How is DNA Used in Forensics?

1
How is DNA Used in Forensics?
2
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.

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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

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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

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DNA Collection Comparison

• Analysis of DNA??
• Uses various DNA Technologies
• RFLP
• PCR
• STR
• Mitochondrial DNA Analysis

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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

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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

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STR Short Tandem Repeat

• Evaluates specific regions (loci) within nuclear
  DNA
• FBI uses 13 standard specific STR regions for
  CODIS

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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

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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.

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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

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Sources of DNA at Crime Scenes

• Cool table at
• http//www.dna.gov/basics/evidence_collection/iden
  tifying

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Sources of DNA at Crime Scenes

• Examples of sources from real cases
• Saliva on the stamp of a stalkers 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 victims throat
• Maggots can contain DNA
• of a perpetrator

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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

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Other Uses of DNA

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

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Other Uses of DNA

• Identifying Stolen Trees, Poached Animals
• www.purdue.edu/UNS/html3month/2004/040701.Woeste.r
  ustling.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)
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Sources

• http//www.safenetwork.org/DNA_Information.html
• http//www.troopers.state.ny.us/Forensic5FScience
  /DNA/
• http//www.ornl.gov/sci/techresources/Human_Genome
  /elsi/forensics.shtml
• http//www.howstuffworks.com/dna-evidence3.htm
• http//www.howstuffworks.com/csi4.htm
• www.ncjrs.gov/txtfiles1/nij/bc000614.txt

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Sources

• http//images.google.com/imgres?imgurlhttp//www.
  tritechusa.com/kits/_derived/Forensic2520Kits252
  0cover.htm_txt_Forensic-Kits-cover.gifimgrefurlh
  ttp//www.tritechusa.com/kits/Forensic2520Kits25
  20cover.htmh358w269sz57hlenstart5um1t
  bnidmF81oicmfKL1eMtbnh121tbnw91prev/images
  3Fq3Ddna2Bevidence2Bcollection26svnum3D1026
  um3D126hl3Den26rls3DGGLG,GGLG2005-35,GGLGen
  
• www.cji.net/CJI/CenterInfo/fscec/Contamination.htm
  
• http//www.courttv.com/trials/westerfield/071702_c
  tv.html