How DNA Changed the World of Forensics
 What did Forensic Scientists use before DNA
 What does the Innocence Project do?
How DNA Changed the World of Forensics
 How did/do lay jury view hair evidence?
 Why is DNA evidence so important?
The Killer’s Trail Video
 What tool is Sam Sheppard’s son using to exonerate
his father?
 What is compared in DNA analysis?
The Killer’s Trail Video
 What is the DQA1 test?
 How is DNA replicated in the lab?
The Killer’s Trail Video
 How many alleles are compared in the DQA1 Test?
 What did the DQA1 Test show?
DNA Analysis
By the end of this unit you will be able to:
 Explain how crime scene evidence is
collected and processed to obtain DNA
 Describe how radioactive probes are used in DNA
 Explain how DNA evidence is compared for matching
 Explain how to use DNA fingerprinting to identify DNA
from a parent, child, other relative, or a non-related
Introduction and History of Biological
Evidence in Forensics
DNA fingerprinting, also known as DNA profiling, is
used in criminal or legal cases with a high degree of
Biological evidence such as blood, saliva, urine,
semen, and hair is examined for the presence of
genetic traits.
Basic Vocabulary
 Genome- a full set of chromosomes; all the
inheritable traits of an organism.
 Gene- segment of DNA in a chromosome that have
instructions that determine our inherited
characteristics or traits.
Ex. Blood type, hair color
 Allele- an alternate form of a gene
 Ex. A gene for human hair color may have alleles that cause
red or brown hair
DNA Structure
 DNA = deoxyribonucleic acid
 DNA is the blueprint of life and contains
the genetic material of a cell.
 Found in the nucleus of cells in the
human body.
 DNA is a polymer consisting of
thousands of smaller, repeating units
called nucleotides (monomer)
 The order of the bases determines the
genetic code.
DNA Structure
 DNA is made up of two strands of nucleotides that form
a structure that looks like a twisted ladder which we call
a double helix.
DNA Structure
 Double helix is made up
Sugar/Phosphate Backbone
(sides of ladder)
Strands of the double helix
are connected by the
nitrogenous bases held
together by hydrogen bonds
(rungs of the ladder )
H Bond
Figure 9–1 How nucleotides can be linked to form a DNA strand. S
designates the sugar component, which is joined with phosphate groups
(P) to form the backbone of DNA. Projecting from the backbone are
four bases: A, adenine; G, guanine; T, thymine; and C, cytosine.
The Bases
 Four types of bases are associated with the DNA
structure: adenine (A), guanine (G), cytosine (C), and
thymine (T).
 Complimentary Base Pairings: every base pair consists of
one purine (A or G) and one pyrimidine (T or C)
Adenine (A) always pairs with thymine forming 2 hydrogen bonds
 A-T
Guanine (G) always pairs with cytosine forming 3 hydrogen bonds
 G-C
 The order of the bases is what distinguishes different
DNA strands.
Base Pairings
Label the DNA Strand Below
H Bonds
DNA Structure
 DNA makes up structures
known as chromosomes
DNA coils into chromosomes
around proteins
Chromosomes are found in a
cell’s nucleus
Human DNA
 There are 23 pairs (total of
46) of chromosomes in the
nucleus of most human
cells (except sperm and egg
One chromosome in each
pair is inherited from the
mother and the other is
inherited from the father
50% of a person’s DNA
come from each parent
DNA Replication
 DNA duplicates itself prior to cell division.
 DNA replication begins with the unwinding
of the DNA strands of the double helix.
Helicase “unzips” the DNA at origins of
replication by breaking the Hydrogen bonds
between the Nitrogen bases.
 Each strand is now exposed to a collection of
free nucleotides that will be used to recreate
the double helix, letter by letter, using base
DNA poylmerase bonds the nucleotides
together to form two strands.
DNA polymerase also “proofreads” each new
DNA strand, ensuring that each molecule is a
perfect copy of the original .
Coding vs Non-coding DNA
 Coding DNA
 5% of your DNA
 Recipe that “codes” for proteins that make you unique
 Non-coding DNA (Junk DNA)
 95% of your DNA
 Doesn’t seem to code for anything
 Repeats the same base pair sequences over and over (aka
Tandem Repeats)
 Best for ID’ing people forensically because it varies most in
DNA Identification
 Junk DNA (noncoding) contains many of the unique
patterns of repeated base sequences that identify
individuals called polymorphisms.
 In 1984 a technique was developed for isolating and
analyzing these tandem repeats.
 To a forensic scientist, these tandem repeats offer a
means of distinguishing one individual from another
through DNA typing.
 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.
DNA Fingerprinting
Who Invented it?
 The process of DNA
fingerprinting was invented
by Alec Jefferys at the
University of Leicester in
 He was knighted in 1994.
