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DNA = Deoxyribonucleic Acid
Located in CHROMOSOMES in the nucleus of
cells
◦ What is a chromosome?
 Tightly packed genetic information
◦ Where do we get them from?
 One from each parent!
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Genes – portions of DNA that code for traits
and functions
DNA
Timeline
1868 Miescher “discovers” DNA
1953 Watson and Crick report
double helix structure
1977 First human gene cloned.
1984 Jeffreys reports DNA
sequences
1985 First report of PCR method
1986 Jeffreys uses DNA to solve
first murder case
1987 First conviction on DNA
evidence
1991 STRs first reported
1998 FBI starts CODIS database
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Discovery
• Discovered in 1984 by
Dr. Alec Jeffreys at
the University of
Leicester
• He was knighted for
his discovery
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Case Study: The First Use of DNA Evidence
• Two teenage girls raped and murdered in Leicestershire,
England
• Semen from the victims indicated a male with Type A blood and
a rare enzyme (10% of the local male population)
• A local boy, Richard Buckland, confesses upon interrogation
• Police use DNA fingerprinting to confirm, but DNA profiles of
Buckland and crime scene DNA do not match
• Ironically, Buckland becomes the first person exonerated by
DNA evidence
• Police request DNA samples from all adult males in 3 nearby
villages (5000 men)
• 6 months later – no results!
• A year later, police are informed by a bakery worker that they
overheard a co-worker bragging they had given a DNA sample
for another man
• Police obtain DNA from Colin Pitchfork and obtain a perfect
match
• In 1988, Colin Pitchfork was tried and convicted and sentenced
to life in prison for the double rape and homicide based in large
part to the DNA evidence
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DNA is a POLYMER made up of
Nucleotides are made up of three parts
Sugar (deoxyribose)
Phosphate
A nitrogen containing base (A,T,G,C)
There are approximately 100 million nucleotides in the
average DNA molecule
Double helix—two coiled DNA strands
 Composed of nucleotides connected together

 Four bases make the rungs:
 Adenine
 Cytosine
 Guanine
 Thymine
Bases always bind with specificity (complementary
base pairing)
A-T
G–C
A, G, C or T
Forms sugar
Phosphate
Backbone
What makes DNA
Different from
RNA?

Nucleic Acid Polymer (DNA)
Phosphate
Base
Sugar
Phosphate
Base
Sugar
Polymer
Phosphate
Base
Sugar
Phosphate
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Can predict sequence of one strand based on
the sequence of the other.
Responsible for Replication and Transcription
.
◦ If a nucleotide is changed the “wrong” amino acid is placed in the
protein and the protein may not function correctly and this is the
basis for many diseases and health issues.
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The specific sequence of nucleotides of all
human beings is 99.9% the SAME!!
It is that 0.1% difference that makes each
person unique
◦ What is the exception to this rule?
Identical twins
◦ What is so important about the sequence?
It is the coding for proteins
A
TCGACTAACCGAC
T A G C T G A T T G G C T G
 Forensic cases – matching suspects with evidence
 Exonerate individuals
 ID crime and catastrophe victims (ex. 9/11)
 Establish paternity and other familial relations
 Match organ donors with recipients in transplant
programs
 Missing persons investigations
Bodily sources of DNA
All Cells that have a nucleus:
 white blood cells (NOT RED BLOOD
Cells-they don’t have a nucleus)
 Semen
 saliva –contains buccal (cheek cells)
 hair root
 Teeth
 bone
 Any tissue
All cells contains thousands of
mitochondria (which contain maternal
DNA)
DNA “Typing” (Fingerprinting/Analysis)
3 main technologies have been used:
1) RFLP –Restriction Fragment Length Polymorphisms
- Developed in 1985 & used until mid 1990’s
2) PCR –Polymerase Chain Reaction
- Developed in the 1980’s, but perfected in mid 1990’s
- Still used today as a step in the STR process
3) STR-Short Tandem Repeats
- Developed in the 1990’s
- Used almost exclusively today
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Restriction Fragment Length Polymorphism
◦ Characterize fragments and calculate the statistical
probability that two people could have the same
fragment sequence
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Portions of DNA contain sequences of bases that are repeated
numerous times, known as
These tandem repeats offer a means of distinguishing one individual
from another.
They act as filler or spacers between the coding regions of DNA.
What is important to understand is that all humans have the same type
of repeats, but there is tremendous variation in the number of repeats
each of us have.
cut DNA into fragments. They
are thought of as highly specialized scissors that cut
a DNA molecule when it recognizes a specific
sequence of bases.
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Length differences associated with relatively long
repeating DNA strands are called
Typically, a core sequence consists of 15 to 35 bases
in length and repeats itself up to a thousand times.
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General Overview:
1)Isolate—separate DNA from the cell of desired
individuals & evidence
2)Cut—using restriction enzymes to cut DNA into
smaller fragments
3)Separate/ Sort—by size using electrophoresis which
creates banding pattern.
4)Analyze—the specific allele patterns for
identification
DNA “Fingerprinting” RFLP
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Restriction enzyme cuts at ….GAATTC…..
Spacing between cutting sites for enzyme is different for
different individuals
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Once the DNA molecules have been cut up by a
restriction enzyme, the resulting fragments are
sorted out by
The smaller DNA fragments will move at a faster
rate on the gel plate than the larger ones.
The fragments are then transferred to a nylon
membrane
The nylon sheet is treated with radioactive probes
to visualize the RFLPs
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RFLP: Electrophoresis
• DNA is visualized using electrophoresis
• Negatively charged DNA moves through a gel with a current
• Smaller DNA moves faster than larger DNA fragments
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Which Suspect, A
or B, cannot be
excluded from the
class of potential
perpetrators of
this assault?
Three Possible Outcomes
 Match —The DNA profile appears the same.
 Excluded—The two samples originate from different
sources.
 Inconclusive—The data does not support a conclusion
as to whether the profiles match.
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Need a relatively large amount of DNA
Large sequence size/lengths (do not allow for
copying)
Requires a good sample (undegraded)
Relatively slow
Crime scene DNA is often in adverse conditions
What to do
when
there’s not
much
there…
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Technique used for making copies of DNA
molecule
PCR can amplify minute quantities of DNA many
millions of times.
Uses high temperature enzyme that “mindlessly”
copy DNA
After just 20 cycles a single copy of DNA is
amplified over 2,000,000 fold.
Can be automated
Can use very small samples!
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PCR process -
DNA “Fingerprinting”
Steps:
Isolate DNA piece
1. Heat/Denature: separation of DNA Strands
(unzip)
2. Annealing: use primers [“start” and “stop”
switches] to anneal [bind to complimentary DNA
sequences]
3. Extension: use polymerases to build complimentary
strand between “switches”
DNA Amplification - PCR

