Chapter 5:
DNA Fingerprinting
MS. COLABELLI
O.J. Simpson
A Mountain of Evidence
On June 12, 1994, police arrived at the home of Nicole Simpson only
to view a horrific scene. The bodies of O.J. Simpson’s estranged wife
and her friend Ron Goldman were found on the path leading to the
front door of Nicole’s home. Both bodies were covered in blood and
had suffered deep knife wounds. Nicole’s head was nearly severed
from her body. This was not a well-planned murder. A trail of blood
led away from the murder scene. Blood was found in O.J. Simpson’s
Bronco. Blood drops were on O.J.’s driveway and in the foyer of his
home. A blood-soaked sock was located in O.J. Simpson’s bedroom,
and a bloodstained glove rested outside his residence.
O.J. Simpson
A Mountain of Evidence
As DNA was extracted and profiled from each bloodstained article, a
picture emerged that seemed to irrefutably link Simpson to the
murders. A trail of DNA leaving the crime scene was consistent with
O.J’s profile, as was the DNA found entering Simpson’s
home. Simpson’s DNA profile was found in the Bronco along with that
of both victims. The glove contained the DNA profiles of Nicole and
Ron, and the sock had Nicole’s DNA profile. At trial, the defense team
valiantly fought back. Miscues in evidence collection were craftily
exploited. The defense strategy was to paint a picture of, not only an
incompetent investigation, but one that tinged with dishonest police
planting evidence. The strategy worked. O.J Simpson was acquitted
of murder.
https://www.youtube.com/watch?v=-qxrDkMieQs
Introduction
No two people on Earth have the same DNA
◦ Except for identical twins
DNA technology has allowed criminal cases to be solved that were once not
possible to conclude
Since 1980s, DNA evidence has been used to
◦
◦
◦
◦
◦
Investigate crimes
Establish paternity
Link a suspect to a crime
Eliminate a suspect
Identify victims of war/large-scale disaster
DNA evidence is individual evidence because it can track back to one person
◦ Can also be trace evidence when a small amount is left behind
The Function & Structure of DNA
DNA contains the genetic material that holds the information to produce
proteins and replicate itself
DNA is stored in animal cells into a structure called a chromosome
DNA is made up of two strands tightly coiled called a double helix
The backbone of DNA is alternating sugar (deoxyribose) and phosphate
Made up of four bases
◦ Adenine to thymine
◦ Guanine to cytosine
Every organism has a specific number of chromosomes
◦ Humans have 23 pairs
◦ Dogs have 39 pairs
The Function & Structure of DNA
Nuclear DNA
◦ Double helix shape
◦ Inherited from BOTH parents
◦ Half of genetic information from each parent
◦ DNA from egg cell from mom
◦ DNA from sperm cell from dad
mtDNA
◦
◦
◦
◦
◦
Found only in mitochondria organelles
Animal cells contain multiple mitochondria
Loop shaped chromosome
Inherited ONLY from mother
Passed to offspring in cytoplasm from egg cell
Genes & Alleles
Genes
◦ DNA sequences with instructions for traits
◦ Ex: blood type
Allele
◦ Alternative forms of a gene
◦ Ex: if the gene is eye color, the alleles
would be brown and blue
Genome
◦ Complete set of DNA in a cell
◦ Chromosomal and mitochondrial
Genes & Alleles
Exons
◦ DNA encoded to make proteins and other molecules
◦ Exit the nucleus to make proteins (expressed)
◦ Less than 1.5% of your genome
Introns
◦ Non-coding regions of DNA
◦ Stay in the nucleus and are never coded
◦ Some functions for gene splicing and gene regulation
◦ Remainder of introns has an unknown function
DNA Identification
Most of human genome is the same
The areas of variation are in the introns or the non-coding
regions of DNA
◦ Repeated base sequences
◦ Individuals have unique patterns or repeat sequences
◦ Different lengths and sequences of the bases
◦ Called polymorphisms
DNA Fingerprinting or DNA Profiling
◦ Takes the pattern of bands on X-Ray film
◦ Polymorphisms produce unique band patterns
◦ Forensic scientists focus on STR sections
STR – Short Tandem Repeats
Short sequence of DNA between 2-5 bases in length
Found in great abundance in
