How is DNA Used in Forensics?

DNA Collection & Comparison Overview: –Investigators gather samples from the crime scene and from suspects and then analyze it for a set of specific DNA regions or markers. –A match of one marker is not usually unique, but if a sample matches four or five markers, there is a very good chance it is a match.

DNA Collection & Comparison DNA is collected at crime scenes in a variety of ways using tools such as: –Smear slides, scalpels, tweezers, scissors, sterile cloth squares, UV light, luminol and/or blood collection kits (for sample collection of suspects or living victims) DNA samples can be from: –Saliva, blood, hair strands, skin, finger or toe nails, and/or a tooth with root material

DNA Collection & Comparison How is blood collected? –Blood on Clothing? Investigators submit whole pieces of clothing or they may use a sterile cloth square and a small amount of distilled water –Dried blood on furniture? Investigators send the whole object to the lab –Dried blood on a wall, tub or some other object too big or difficult to move to the lab? Investigators scrape the blood sample into a sterile container for further analysis

DNA Collection & Comparison Analysis of DNA?? –Uses various DNA Technologies RFLP PCR STR Mitochondrial DNA Analysis

RFLP: Restriction Fragment Length Polymorphism Analyzes variable lengths of DNA fragments One of the original applications of DNA analysis Not used as much anymore because it requires a large quantity of DNA sample and samples degraded by the environment do not work well with RFLP

PCR: Polymerase Chain Reaction Used to make millions of exact copies of DNA from a biological sample Allows very small samples to be analyzed, such as a sample of a few skin cells Must be very careful about contamination in this process

STR: Short Tandem Repeat Evaluates specific regions (loci) within nuclear DNA FBI uses 13 standard specific STR regions for CODIS

Mitochondrial DNA Analysis Used for samples that cannot be analyzed using RFLP or STR Uses DNA extracted from mitochondrion rather than nuclear DNA Especially useful in old cases and old samples

DNA Collection & Comparison What happens after the samples are collected? –A DNA profile is created….how?? Markers are found by designing small pieces of DNA (probes) that will seek out and bind to complementary DNA sequences. This creates a distinct pattern. Again, one marker is not usually unique, but with four or five regions the match is likely –The DNA profiles are compared with samples from suspects to find possible matches. –If there are no suspects, a national database called CODIS may be used to find potential suspects.

DNA Collection & Comparison More on CODIS: –Stands for Combined DNA Index System –National Network that helps identify leads for crimes with no suspects –Three tiers: Local (LDIS), State (SDIS), National (NDIS) –Uses 13 DNA regions that vary from person to person –Looks for matches at more than one location on a genome for more accurate results

Sources of DNA at Crime Scenes Cool table at: – n/identifyinghttp:// n/identifying

Sources of DNA at Crime Scenes Examples of sources from real cases: –Saliva on the stamp of a stalker’s threatening letter –Skin cells shed on a ligature of a strangled victim –Perspiration on a baseball cap discarded by a rapist was compared with the DNA in the saliva swabbed from a bite mark on a different rape victim –DNA analysis of a single hair (without the root) found deep in a victim’s throat –Maggots can contain DNA of a perpetrator

How can DNA evidence be planted?? Sneezing or coughing over evidence Person touches their mouth, nose or other part of the face and then touches the area that may contain the DNA to be tested. Scene personnel can deposit hairs, fibers, or trace material from their clothing Wind can carry in contaminants

Other Uses of DNA Paternity Testing and Proving Family Relations Identification of John or Jane Does Study of evolution and ancestry Studying Inherited Disorders

Other Uses of DNA Identifying Stolen Trees, Poached Animals – 01.Woeste.rustling.htmlwww.purdue.edu/UNS/html3month/2004/ Woeste.rustling.html Molecular geneticist Keith Woeste at Purdue University was able to prove this chunk of walnut wood came from a tree stolen in Warren County. Working in the USDA Forest Service Hardwood Tree Improvement and Regeneration Center at Purdue, Woeste ground the wood into a fine powder to extract enough DNA evidence to convince investigators the wood matched that of the stolen tree, which had been sold to a sawmill. (Purdue Agricultural Communication photo/Tom Campbell)

Sources mlhttp:// ml ence/DNA/ ence/DNA/ enome/elsi/forensics.shtmlhttp:// enome/elsi/forensics.shtml evidence3.htmhttp:// evidence3.htm

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