal locations more easily to ensure closely linked loci are not chosen. [31] |
3. STR alleles also have lower mutation rates, which makes the data more stable and predictable.
4. Because of these characteristics, STRs with higher power of discrimination are chosen for human identification in forensic cases on a regular basis. It is used to identify victim, perpetrator, missing persons, and others. [31]
4. Because of these characteristics, STRs with higher power of discrimination are chosen for human identification in forensic cases on a regular basis. It is used to identify victim, perpetrator, missing persons, and others. [31]
PROCESS-
- The "STR" or the Microsatellites are AmpFLP loci.
- The advantages with the STRs are that they are easy to amplify, they have relatively small allele sizes, and can be separated using Polyacrylamide Gel Electrophoresis (PAGE).
- Following PAGE, detechtion of DNA fragments can be achieved using non- isotopic Methods, without the need of the hybridization step.
- A horizontal, discontinuous PAGE system to resolve AmpFLP and STR alleles, and a silver stain method for the detechtion of the amplification products has been developed.
- A discontinuous, vertical PAGE also exists.
- Using any of the two methods, DNA extraction to data analysis can be performed in one or two days.
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- Multiple Flourophors allow for internal lane standards andfor multiplexing many AmpFLP or STR systems in the same lane.
- Florescently labeled DNA fragments are detected in real time using Laser technologies to visualize the alleles.