the human genome
Becoming a preferred method
of analysis
◦ More accurate
◦ Small & partially degraded DNA
samples can be analyzed
Hundreds of different types in
human genome
DNA Fingerprinting
◦ DNA can be extracted from small samples of biological evidence
◦ Saliva – envelope, toothbrush, bite wound
◦ Hair follicle
◦ Blood
◦ Seminal fluid
◦ Skin cells
◦ DNA fingerprint developed when several different STRs are examined
◦ Tissue matching
◦ When two samples have the same band pattern from the same person
◦ Crime-scene evidence matches
DNA Fingerprinting
Preparing DNA Samples
◦ When DNA is mixed with special enzymes that cut DNA in
specific places, it will form different sized DNA fragments
◦ Using a technology called Gel Electrophoresis, these DNA
fragments can be separated within a gel
◦ The results will form band patterns (or fingerprint) in the
gel specific to that individual
◦ Relatives may share some bands and this is how this
technology can be used to identify inheritance and
paternity
https://www.youtube.com/watch?v=ZxWXCT9wVoI
DNA Fingerprinting Steps
1. Extraction
◦ Cells isolated from tissue
◦ DNA is extracted from the nucleus of the cells collected
2. Restriction Enzymes
◦ DNA is cut into fragments using biological scissors known as restriction
enzymes
◦ These molecules recognize specific sequences in the DNA sample and cut
in those specific locations
◦ Ex: Hind III recognizes the AACGTT base sequence and cuts between the two AA bases
◦ These sequences of tandem repeats are known as VNTR sequences
(Variable Number Tandem Repeats)
◦ Shorter sequences about 2-5 bases in length are known as STR sequences
(Short Tandem Repeats)
DNA Fingerprinting Steps
3. Amplification
◦ Small samples of DNA evidence can be amplified or copied in a
process called the Polymerase Chain Reaction (PCR)
◦ Multiplexing – simultaneously extracting DNA and amplifying
4. Electrophoresis
◦
◦
◦
◦
DNA is loaded into wells of an agarose gel
Separate molecules within an electric field
Negatively charged DNA fragments move to positive end of the gel
The process will separate the fragments of DNA by their size
◦ Smaller fragments of DNA will travel faster and farther down the gel
DNA & Probes
If there are too many DNA restriction fragments, scientists use probes
to identify specific DNA sequences
Helps to make any DNA on the gel visible
Made of synthetic DNA that is complementary to a fragment
For example, a probe AAGCTTA will find a TTCGAAT fragment and attach
If probe is fluorescent, then you use a UV light to see the probes
If probe is radioactive, then you use X-Ray film to see the probes
◦ Called autoradiograph
Population Genetics
& DNA Databases
Study of variation in genes among a group of individuals
Proportion of people in a population with a particular
characteristic is determined by the proportion of the alleles
for these traits in a population
Example:
◦ Asian population - blue eyes rare, few people have allele that codes
for blue eyes
◦ Northern Europeans - blue eyes common, more people have allele
that codes for blue eyes
◦ Europe/Great Britain/US - 46% have type O Blood
◦ South America - almost 100% type O blood
Collection & Preservation of
DNA Evidence
Extremely small samples can become contaminated
◦ Sneezing
◦ Coughs
◦ Evidence collector touches own mouth or nose
To avoid contamination
◦ Wear disposable gloves and change often
◦ Use disposable instruments for handling each
sample
◦ Avoid touching area where DNA may exist
◦ Avoid talking, sneezing, coughing over evidence
◦ Avoid touching face, nose, mouth when collecting
and packaging
◦ Air-dry evidence thoroughly before packaging
◦ Place evidence in new paper bags/envelopes
◦ If wet evidence can not be dried, may be frozen
to preserve
◦ Keep evidence cool and dry during transportation
and storage
◦ Mold can damage DNA
◦ Avoid plastic bags for DNA evidence
◦ Direct sunlight and warm conditions can harm
DNA
Analysis of DNA Fingerprints
Which man is the father?
◦ Can either man be excluded as the father?
◦ Which man may be the father of the child?
◦ Is this DNA profile sufficient to establish paternity?
