Term
Chapter 10
Forensic Toxicology |
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Definition
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Term
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Definition
Toxicological examinations involve the
identification and often quantitation of drugs &
toxic materials in the human body |
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Term
| role of the forensic toxicologist |
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Definition
– determination of the presence of drugs or alcohol in the
body
– identification of substances causing unnatural death |
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Term
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Definition
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Term
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Definition
Alcohol is a general term for a family of
organic compounds
The term alcohol will be taken to mean
ethanol (ethyl alcohol) |
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Term
| Chemical Properties of Ethanol |
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Definition
• A clear volatile liquid
• Burns easily
– oxidizes easily
• Slight, characteristic odor
• Is very soluble in water
– miscible in all proportions |
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Term
Physiological Properties of
Ethanol |
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Definition
• A central nervous system (CNS) depressant
• CNS is the bodily system which is most
severely affected by alcohol
• The degree to which the CNS function is
impaired is directly proportional to the
concentration of alcohol in the blood |
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Term
What Does the Body Do With
Alcohol? |
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Definition
When an alcoholic beverage is swallowed,
it is diluted by stomach juices & quickly
distributed throughout the body
• Alcohol does not require digestion before
its absorption into the bloodstream
– some diffuses into bloodstream directly through
the stomach wall
– remainder passes into the small intestine
• rapidly absorbed & circulated |
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Term
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Definition
• Alcohol is absorbed from all parts of the
gastrointestinal tract largely by simple
diffusion into the blood
– small intestine is the most efficient region for
absorption because of its large surface area
• The rate of absorption varies according to
the particular beverage & the state of the
consumer’s stomach |
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Term
| The most awesome person I know |
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Definition
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Term
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Definition
• Fasting individual
– 20-25% of a dose of alcohol is absorbed from
the stomach
– 75-80% is absorbed from the small intestine
– peak blood alcohol concentrations occur in 0.5-
2.0 hrs |
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Term
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Definition
• Non-fasting individuals
– presence of food in stomach delays absorption
• peak alcohol concentrations 1.0-6.0 hrs
• Alcohol ingested with carbonated beverages
– ordinarily absorbed more rapidly than straight
alcohol |
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Term
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Definition
• Alcohol has a high affinity for water
• Is diffused in the body in proportion to the
water content of the various tissues &
organs
– greater concentration in blood & brain
– lesser concentration in fat & muscle |
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Term
| Blood Alcohol Concentration |
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Definition
BAC is the amount of alcohol in the
bloodstream measured in percentages
– BAC = 0.10%
• means a person has 1 part alcohol per 1,000 parts of
blood in the body |
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Term
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Definition
Liver eliminate ~95% of the alcohol through
metabolism
Remainder of eliminated (chemically unchanged)
through excretion in breath, urine, sweat
• Rule of Thumb
– 0.5 oz (15 mL) or 0.015 percent alcohol eliminated
per hr |
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Term
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Definition
0.04% Feelings of contentment, happiness,
relaxation
0.06% Judgement somewhat impaired
Some loss of coordination
Less able to make rational decisions
0.08% Muscle coordination & driving
impaired
0.30% Loss of consciousness may occur
Breathing may stop (death may
result)
Alcohol in the bloodstream continually circulates to the brain |
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Term
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Definition
• ~half of traffic injuries involve alcohol
• ~1/3 of fatally injured passengers &
pedestrians have elevated blood alcohol
levels
• ~half of homicides involve alcohol
• 1/2 to 1/3 of suicides involve alcohol
• ~30,000 unintentional injury deaths are
directly attributable to alcohol |
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Term
| Alcohol in Blood vs Breath |
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Definition
• The ratio of alcohol in blood to alcohol in
breath is 2100 to 1
• 1 mL of blood will contain about the same
amount of alcohol as 2100 mL of breath
• Breath test reflects alcohol concentration in
the pulmonary artery (reflects what reaches
the brain |
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Term
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Definition
• Measures the alcohol content of breath
• Subject blows into a mouthpiece until 52.5
mL of breath has been collected
Measures alcohol concentration of 52.5 mL of
breath, which is equivalent to 0.025 mL of blood |
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Term
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Definition
• Breath testers that operate on the principle of
infrared light absorption are becoming
increasingly popular within the law enforcement
community.
• Many types of breath testers are designed to
capture a set volume of breath.
• The captured breath is exposed to infrared light.
• It’s the degree of the interaction of the light with
alcohol in the captured breath sample that allows
the instrument to measure a blood alcohol
concentration in breath |
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Term
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Definition
• Law enforcement officers typically use field
sobriety tests to estimate a motorist’s degree
of physical impairment by alcohol and
whether or not an evidential test for alcohol
is justified.
• The horizontal gaze test, walk and turn, and
the one-leg stand are all considered reliable
and effective tests.
• A portable, handheld, roadside breath tester
may be used to determine a preliminary
breath-alcohol content. |
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Term
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Definition
• A portable, handheld, roadside breath tester may
be used to determine a preliminary breath-alcohol
content. The results are nonevidential.
• The results from a portable breath tester are
subject to further infrared breath testing or gas
chromatography blood testing.
• Roadside breath tester utilizing a fuel cell detector
are becoming popular. |
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Term
| Gas Chromatography Testing |
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Definition
• Gas chromatography offers the toxicologist the most
widely used approach for determining alcohol levels
in blood.
