Search

“The science of deduction and analysis is one which can only be acquired by long and patient study.”

Multiplex polymerase chain reaction (PCR) enables examination of many loci simultaneously, and this multi-locus genotyping information can increase interpretation capabilities beyond what is available from a single locus. Short tandem repeat (STR) profiles can be illustrated with simple triangles, line drawings, or more sophisticated tools for illustrating electropherograms. This chapter addresses single-source samples. An understanding of expected behavior of single-source DNA samples is important prior to attempting DNA mixtures containing multiple contributors.

Short tandem repeats or STRs are the mainstay of forensic DNA analysis. This article concentrates on the established autosomal identification STR markers as those of the Y and X are detailed in other articles in this encyclopedia. Eighteen core forensic identification STRs were expanded in 2010 to a total of 23 markers available in validated, off-the-shelf kits and in near-universal use worldwide. The genomic characteristics of these core STRs are outlined along with the standard typing procedure including an overview of irregular STR profile patterns plus a brief survey of future technical developments and new STRs.

Commercial multiplex kits capable of amplifying multiple DNA loci in a single tube reaction are essential to solving cases involving DNA evidence in crime labs. In this experiment, a selected commerical short tandem repeat (STR) multiplex kit will be used to amplify previously extracted DNA from buccal swabs and simulated evidence samples using the polymerase chain reaction following the manufacturer’s instructions. The amplified loci will be determined by the kit employed.

Publisher Summary STRs (short tandem repeat) have become popular DNA repeat markers because they are easily amplified by the polymerase chain reaction (PCR) without the problems of differential amplification. This is because both alleles from a heterozygous individual are similar in size since the repeat size is small. The number of repeats in STR markers can be highly variable among individuals, which makes these STRs effective for human identification purposes. STR repeats sequences are named by the length of the repeat unit. STR markers are usually identified in one of the two ways: searching DNA sequence databases such as GenBank for regions with more than six or so contiguous repeat units or performing molecular biology isolation methods. STRs are often divided into several categories based on the repeat pattern. Simple repeats contain units of identical length and sequence; compound repeats comprise two or more adjacent simple repeats; and complex repeats may contain several repeat blocks of variable unit length as well as variable intervening sequences. An STR typing kit consists of five components: a PCR primer mixture containing oligonucleotides designed to amplify a set of STR loci, a PCR buffer containing deoxynucleotide triphosphates, MgCl2, and other reagents necessary to perform PCR, a DNA polymerase, which is sometimes premixed with the PCR buffer, an allelic ladder with common alleles for the STR loci being amplified to enable calibration of allele repeat size, and a positive control DNA sample to verify that the kit reagents are working properly.

Short tandem repeat (STR) sequences are named by the length of DNA regions with the repeat unit. STRs have become popular DNA repeat markers because they are easily amplified by the polymerase chain reaction (PCR) without the problems of differential amplification. This chapter discusses the types of STR markers. STR allele nomenclature, a common nomenclature, is developed to aid in interlaboratory reproducibility and comparisons of data in the forensic DNA community. An allelic ladder is an artificial mixture of the common alleles present in the human population for a particular STR marker. For DNA typing markers to be effective across a wide number of jurisdictions, a common set of standardized markers must be used. Today both Applied Biosystems and the Promega Corporation have STR kits that address the needs of the DNA typing community and cover a common set of STR loci. Each manufacturer of STR kits provides allelic ladders that may be used for accurate genotyping. Applied Biosystems introduced two new technologies with their AmpF l STR Identifiler kit when it was released in 2001. By far the most popular method for sex typing today is the amelogenin system because it can be performed in conjunction with STR analysis. STR Base, an Internet-accessible informational database, officially launched in July 1997 in anticipation of the impact of STR markers on DNA typing and the need for a common source of information that could evolve as the process improved.