DNA
Fingerprinting
and
Forensics
What Is a DNA Fingerprint?
 Every individual carries a unique set of
genes
 Chemical structure of DNA is same, but the
order of the base pairs differs
 Every cell contains a complete set of DNA
that identifies the organism as a whole
 Only one tenth of 1% of DNA differs from
person to person
What Is a DNA Fingerprint?
 Two Main Types of Forensic DNA Testing
 RFLP (restriction fragment length polymorphism)
 Requires larger amounts of DNA
 DNA cannot be degradedRFLP
 was one of the first applications of DNA analysis to forensic
investigation. With the development of newer, more efficient
DNA-analysis techniques, RFLP is not used as much as it once
was because it requires relatively large amounts of DNA. In
addition, samples degraded by environmental factors, such as
dirt or mold, do not work well with RFLP.
Log onto: http://www.pbs.org/wgbh/nova/sheppard/analyze.html
 PCR (polymerase chain reaction)
 Less DNA and DNA can be partially degraded
 Extremely sensitive to contaminating DNA
Lets try a virtual DNA fingerprint lab!
Want to see the evidence from the OJ case?
http://www.law.umkc.edu/faculty/projects/ftrials/Simpson/simpson.htm
Click on incriminating evidence
Then DNA evidence (on the top in the center)
What Is a DNA Fingerprint?
 DNA profiling depends on a small
portion of the genome
 Exons code proteins (proteins that are expressed)
These are the 99.9% that makes us similar
 Introns do not code for proteins (these are what make
us different)
 The introns contain repeated sequences of between
1 and 100 base pairs
 Called variable number tandem repeats (VNTR’s)
 Some VNTR’s are inherited from mother and
some from father
What Is a DNA Fingerprint?
 DNA fingerprinting is restricted to the
detection of microsatellites
 1 to 6 nucleotide repeats dispersed throughout
the chromosomes
 Probes used to identify the microsatellite
surround the specific microsatellite being
analyzed
 Also called short tandem repeats (STR)
 FBI has chosen 13 unique STRs for testing
 Combined DNA Index System (CODIS)
Preparing a DNA Fingerprint
 Specimen Collection
 Search for sources of DNA
 Collection requires scrupulous attention to
detail
 Wear disposable gloves; change them frequently
 Use disposable instruments
 Avoid talking, sneezing, and coughing
 Avoid touching any item that might contain DNA
(face, nose, or mouth)
 Air-dry evidence before packaging; mold can
contaminate a sample
Preparing a DNA Fingerprint
 Enemies of Evidence
 Sunlight and high temperature
 Bacteria
 Moisture
 DNA fingerprinting is a comparative process
 Samples from crime scene must be compared to suspect
DNA
 Best sample from suspect DNA is fresh, whole blood
 Extraction of DNA
 DNA can be purified
 Chemically (using detergents)
 Mechanically (pressure to force DNA out of cell)
Preparing a DNA Fingerprint
 RFLP Analysis: Restriction Fragment
Length Polymorphism
 Treat DNA with restriction enzyme
 Restriction enzyme cuts DNA at restriction sites
 Use several restriction enzymes in sequence or
combined
 Use agarose gel electrophoresis to separate
the pieces
 Gel is chemically treated or heated to denature
the DNA
 Allows the binding of a single-stranded probe
Preparing a DNA Fingerprint
 Southern Blot Technique
 Transfer DNA fragments from gel to nitrocellulose or
nylon membrane
 Membrane incubated with a probe
 Short strand of complementary DNA with a radioactive or
fluorescent tag
 Targeted area on the DNA fragment is called a locus
 Expose X-ray (photo) film to membrane to obtain
permanent record of results
Preparing a DNA Fingerprint
 PCR – used to amplify DNA found at crime
scene into an amount that can be analyzed
 DNA produced is identical to the original
sample
 Use amplified DNA in a Dot Blot Analysis
 DNA amplified by PCR is blotted onto specially
prepared blot strips
 Each dot on the strip is a different DNA probe from human
DNA
Try a virtual one for yourself!
