Chapter 7
DNA Fingerprinting
Forensic Science

Chapter 7 DNA Fingerprinting
Objectives and Vocabulary
Objectives
– Explain how crime scene
evidence is collected for DNA
analysis
– Describe how crime scene
evidence is processed to obtain
DNA
– Describe how radioactive probes
are used in DNA fingerprinting
– Explain how DNA is compared for
matching
– Explain how DNA fingerprinting is
used to determine if specimens
come from related or unrelated
individuals
– Explain how to use DNA
fingerprinting to identify DNA from
a parent, child, or relative of
another person
Vocabulary
– Allele
– Chromosome
– DNA fingerprint
– DNA probe
– Electrophoresis
– Gene
– PCR (polymerase chain reaction)
– Restriction enzyme
– STR (short tandem repeat)
– VNTR (variable number of tandem
repeats)

Chapter 7 DNA Fingerprinting
Introduction
Except for identical twins,
not two people have the
same DNA.
Since the 1980’s DNA
evidence has been used
to investigate crimes,
establish paternity, or
identify victims of large
scale disasters.
DNA evidence can only
be linked to only one
person

Chapter 7 DNA Fingerprinting
History of Biological Evidence in
Forensics
Types of biological evidence
– Skin
– Blood
– Saliva
– Urine
– Semen
– hair
Their chromosomes can be examined to determine,
karyotyping
Ex. blood typing
DNA fingerprinting is used in criminal and legal cases to
determine identity and parentage.

Chapter 7 DNA Fingerprinting
The Function and Structure of
DNA
DNA is a blueprint of life, contains the genetic
material of the cell
Makes proteins and copies of itself
Genetic information is stored in molecules of
DNA making up structures called chromosomes
Made up of two strands, know as a double helix
James Watson and Francis Crick received the
1953 Nobel Prize for describing the structure of
DNA

Chapter 7 DNA Fingerprinting
The Function and Structure of
DNA
1. Sugar phosphate
backbone
2. Phosphate Group
3. Nitrogenous bases
joined by hydrogen
bonds
Adenine
Thymine
Guanine
Cytosine

Chapter 7 DNA Fingerprinting
The Different DNA Bases
DNA strands are considered to be
complementary
Following complementary base pairing
rules
– A pairs with T
– G pairs with C
A G C G C C G A T T A G C G C T A
AT AAC G C C C CG G T T G G T

Chapter 7 DNA Fingerprinting
The Different DNA Bases
There are 23 pairs of
chromosomes (a total of
46)
One chromosome of each
pair is inherited from the
mother and one from the
father
Two types of DNA
– Nuclear – virtually identical
in all cells of the human
body
– Mitochondrial – is passed
in the cytoplasm of the egg,
found in the mitochondrial,
circular

Chapter 7 DNA Fingerprinting
Genes and Alleles
Genes are DNA
sequences that have
instructions that
determine our inherited
characteristics or traits
Genes also make up
RNA
– RNA single stranded
– Does not include T, has
uracil (U)
An allele is one of two or
more alternative forms of
a gene (one allele from
the mother one allele
from the father)

Chapter 7 DNA Fingerprinting
Genes and Alleles
Entire human genome is contained in the
nucleus
Approx. 3 billion base pairs
DNA  RNA  proteins
Has exons (produces RNA and proteins)
and introns (“junk” DNA, may be useful in
gene splicing)
The nucleus of each human cell contains
23,688 genes, averaging 3,000 bp
Central
Dogma

Chapter 7 DNA Fingerprinting
Genes and Alleles

Chapter 7 DNA Fingerprinting
DNA Identification
Most of the human genome is the same,
but there are some variations, this allows
us to identify individuals
Have unique sequences in non-coded
DNA
DNA sequences have different lengths
and different sequences, these differences
are called polymorphisms

Chapter 7 DNA Fingerprinting
DNA Identification
1984, Dr. Alec Jeffreys
developed a technique for
isolating and analyzing
these variable areas,
knows as DNA
fingerprinting or DNA
profiling
Unique patterns look like
bands
The examination of DNA
profiles can help forensic
scientists decide if two or
more DNA samples are
from the same individual

Chapter 7 DNA Fingerprinting
VNTR
Variable number of tandem repeats
(VNTR)
Certain short sequences of DNA are
repeated multiple times
The number of repeats differ from person
to person
Ex.
CATACAGACCATACAGACCATACAGAC

Chapter 7 DNA Fingerprinting
STR
Short tandem repeat (STR)
High degree of polymorphism and most useful
for DNA analysis
Much shorter than VNTR, usually only two to five
bp in length
Different number of copies of the repeat element
Ex.
GATAGATAGATAGATAGATAGATAGATAGATA
Preferred because of its accuracy and because
small and partially degraded DNA samples may
be analyzed still

Chapter 7 DNA Fingerprinting
DNA Profile
A DNA fingerprint can be developed with
several different VNTRs and STRs
For tissue matching, two samples that
have the same band pattern are from the
same person
For inheritance matching, each band in a
child’s DNA fingerprint must be present in
at least one parent

Chapter 7 DNA Fingerprinting
DNA Profile
Two main purposes
– Tissue matching
Exact same pattern
– Inheritance matching
Follows the rules of
inheritance

Chapter 7 DNA Fingerprinting
DNA Profile

Chapter 7 DNA Fingerprinting
Population Genetics and DNA
Databases
Population genetics – the study of
variation in genes among a group of
individuals
Ex.
– Asian populations, blue eyes are rare (more
common among northern Europeans)
– Great Britain/US, 46% of people have O blood
but native South Americans, almost of 100%
of population has O blood

Chapter 7 DNA Fingerprinting
Collection and Preservation of DNA
Evidence
Attention to contamination issues is
necessary
Can occur if DNA from another source is
mixed into the DNA from the crime scene

Chapter 7 DNA Fingerprinting
Preparing DNA Samples for
Fingerprinting
Use gel
electrophoresis
– When different sized
DNA fragments are
separated with an
agarose gel

Chapter 7 DNA Fingerprinting
Steps of DNA Fingerprinting
1. Extraction.
extraction from the nucleus
2. Restriction fragments.
cuts into smaller pieces at specific sequences
3. Amplification.
PCR (polymerase chain reaction)
4. Electrophoresis

Chapter 7 DNA Fingerprinting
Electrophoresis
Movement based on
size and charge
– DNA is negatively
charged
Sample is placed in a
well