Forensic Science
Name: TEACHER KEY
A. Intro
a. DNA, or Deoxyribonucleic Acid, is the genetic material in our cells.
i. No two people (except identical twins) have the exact same DNA.
b. DNA fingerprinting is also known as DNA profiling. It is a technique used by scientists to
distinguish between individuals of the same species using only samples of their DNA.
i. The process of DNA fingerprinting was invented by English geneticist Alec Jeffreys
at the University of Leicester in 1985.
c. DNA fingerprinting has been used to help solve crimes by linking suspects to crimes and by
helping identify bodies of victims.
i. In addition to crime investigation, DNA fingerprinting can also be used to:
1. establish paternity and parentage
2. identify victims of war and large scale disasters
3. study biodiversity of species
4. track genetically modified crops
5. settle immigration disputes
ii. Most lab techniques used for DNA fingerprinting were not intended for
these purposes, but instead were developed for use in the medical field for
diagnosis and treatment of diseases.
d. Small amounts of biological evidence left at crime scenes, called trace evidence, are
the source of DNA needed for DNA fingerprinting.
i. Biological evidence includes saliva, blood, semen, skin, hair roots, body
tissue cells, and even urine.
ii. Because this evidence is capable of identifying a specific person, it is
considered individual evidence.
B. Structure and function of DNA
a. DNA is a nucleic acid, found in chromosomes, in the nucleus of your cells.
i. Most human cells have 23 pairs of chromosomes, or 46 total.
1. 23 chromosomes come from the mother, 23 come from the father.
2. Exception is sperm and egg cells which have
only 23 chromosomes because they are not in
pairs.
ii. Each chromosome pair can be broken into smaller
segments called genes.
iii. Genes control the traits of the organism, and therefore
can vary between individuals.
b. Genetic code
i. DNA has four different nitrogenous bases:
1. Adenine (A),
2. Thymine (T),
3. Guanine (G),
4. Cytosine (C).
ii. These bases form pairs according to the base pairing rule:
1. Adenine binds only with Thymine
2. Cytosine binds only with Guanine
3. These pairs are considered to be ‘complementary’.
iii. If one strand of DNA has the genetic code: A T C T G C
its complementary strand would read:
T A G A C G
iv. DNA is made when these two strands twist together in a shape called the
double helix.
1. The sides of the helix, referred to as the backbone of DNA, are made
up of alternating sugar and phosphate molecules.
a. The sugar in DNA is called deoxyribose, from which DNA got
its name.
2. The rungs of the helix are made up of the paired nitrogenous bases
(A-T, C-G), which help to code the DNA with instructions for the cell.
c. Types of DNA
i. DNA in chromosomes is called nuclear DNA.
1. Nuclear DNA is inherited from both the mother and father, and is virtually identical in
all cells of an individual’s body.
ii. However, another type of DNA can also be found in the mitochondria of cells.
1. Mitochondrial DNA is in the form of a circular loop, and unlike nuclear DNA, is inherited
only from the mother.
2. Mitochondrial DNA is passed to offspring in the cytoplasm of the egg cell at fertilization;
no mitochondria come from the sperm cell.
3. Both Nuclear DNA and Mitochondrial DNA can be useful in forensic science.
d. Genome
i. The human genome is the total amount of DNA in a cell. It is contained in both the nucleus and
mitochondria.
ii. The human genome consists of approximately 3 billion base pairs.
iii. The purpose of the genome is to code the blueprint for the human body and how it works. It is
responsible for telling the body to make proteins and other needed molecules.
iv. However, not all of the 3 billion bases are useful code.
1. Chromosomes have regions called exons and introns.
a. Exons are encoded DNA, with directions to build molecules
i. Exons made up only 1.5% of the entire genome.
ii. Together, they code for about 24,000 different genes.
iii. The rest of the genome is made of introns.
b. Introns are un-encoded DNA, that do not code for the production of molecules.
i. These segments make up 98.5% of DNA and are often referred to as
‘junk DNA’.
ii. Despite its nickname, introns may function in gene splicing and
therefore may have an important role in biology after all.
iii. Introns are also very useful in forensic science!
C. DNA Identification
a. Most of the human genome is the same in all humans, but some variation exists among individuals.
i. Scientists can identify individuals based on the variations
ii. Most of the variation in DNA is found in the non-coding segment, or introns.
iii. Much of the non-coding DNA is in the form of repeated base sequences, and some of these
sequences can be repeated many times.
1. Polymorphisms are the non-coded DNA segments that contain unique patterns of
repeated base sequences that that are unique to individuals.
