 |
 |
|||
|
Site Menu Molecular Biology |
Randomly Amplified Polymorphic DNA | Amplified Fragment Length Polymorphism | Trace Alkaloids and Stable Isotope Ratios |
|||
Botanical samples may be too small or deteriorated to supply definitive morphological or histological features. In such cases, DNA technology may be used to identify the species. Useful loci include the nuclear ribosomal DNA internal transcribed spacer regions 18S, ITS1, and ITS2, and the chloroplast genes rbcL, atpB, and ndhF(Miller Coyle et al., 2001). Plant material can be identified to species with as little as a 1 mm fragment of dried tissue, 5 grains of fresh pollen, or 10 picograms of DNA (* IDENTIFICATION OF PLANT TRACE EVIDENCE USING MOLECULAR TECHNIQUES). Molecular methods have been used in drug investigations, patent cases, and have played a key role in at least one homicide case, discussed below. (Miller Coyle et al., 2001). | ||||
|
Developed in the early 1990s (* DR. DENISE GARCIA), randomly amplified polymorphic DNA is based on the polymerase chain reaction (PCR). Where PCR uses genetic primers to target and amplify specific sequences of DNA, RAPD uses numerous short primers (10 base pairs) to amplify the genome in various random locations (* UNITED STATES DEPARTMENT OF AGRICULTURE). The result is a unique "genetic fingerprint" of the sampled organism. The "Maricopa Case" marked the first time plant DNA has been admitted into evidence in a criminal court in the United States (Yoon, 1993). In May of 1992, a woman was murdered and her body dumped at an abandoned factory in Maricopa County, Arizona, U.S. (Mestel, 1993). A suspect was apprehended but denied ever having been at the site where the body was found. Seed pods from the palo verde tree, several of which grew near the factory, were recovered from the back of the suspect's truck. |
Palo verde tree, |
|||
|
The Maricopa County Sheriff's office turned to Tim Helentjaris, a plant geneticist at the University of Arizona, hoping that the seed pods could be matched to an individual tree at the crime scene through DNA typing. Helentjaris was initially skeptical, but through RAPD analysis he found that the palo verde trees at the site showed high genetic variability, and could be easily distinguished from one another (Mestel, 1993). In blind tests that included all the palo verde trees from the crime scene plus several chosen at random from around the city of Phoenix, Helentjaris was able to match the seed pods to the crime scene tree. While the seed pods did not prove that the suspect was at the crime scene, they did prove that his truck was there, and the evidence was strong enough to be admitted in court (Yoon, 1993). |
||||
|
Like RAPD, amplified fragment length polymorphism analysis is based on PCR, but it is more sensitive and more reproducible than RAPD. The technique is useful in situations where large numbers of polymorphic markers are required to distinguish species and where few taxon-specific primers are available (* UNIVERSITY OF FLORIDA INTERDISCIPLINARY CENTER FOR BIOTECHNOLOGY RESEARCH). The Connecticut State Forensic Laboratory in the United States is currently validating the AFLP technique for use in typing marijuana (Miller Coyle et al., 2001). DNA analysis is useful not only in the identification of the drug, but also in establishing distribution patterns and links among growing operations. One method of marijuana propagation is by cuttings; all cuttings will have DNA identical to that of the original "mother" plant. Clonal material can be tracked and traced back to its source through DNA typing (Miller Coyle et al., 2001). For those crops propagated by seed, suspects can be linked to a specific crop or shipment if the DNA of fragments recovered from the suspect's person or premises matches the DNA of a siezed crop (Miller Coyle et al., 2001). |
||||
|
Recently, investigators have shown that cocaine can be traced to specific coca-growing regions in South America through analysis of stable carbon (13C) and nitrogen (15N) ratios combined with trace alkaloid analysis. There is wide variation among the different coca-growing regions along the Andean Ridge. Soil differences affect 15N, while humidity and the length of the wet season affect 13C. The variation |
Coca leaves and cocaine powder. Source: Indiana Prevention Resource Center. |
|||
|
in these isotopes from region to region can be used to predict the natural isotopic variation in crops grown there. Of 200 cocaine samples, stable isotope analysis correctly matched 90% to their source region. When combined with analyses of detectable trace alkaloids (truxilline and trimethoxycocaine), this figure rose to 96%. Stable isotope and trace alkaloid data is a feasible means of tracking cocaine trafficking routes and identifying existing and new coca-producing regions. |
||||
|
palynology | anatomy and dendrochronology | ecology and systematics | molecular biology The Forensic Botany site was created in 2002 by Jennifer Van Dommelen as a project in the Web Literacy For the Natural Sciences class at Dalhousie University, Halifax, Canada. Terms in bold are defined within the body of the text. Highlighted terms and author citations are linked to a glossary and reference list, respectively, which open in new windows. All images have been used with permission. Header banners created by Jennifer Van Dommelen. Last content update: April 2002 Contact: | ||||