http://www.wiley.com/college/boyer/0470003790/animations/animations.htm
http://learn.genetics.utah.edu/content/labs/pcr/
STR Analysis
 STR: Small Tandem Repeats
 Use primers to amplify STR’s in DNA using
PCR
 FBI uses 13 STR regions
 Odds that two individuals will have the
same 13-loci DNA profile are more than
one in a billion
Check out the 13 loci profiles that get loaded into CODIS
(the FBI’s DNA Database!)
http://www.cstl.nist.gov/strbase/fbicore.htm
Putting DNA to Use
 DNA fingerprinting is a comparative
process
 Looking for alignment of bands or dots in the
fingerprint
 All tests are based on exclusion
 Testing continues only until a difference is found
 If no difference is found after a statistically
acceptable amount of testing, the probability of a
match is high
Narborough Village Murders
 1983 first reported use of genetic fingerprinting in
a criminal case
 Sexual assault and murder of girl in United
Kingdom
 The prime suspect’s DNA did not match that found
at crime scene
 Police collected 5500 samples from the area’s
population of likely suspects
 None of the DNA profiles matched the crime scene
 Friend of suspect had given a sample in the name
of the suspect
Forest Hills Rapist
 DNA first used in United States – 1987
 Victor Lopez tried for the sexual assault of
three women
 Reported assailant was a black man; Lopez was not
a black man
 DNA was a match to crime scene
Terrorism and Natural Disasters
 They have forced development of new
technologies
 World Trade Center September 11, 2001
 Used DNA techniques to identify the remains of
victims
 Tremendous amount of debris, heat, and microbial
decomposition of remains
 Hundreds of thousands of tissue samples from
nearly 3,000 individuals
World Trade Center
Evident that new strategies would be
necessary to quickly prepare and
organize DNA profiles and compare them
with DNA profiles from relatives
 Within 24 hours, collection points had been
established around the city
 Family filed missing person reports and provided
cheek cell swabs for DNA isolation; personal
items from the missing were also collected
 World Trade Center
 Several companies were involved in
developing new software programs to help
match DNA samples from family members to
DNA profiles of victims
 Used primarily STR, mtDNA, and SNP
analysis
South Asian Tsunami
 December 2004
 Lost over 225,000 lives
 mtDNA (mitochondrial), Y-STR’s (Y-DNA)
 Within 3 months, 800 victims had been identified
DNA and the Rules of Evidence
 DNA fingerprinting had to meet legal
standards regarding the admissibility of
evidence
 5 different standards used
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Relevancy test
The Frye standard
The Coppolino standard
The Marx standard
The Daubert standard
Simpson/Goldman Murders
 Collected 45 samples for DNA analysis, including known
blood samples from the two victims and the suspect;
blood drops found at the crime scene, in the suspect’s
home, and in his car
 Pre-trial hearings announced that the DNA collected at
the crime scene matched that of O.J. Simpson
 Defense lawyers attacked the procedures used in
collecting, labeling, and testing the evidence
 Possibility that evidence was tainted became obvious
 DNA evidence not effective
Want to see more about the case?
http://investigation.discovery.com/videos/crimes-of-the-century-oj-simpson-videos/
DNA in the News
 Tsunami in Japan
 Tornados in Alabama and across the
Midwest
 Osama Bin Ladden Identification
Human Error and Sources of
Contamination
 Chain of custody of samples is compromised
 Collection of evidence must be systematically
recorded and access to evidence must be controlled
 Follow defined standards of laboratory practice
and procedures to prevent DNA damage
during the analysis
 DNA and Juries
 Must make sense to the jury
 Statistics can be confusing
Familial Relationships and DNA Profiles
 Paternity Testing
 Analyze samples from child and adults involved
 Mitochondrial DNA
 Used to examine samples that cannot be analyzed by
PCR or RFLP
 Older samples that lack nucleated cellular material
(hair, bones, and teeth)
 Inherited from the mother only
 Changes only about 1% every million years due to
random mutation
Check out some crimes that have been solved using mtDNA!
http://findarticles.com/p/articles/mi_m2194/is_8_71/ai_90819640/pg_5/
 Y-Chromosome Analysis
 Passed directly from father to son
 Useful for tracing relationships among males or
analyzing biological evidence involving multiple
male contributors