2. DNA Fingerprinting isolates and analyzes polymorphisms.
3. DNA fingerprints appear as a pattern of bands.
D. Repeating DNA sequences
a. Within the non-coding sections of DNA are short sequences of bases repeated multiple times.
i. The number of times the sequence repeats varies between individuals.
ii. Example: If the repeated sequence is CATACAGAC
1. One individual might have three copies of this in their DNA:
a. CATACAGAC CATACAGAC CATACAGAC
2. While another individual might have seven copies.
b. Two types of repeating DNA sequences
i. Variable Numbers of Tandem Repeats (VNTR) are 9 to 80 bases in length
ii. Short Tandem Repeats (STR) are 2 to 5 bases in length
1. A STR is much shorter than a VNTR and therefore is usually preferred for DNA
fingerprinting.
c. VNTR and STR data from DNA fingerprints can be analyzed for two main purposes:
i. Tissue matching
1. Comparing DNA evidence from a crime scene with DNA taken from a suspect
2. How it looks: Two samples that have the same band pattern are from the same person
ii. Inheritance matching
1. Comparing family members’ DNA for proof of familial relationships
2. How it looks: Each band in a child’s DNA fingerprint must be present in at least one
parent (50% from mom, 50% from dad)
iii. Practice: What can we tell about the familial relationships from the following family?
D1: Mom and dad are biological parents.
D2: Mom is biological parent; has a different biological father.
S1: Mom and dad are biological parents.
S2: Neither mother or father is biological.
E. Trace evidence
a. Often the amount of evidence left at a crime scene is very small and therefore considered to be trace
evidence.
b. A problem with analyzing trace evidence is that many forensic tests will destroy the evidence sample, as
is the case with DNA.
c. Therefore, prior to DNA fingerprinting, PCR is used.
i. PCS stands for Polymerase Chain Reaction.
ii. PCR is a technique that makes thousands of copies of segments of DNA that investigators want
to analyze.
iii. The DNA produced by PCR can then be used to make a DNA fingerprint, without worry of
destroying the evidence.
iv. How PRC Works:
1. Crime scene (template) DNA is mixed with nucleotides, an enzyme known as DNA
polymerase, and primers.
a. Primers are short segments of complimentary DNA that base-pair with the
template DNA upstream of the region of interest and serve as recruitment sites
for the polymerase
2. Cycles of denaturation, annealing and extension are repeated to achieve exponential
amplification of the target sequence, allowing for billions of DNA copies to be produced
in just a few hours.
F. Collection and Preservation of DNA Evidence
a. Because DNA is found in all cellular material and cells are very small, contamination is an important
issue to address when collecting and preserving DNA evidence.
b. To avoid DNA evidence contamination, crime scene investigators should:
i. Wear disposable gloves and change them often
ii. Use disposable instruments for handling each sample
iii. Avoid talking, coughing, or sneezing over evidence
iv. Do not touch your face or body when collecting or packaging evidence
v. Air-dry evidence before packaging. If evidence cannot be dried, it may be frozen.
vi. Avoid using plastic bags to store evidence that contains DNA; use paper bags or envelopes
vii. Keep evidence cool and dry during transportation and storage. Avoid direct sunlight which can
damage DNA.
G. Steps of DNA Fingerprinting
a. Extraction of the DNA
i. Cells are isolated from the tissue and are disrupted to release DNA
b. Cutting the DNA into restriction fragments
i. DNA contains long strands of genetic information. In order to
analyze the information, those strands need to be cut into shorter
fragments.
ii. Restriction enzymes are “molecular scissors” that cut DNA at specific locations.
iii. After adding restriction enzymes to DNA, the DNA contains smaller fragments of different
lengths. These are referred to as RFLPs, Restriction Fragment Length Polymorphisms.
c. Amplification (making many copies of those fragments)
i. Polymerase Chain Reaction (PCR) is used to create many copies of the RFLPs.
d. Electrophoresis
i. Electrophoresis is a process that separates RFLPs according to their length, creating a DNA
Fingerprint.
ii. How gel electrophoresis works:
i.
ii.
Note: one well should contain a control, a solution containing
DNA fragments of known lengths, called Marker or Standard
DNA.
Other wells should contain DNA from crime scene, victim,
and suspects.
e. Analysis of DNA Fingerprints
i. DNA fingerprints look like a striped column. In order for DNA
fingerprints to match, the columns must have bands in the
exact same places with the exact same widths.
ii. DNA fingerprinting can:
1. match crime scene DNA with a suspect
2. eliminate a suspect
3. free a falsely imprisoned individual
4. determine maternity, paternity, or match to another relative
5. identify human remains
H. DNA Profile Databases
a. Each state keeps DNA profiles of individuals who have been
convicted of certain kinds of crime such as rape, murder, and
child abuse.
b. The military maintains DNA profiles of all service men and
women.
c. The United States’ Electronic database of DNA profiles is
called CODIS, Combined DNA Index System.
i. DNA profiles from a crime scene can be entered into
CODIS to identify possible suspects.
ii. DNA profiles are so unique, there is a 1 in a
Quadrillion possibility of a match
iii. As of February 2011, CODIS contained 9.4 million
profiles from DNA from convicted felons of violent
crimes. Of 360,000 profiles from DNA at crime scenes, CODIS made 138,700 hits.