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Chapter 7 DNA Fingerprinting
By the end of this chapter you will be able to:
z explain how crime scene evidence is
collected and processed to obtain DNA
z describe how radioactive probes are used in DNA
fingerprinting
z explain how DNA evidence is compared for
matching
z explain how to use DNA fingerprinting to identify
DNA from a parent, child, other relative, or a nonrelated individual
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Forensic Science: Fundamentals & Investigations, Chapter 7
Introduction and History of
Biological Evidence in Forensics
z DNA fingerprinting, also known as DNA
profiling, is used in criminal or legal cases with
a high degree of accuracy.
z Biological evidence such as blood, saliva,
urine, semen, and hair is examined for the
presence of inherited traits.
z What examples can you give of how
laboratory techniques used in forensics were
originally developed for other purposes?
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Forensic Science: Fundamentals & Investigations, Chapter 7
The Function and Structure of DNA
z
DNA molecules make up chromosome
structures and are found in the nucleus of cells
in the human body.
z How would you describe and explain the
double helix, twisted-ladder structure of a
chromosome?
z Can you explain the terms allele, genome, and
junk DNA?
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Forensic Science: Fundamentals & Investigations, Chapter 7
DNA Identification
z Junk DNA contains many of the unique
patterns of repeated base sequences that
identify individuals.
z In a human population, these are called
polymorphisms.
z In 1984 a technique was developed for
isolating and analyzing these variable areas.
z This DNA Fingerprinting appears as a pattern
of bands on X-ray film. These patterns can be
used for identification of individuals.
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Forensic Science: Fundamentals & Investigations, Chapter 7
DNA Identification
The number of copies of the same repeated base
sequence in DNA varies among individuals.
Variable Number of Tandem Repeats (VNTR)
z
z
Within junk DNA, sequences of DNA are repeated
multiple times.
Some can be 9-80 bases in length.
Short Tandem Repeats (STR)
z
z
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Within junk DNA, other sequences of DNA also are
repeated multiple times.
These usually are only 2-5 bases in length and are
becoming the preferred sequences for analysis.
Forensic Science: Fundamentals & Investigations, Chapter 7
DNA Profiling and DNA
Population Databases
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z
VNTR and STR data are analyzed for (a) tissue
matching and (b) inheritance matching.
z
Population genetics is the study of variation
in genes among groups of individuals.
z
Calculations can be made based on these
groups to determine the probability a random
person would have the same alternative form
of a gene (an allele) as (a) a suspect in a crime
or (b) an alleged father in a paternity case.
Forensic Science: Fundamentals & Investigations, Chapter 7
Sources of DNA
z
A perpetrator may leave biological evidence, such as
saliva or blood, at a crime scene.
z This individual evidence is capable of identifying a
specific person.
z But a small amount of biological evidence might be
considered only trace evidence, and it may be
consumed during forensic testing.
z In 1993, however, the polymerase chain reaction
(PCR) technique was invented.
z A method used to rapidly make multiple copies of a specific
segment of DNA.
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Forensic Science: Fundamentals & Investigations, Chapter 7
Avoiding contamination in
the collection and preservation of DNA
1. Use disposable gloves and collection
2.
3.
4.
5.
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instruments.
Avoid physical contact, talking, sneezing, and
coughing in the evidence area.
Air-dry evidence and put it into new paper
bags or envelopes.
If evidence cannot be dried, freeze it.
Keep evidence cool and dry during
transportation and storage.
Forensic Science: Fundamentals & Investigations, Chapter 7
Preparing DNA Samples
for Fingerprinting
1. DNA is mixed with special enzymes.
1. Restriction enzymes are the molecular scissors that
cut DNA at specific base sequences.
2. The enzymes cut apart the DNA in specific
places forming different sized fragments.
3. The DNA is loaded into the chambers found
on an agarose gel.
4. An electric current is passed through the gel
separating the fragments by size.
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1. The smallest fragments travel farthest from the well.
Forensic Science: Fundamentals & Investigations, Chapter 7
Preparing DNA Samples
for Fingerprinting--Extraction
1. Cells are isolated from biological evidence
such as blood, saliva, urine, semen, and
hair.
2. The cells then are disrupted to release the
DNA from proteins and other cell
components.
3. Once released, the DNA can be extracted
from the cell nucleus.
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Forensic Science: Fundamentals & Investigations, Chapter 7
Preparing DNA
Samples for
Fingerprinting
Amplification
z With some VNTR analysis, polymerase chain reaction (PCR)
can be used to amplify the DNA that contains the VNTRs.
z In STR profiles, restriction enzymes are unnecessary; PCR
allows the amplification of the strands with STR sequences.
Electrophoresis
z DNA samples are placed in gels through which electronic
currents are passed.
z DNA fragments line up in bands along the length of each gel.
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Forensic Science: Fundamentals & Investigations, Chapter 7
Probes
z
z
z
z
z
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A molecule labeled with a radioactive isotope, dye,
or enzyme that is used to locate a particular
sequence or gene on a DNA Molecule.
DNA probes are used to identify the unique sequences
in a person’s DNA.
Different DNA probes are made up of different synthetic
sequences of DNA bases compli-mentary to the DNA
strand.
The probe binds to complimentary bases in the strand
(see the fragmentary DNA bands above).
In most criminal cases, 6-8 probes are used.
Forensic Science: Fundamentals & Investigations, Chapter 7
Analysis of DNA Fingerprints and
Applications
z Bands and widths are significant in matching samples of
DNA.
z DNA fingerprinting can (a) match crime scene DNA with a
suspect, (b) determine maternity, paternity, or match to
another relative, (c) eliminate a suspect, (d) free a falsely
imprisoned individual, and (e) identify human remains.
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Forensic Science: Fundamentals & Investigations, Chapter 7
Important People
z
Dr. Alec Jeffreys – p. 158, 162, & 163
–
z
Dr. Kary Banks Mullis – p. 164 & 172
–
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1984 developed DNA Fingerprinting Technique
1993 developed the PCR Technique
Forensic Science: Fundamentals & Investigations, Chapter 7
Case Studies p. 170 & 171
z
z
z
z
z
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Colin Pitchfork (1983)
Tommie Lee Andrews (1986)
Ian Simms (1988)
Kirk Bloodsworth (1984)
ID of Human Remains: Hurricane Katrina
Forensic Science: Fundamentals & Investigations, Chapter 7
. . . . . . . . . . Summary . . . . . . . .
z DNA contains the information needed for replication in
a sequence of nitrogenous bases.
z DNA analysis allows even a small sample of tissue to
be identified with a single individual.
z DNA contains, in non-coding regions called junk DNA,
many repeated sequences that vary in number
between individuals.
z These differences between individuals can be used to
produce a DNA fingerprint for an individual.
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Forensic Science: Fundamentals & Investigations, Chapter 7
. . . . . . . . . . . . . . . . . Summary
z Polymerase chain reaction (PCR) for DNA
amplification has largely eliminated the problem
resulting from the tiny samples usually available.
z DNA evidence must be collected carefully to avoid
contamination with other DNA.
z DNA analysis involves extraction, electrophoresis, and
visualization.
z DNA profiles are kept by police agencies in electronic
databases.
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Forensic Science: Fundamentals & Investigations, Chapter 7