Separated into 2 strands (Denaturation)
◦ Heat or chemically
A
G
A
T
A
G
T
C
T
A
T
C
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- attaching primers (“start” and “stop”
switches for DNA replication.
Primer - fixed
DNA binding
points.
T
- using polymerases
T
Completes first cycle - cycles repeated many times.
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PCR technology cannot be applied to RFLP
since the strands are too long (often
numbering in the thousands of bases)
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Typically 28-32 PCR cycles are run resulting in
billions of copies of DNA
Each cycle that doubles the DNA takes about 2
minutes.
Minute amounts of DNA may be used for
amplification (less than 1 billionth of a gram).
Can get enough DNA from envelopes, stamps, soda
cans, & cigarette butts to run PCR
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The latest method of DNA typing uses
analysis.
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STRs are locations on the chromosome that contain short
sequences of 3-7 bases that repeat themselves within the
DNA molecule (100-200x).
Serve as useful markers for identification since they are
found in great abundance throughout the human genome
Because they are short, STRs are ideal candidates for
multiplication by PCR
STRs are much less susceptible to degradation and may
often be recovered from stains that have been subjected
to decomposition
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Short Tandem Repeats (STRs)
STR typing is visualized by peaks shown on a graph. Each represents the
size of the DNA fragment.
The possible alleles are numbered for each loci.
Norma's genotype is 15 repeats, 15 repeats at the locus D3S1358,
14 repeats, 16 repeats at vWA,
24 repeats, 25 repeats at FGA
(gets on from each parent)
David & Karen are parents of a missing child:
DNA Profile from remains found in a shallow grave:
Could this be
their child?
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Another tool available for crime scene
investigations are STRs located on the Y
chromosome, which is male specific.
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More than 20 different Y-STR markers have
been identified
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Y-STRs will prove useful when multiple males
are involved in a sexual assault or in question
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The Y-STR is less complicated in appearance
and interpretation.
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Shorter DNA strands tend to be more stable and
less susceptible to degradation
It can amplify minute quantities of DNA- need only
18 DNA bearing cells (100 times less than that
normally required for RFLP analysis)
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Works well on fragmented DNA
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More rapid analysis
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Probability of a person matching a random DNA sample
at any 1 STRS site is roughly 1/10
3 STRS sites?
1/10 x 1/10 x 1/10 = 1/1000
All 13 STRS sites would mean that the chances of
matching a random DNA sample are about 1 in 10
trillion:
1/10 x 1/10 x 1/10 x 1/10 x 1/10 x 1/10 x
1/10
x 1/10 x 1/10 x 1/10 x 1/10 x 1/10 x 1/10 x =
1/10,000,000,000,000
Probability of two different people matching at all 13
STRS sites is virtually zero.
A Sample Profile
• By combining the frequency information for all 13 CODIS loci, the
frequency of this profile would be 1 in 7.7 quadrillion Caucasians
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Nuclear DNA
◦ Located in the nucleus of cells
◦ Inherited ½ from mother and ½ from father
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Mitochondrial DNA
◦ Located in mitochondria (found in cytoplasm)
◦ Inherited solely from mother
◦ Used when nDNA typing is not possible (old, degraded
samples)
◦ More costly and time consuming
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Shows maternal lineage (will be identical if
related through the mother)
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FBI’s CODIS DNA Database
Combined DNA Index System
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Used for linking serial crimes and unsolved cases with repeat
offenders
Launched October 1998
Requires the
markers
The Forensic Index contains DNA profiles from crime scene
evidence.
The Offender Index contains DNA profiles of individuals convicted
of sex offenses and other violent crimes with many states now
expanding legislation to include other felonies
◦ Forensic Profiles in NDIS (National DNA Index system – a component of CODIS): 119,782
◦ Convicted Offender Profiles in NDIS: 2,643,409
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