forensics chapter 8. central points dna testing can determine identity dna profiles are...
TRANSCRIPT
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Forensics
Chapter 8
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Central Points
DNA testing can determine identity
DNA profiles are constructed in specialized laboratories
DNA profiles used in courts
Ancestry can be determined by testing DNA
May refuse to give sample to the police
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Case A: DNA Frightens Victim
Five years ago, Margaret was attacked and beaten
ID William in a line up by his “eyes”
William claimed innocence but was convicted and sentenced to 25 years
New DNA testing using PCR • Blood on victim’s clothing not William’s
• He is asking to be released from prison
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8.1 How Is DNA Tested?
In 1975, in England
Dr. Alec Jeffreys developed DNA fingerprinting to compare DNA profiles from different individuals
Used to solve murders (see Narborough Village Murders)
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8.2 DNA Profile (DNA Fingerprint)
Variations in:• Minisatellites (repeated sequences, 10–100 base
pairs)
• Short tandem repeats (STRs) (2–9 base pairs)
• Single nucleotide polymorphisms (SNPs)
DNA variations can be used to create DNA profile and ID individuals
Used in criminal cases, paternity lawsuits, and studies of human evolution
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Minisatellites
Short tandem repeats (STRs)
Single nucleotide polymorphisms (SNPs)
Variations in DNA
...CCTGACTTAGGATTGCCA...
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Methods to Prepare DNA Profiles
Two common methods:
Restriction fragment length polymorphism (RFLP) analysis
Polymerase chain reaction (PCR)
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RFLP Analysis
Large amount of blood or tissue
Steps:
1.DNA extracted
2.Cut into fragments using restriction enzymes
3.Fragments loaded on agarose gel
4.Electrical current passed through gel, DNA fragments migrate and separating by size
5.Pattern fragments photographed and compared to others
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p. 132
4 The gel is immersed in a liquid, and an electric current is applied across the gel from top to bottom.
Stepped Art
3 The DNA is loaded onto a gel.
5 DNA fragments move through the gel and separate by size. Smaller fragments migrate furthest.
1 DNA is extracted from cells belonging to three different people.
2 Each sample is cut with a specific restriction enzyme.
6 The DNA bands are visualized on film and analyzed.
Steps in RFLP
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RFLP
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PCR
Small amount of DNA sample
Steps in one cycle:
1.DNA placed in solution
2.Heated, double-stranded DNA separates
3.Temperature lowered, primers mixed with DNA
4.Primers pair with complementary regions on DNA
5.DNA polymerase uses nucleotides to synthesize double-stranded DNA molecule
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2 DNA is heated and separates into single strands.
p. 133
1 DNA is recovered for PCR testing. Primers
3 Primers are added and bind with complementary regions in the DNA.
4 Primers serve as starting points for new DNA.
5 New double stranded DNA is formed.
a. One PCR cycle Stepped Art
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2 One cycle
p. 133
1 Original sample
3 Two cycles
b. Multiple PCR cycles
4 Three cycles
Stepped Art
Steps in PCR
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Using PCR to Increase DNA
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STR: Allele, a Unique Number of Repeats
2–9 nucleotides long
Example of three alleles:• Allele 1: …AGA…
• Allele 2: …AGAAGAAGA…
• Allele 3: …AGAAGAAGAAGAAGA…
Analysis of several STR alleles show frequency of specific combinations
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PCR for DNA Profile
Steps:
1.DNA sample analyzed for STR alleles present in population members
2.Analyze population frequency, how often combinations of alleles present
3.Population frequencies for each STR allele multiplied to estimate probability
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Combined Frequencies
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DNA Samples (1)
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DNA Samples (2)
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Case A Questions
Should William get a new trial?
What are the benefits and challenges of using DNA profiles?
How reliable is DNA evidence?
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Animation: Studying and Manipulating Genomes (DNA fingerprinting)
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Animation: Whodunnit? A DNA fingerprint crime
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8.3 DNA Profiling and U.S. Courts
Landmark case: Frye v. U.S.
DNA entered U.S. court system in 1989
Approved for use in courts all over the country
Evidence may be challenged
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How Are DNA Profiles Used?
Material from crime scene analyzed, compared to other samples or data base
Combined DNA Index System (CODIS) panel of 13 STRs for DNA profiles
~30% of DNA profile results clear innocent people
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DNA Profile Comparison
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DNA Databases
Since 1998, FBI enters DNA profile from felons
More than 1,700,000 profiles
DNA databases becoming important tools in solving crimes
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8.4 Other Uses for DNA Profiles
Czar Nicholas II and family
Remains of military personnel killed in action
Victims of 9/11 and Katrina
Paternity identification
Trace our ancestry
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Czar Nicholas II
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Tracing Ancestry
Mitochondrial DNA testing• Mitochondria organelle with DNA
• Maternal inheritance, passed from mother to children
• Use haplotypes to trace maternal ancestry
Y chromosome testing• Y chromosome passed father to son
• Use haplotypes to trace paternal inheritance
Genographic Project sponsored by National Geographic Society
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Mitochondrial Inheritance
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Y Chromosome Inheritance
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Case B: Samples Asked of All
Two women murdered in a small community
Males > age 17 to provide sample for DNA dragnet
Murderer was identified
What are the important issues in this request?
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8.5 Legal and Ethical Issues