Avoiding Contamination in the Collection and
Preservation of DNA
Use disposable gloves and collection instruments
Avoid physical contact, talking, sneezing and coughing in the
evidence area.
Each stained article should be packaged separately in a paper
bag or in a well-ventilated box.
Avoiding Contamination in the Collection and
Preservation of DNA
 Dried blood is best removed from a surface by using a
sterile cotton swab lightly moistened with distilled water
that is air dried before being placed in a swab box, then a
paper or manila envelope.
 All biological evidence should be refrigerated or stored in
a cool location until delivery to the laboratory.
 Standard/reference DNA specimens must also be
collected, such as blood or the buccal swab (swabbing the
mouth and cheek).
DNA Fingerprinting- STEP 1: Extraction
Cells are isolated from biological evidence such as blood,
saliva, urine, semen and hair.
The cells then are disrupted to release the DNA from proteins
and other cell components.
Once released, the DNA can be extracted from the cell
DNA Fingerprinting- STEP 2: Restriction
The DNA is cut into fragments using restriction enzymes.
Each restriction enzyme cuts DNA at a repeating base
sequence, forming different sized fragments.
DNA Fingerprinting- STEP 3:
Amplification PCR
 For the forensic scientist, PCR offers a distinct advantage in
that it can amplify minute quantities of DNA found at a crime
scene many millions of times.
 PCR technology was created using understanding of how DNA
strands naturally replicate within a cell.
1. The DNA is heated to separate it.
2. Primers (short strands of DNA used to target specific regions of DNA for
replication) are added, which hybridize with the strands.
3. DNA polymerase and free nucleotides are added to rebuild each of the
separated strands.
4. Process is repeated 25 to 30 times.
DNA Fingerprinting- STEP 3:
Amplification PCR
DNA Fingerprinting- STEP 4:
The DNA is loaded into the chambers found on an agarose
2. An electric current is passed through the gel separating the
fragements by size.
Analysis of DNA Fingerprints and Applications
 Bands and widths are significant in matching
samples of DNA.
 DNA fingerprinting can
A. Match crime scene DNA with a suspect
B. Determine maternity, paternity or match to another
C. Eliminate a suspect
D. Free a falsely imprisoned individual
E. Identify human remains
Restriction Fragment Length Polymorphisms (RFLP)
 RFLP is a technique that is not widely used now
but it was one of the first techniques used for
DNA analysis in forensic science and several
other fields. RFLP stands for 'restriction
fragment length polymorphism,' which is a term
that refers to a variation in a sequence of DNA
that is detectable through gel electrophoresis.
Variable Number of Tandem Repeats (VNTR)
 Within junk DNA, sequences of DNA are repeated
multiple time.
 Some can be 9-80 bases in length.
 Example: If a repeated base sequence is CATACAGAC
there might be three copies in the DNA of one
The number of copies of the same repeated base sequence in DNA
varies among individuals
Short Tandem Repeats
 The latest method of DNA typing, short tandem repeat
(STR) analysis, has emerged as the most successful and
widely used DNA profiling procedure.
 STRs are locations on the chromosome that contain
short sequences (usually 2-5 bases in length) that repeat
themselves within the DNA molecule.
 They serve as useful markers for identification because
they are found in great abundance throughout the
human genome and are more common than VNTR.
 Example: if a repeated base sequence is GATA there
might be four copies in the DNA of one individual
DNA Profiling and DNA Population Databases
 VNTR and STR data are analyzed for:
 Tissue Matching
 Inheritance Matching
 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)
A suspect in a crime
An alleged father in a paternity case
Figure 9-16 Every cell in the body contains hundreds of mitochondria, which
provide energy to the cell. Each mitochondrion contains numerous copies of
DNA shaped in the form of a loop. Distinctive differences between individuals in
their mitochondrial DNA makeup are found in two specific segments of the
control region on the DNA loop known as HV1 and HV2.
Mitochondrial DNA Testing
 Mitochondrial DNA typing does not approach STR
analysis in its discrimination power and thus is best
reserved for samples, such as hair, for which STR
analysis may not be possible.
 Forensic analysis of mtDNA is more rigorous, time
consuming, and costly when compared to nuclear
DNA analysis.
 All individuals of the same maternal lineage will be
indistinguishable by mtDNA analysis.
 Perhaps the most significant tool to arise from DNA
typing is the ability to compare DNA types recovered
from crime scene evidence to those of convicted sex
offenders and other convicted criminals.
 CODIS (Combined DNA Index System) is a computer
software program developed by the FBI that maintains
local, state, and national databases of DNA profiles
from convicted offenders, unsolved crime scene
evidence, and profiles of missing persons.
A. Who done it?
C. Identical or not?
Which sets of twins are
identical twins?
Which suspect matches
the bloodstain?
Information & image from
B. Whose your daddy?
Which sample is most
likely to be the father?
F1 or F2

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