Paternity
Child inherits one gene from each parent
If two genes from parents are different, then two bands
appear in child’s DNA fingerprint
If two genes are identical, only one band appears
The more probes used in DNA fingerprint, the greater the
accuracy of the DNA fingerprint
Most criminal cases use 6-8 probes
Analyze both position and width of the bands
All bands have to match exactly
Analysis of DNA Fingerprints
Some countries have databases of DNA profiles
US has CODIS
◦ Combined DNA Index System
◦ Electronic database of DNA profiles
◦ Every state maintains a DNA index of individuals convicted of rape,
murder, child abuse
Probability of identity is a measure of the likelihood that two
individuals selected at random will have an identical STR type
STR’s occur independently, probability of having a particular
combination of STR types is determined by the product of their
frequency in a population
The Break-in…
One afternoon, a break-in occurred at a high school, and several computers were stolen. At the time
of the break-in, the building was empty. A motion detector tripped by movement in one of the
hallways alerted the police. When the police arrived to investigate, they found that one of the doors
leading into the school had been propped open with paper wedged into the door-jamb. The door
appeared to be locked, but it could easily be pushed open. Near the door, police found a cold soft
drink can. Because of the cool temperature of the drink, police suspected that the can was left by one
of the intruders.
The can was bagged as evidence, and in the forensics lab, a DNA sample was obtained from the lip of
the can. The neighborhood was canvassed, and a clerk in a convenience store remembered selling
canned soft drinks to two young males just before the break-in occurred. The surveillance video in the
convenience store was examined, and the clerk provided the police with the names of all males who
were in the store just prior to the break-in. Three suspects were identified from the surveillance video.
Using a DNA sample found from the soft drink can collected at the crime scene, a PCR was run to
amplify the amount of DNA, and then a DNA profile was performed. Cheek swabs were obtained from
the 3 suspects, and their DNA was tested.
Match the crime scene DNA with a DNA sample from the 3 suspects.
The Break-in
DNA Fingerprint Results
1. Does the crime scene DNA match the DNA from any of the suspects?
2. Is there more than one DNA match?
3. Is this DNA profile sufficient to convict a suspect?
Who Are the Parents?
Who are the Parents?
Three baby boys were born on the same morning in the same
hospital. That morning, the hospital had started using new
identification bracelets. When the babies were bathed, the ID bracelets
slipped off and the nurses thought a mix-up might have occurred.
Recall that 50 % of a child's DNA is obtained from each parent.
Use a ruler to align the DNA bands of the baby with any DNA bands of
the parents. Determine if any parents share the same band of DNA with
the babies.
There is only one correct set of parents matching a baby.
1.
Which baby belongs to
the Meanys?
2.
Which baby belongs to
the Glicks?
3.
Which baby belongs to
the Moes?
4.
Is it possible for a child
to have a DNA band that
is not found in the
mother's DNA?
Do Now
A young woman claimed that her son was born as the result
of an affair with a famous athlete who was a
multimillionaire. The athlete denied paternity, so the woman
took her case to court, and sued for an extremely generous
child support payment. The three individuals gave blood
samples for DNA analysis, and the fingerprints are shown
below.
Was the woman's claim justified?
Do Now
Do Now:
The fingerprints to the right are from a Guatemalan family. A
woman claimed that
her daughter was born in Canada and emigrated to Guatemala
to join her father. When she wanted to
return to Canada, the immigration authorities claimed that she
was not the woman's real daughter.
On the basis of the evidence is the woman's claim true?
Jimmy Solo, a famous rock star, returned to his New York mansion from an appearance in
Toronto. As Jimmy entered the side entrance he noticed a great mess in the living room and
den. Drawers had been emptied; couch cushions ripped open; the furniture was in disarray and
the wall safe in the study was open. He stormed upstairs to see if his expensive jewelry
collection had been touched and upon doing so heard someone in his bedroom. He grabbed a
small metal statue and rushed into his bedroom surprising the thief. In the ensuing struggle the
thief's gun discharged killing Jimmy instantly.
The next day the housekeeper found Jimmy dead on the floor and immediately contacted the
police. The subsequent investigation identified seven possible suspects.
1. Willie James - lead guitarist in Jimmy's band who was constantly upstaged and underpaid by Jimmy
2. Samantha Smith - Jimmy's former girlfriend who was dropped by him following three years of
cohabitation
3. Lucifer - Jimmy's bodyguard who is extremely jealous of Jimmy's wealth
4. Jamie Waite - Jimmy's manager who has accumulated massive gambling debts
5. Dicky Schultz - a former acquaintance of Jimmy's who claims he is owed thousands for past services
6. Casparina - Jimmy's new girlfriend who is under pressure from former friends to pay back large
loans
7. Jimmy The Weasel - a former friend of Jimmy who was promised a position in Jimmy's band that
never materialized
It was established that all had a motive for killing Jimmy and none had an ironclad alibi. The
police realized they had a problem and consequently decided to undertake a DNA fingerprinting
of all suspects and from a blood sample taken from the crime scene.