• Blood must always be drawn under medically
accepted conditions by a qualified individual.
• It is important that a nonalcoholic disinfectant be
applied before the suspect’s skin is penetrated with a
sterile needle or lancet.
• Once blood is removed from an individual, its
preservation is best ensured when it is sealed in an
airtight container after an anticoagulant and a
preservative have been added and stored in a
refrigerator. 26 |
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Term
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Definition
• The American Medical Association and the
National Safety Council have been able to exert
considerable influence in convincing the states to
establish uniform and reasonable blood-alcohol
standards.
• Between 1939 and 1964 a person having a bloodalcohol
level in excess of 0.15 percent was to be
considered under the influence, which was
lowered to 0.10 percent by 1965.
• In 1972 the impairment level was recommended to
be lowered again to 0.08 percent. |
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Term
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Definition
• Starting in 2003, states that have not adopted the
0.08 percent level will lose part of their federal
funds for highway construction.
• To prevent a person’s refusal to take a test for
alcohol consumption, the National Highway Traffic
Safety Administration recommended an “implied
consent” law.
• Adopted by all states by 1973, this law states that
the operation of a motor vehicle on a public
highway automatically carries with it the stipulation
that a driver will submit for a test for alcohol
intoxication if requested or be subject to loss of the
license. |
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Term
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Definition
• Beyond the analysis of alcohol, the toxicologist is
confronted with a maze of drugs and poisons.
• The toxicologist is originally presented with body
fluids and/or organs and is normally requested to
examine them for the presence of drugs and
poisons.
• Without supportive evidence, such as the victim’s
symptoms, a postmortem pathological
examination, or an examination of the victim’s
personal effects, the toxicologist is forced to use
general screening procedures with the hope of
narrowing thousands of possibilities to one. |
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Term
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Definition
• Once the drug has been extracted from appropriate
biological fluids (blood, urine), tissues, and
organs, the forensic toxicologist can proceed to
identify the drug substance present.
• Blood levels or urine levels of drugs alone can not
be used to draw definitive conclusions about the
effects of a drug on an individual.
• Blood and urine drug levels, are best used by law
enforcement authorities and the courts to
corroborate other investigative and medical
findings regarding an individual’s conditions. |
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Term
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Definition
• The forensic
toxicologist must
devise an analytical
scheme that will
successfully detect,
isolate, and
specifically identify
toxic drug substances |
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Term
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Definition
Drug extraction is generally based on a
large number of drugs being either
acidic or basic.
• pH scale is used to express the acidity
of a substance. A pH of 7 is neutral;
lower values are acidic and higher
values are basic. |
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Term
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Definition
The strategy used for identifying
abused drugs entails a two-step
approach: screening and confirmation |
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Term
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Definition
• A screening test is normally employed to provide
the analyst with quick insight into the likelihood
that a specimen contains a drug substance.
• Positive results arising from a screening test are
considered to be tentative at best and must be
verified with a confirmation test.
• The most widely used screening tests are thinlayer
chromatography, gas chromatography, and
immunoassay.
• Immunoassay is based on specific drug antibody
reactions. |
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Term
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Definition
• Gas chromatography/mass spectrometry is
generally accepted as the confirmation test of
choice.
• The GC separates the sample into its components,
while the MS represents a unique “fingerprint”
pattern that can be used for identification.
• Once the drug is extracted and identified, the
toxicologist may be required to provide an opinion
on the drug’s effect on an individual’s natural
performance or physical state |
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Term
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Definition
• Drug Recognition Expert is referred to a person who can
advise the toxicologist as to which analytical scheme to use
to specifically identify a toxic substance.
• The Drug Recognition Expert program incorporates
standardized methods for examining automobile drivers
who are suspected of being under the influence of drugs.
• To ensure that each subject has been tested in a routine
fashion, each DRE must complete a standard Drug
Influence Evaluation form.
• The DRE program usually cannot determine which specific
drug was ingested.
• Hence, it is the production of reliable data from both the
DRE and the forensic toxicologist that is required to prove
drug intoxication. |
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Term
Chapter 12
FORENSIC SEROLOGY |
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Definition
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Term
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Definition
• A complex mixture of cells, enzymes,
proteins & inorganic substances
• Fluid portion of blood is called the plasma
(55% of blood content)
– primarily water
• Red blood cells (erythrocytes), white blood
cells (leukocytes), and platelets are the solid
materials (45% of blood content) suspended
in plasma .
• Antigens, usually proteins, are located on
the surface of red blood cells and are
responsible for blood-type characteristics. |
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Term
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Definition
• Transport oxygen from the lungs to the
body tissues
• Transport carbon dioxide from the tissues to
the lungs
• Red cells possess chemical structures on
their surfaces called antigens or
agglutinogens
– impart blood type characteristics
• The white blood cells fight infection.
• The platelets help the blood to clot. |
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Term
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Definition
On average, blood accounts for
8 % of total body weight
5 to 6 liters of blood for males
4 to 5 liters of blood for females
A 40 percent blood volume
loss, internally or/and
externally, is required to
produce irreversible shock
(death).
A blood loss of 1.5 liters,
internally or externally, is
required to cause incapacitation |
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Term
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Definition
• A term which describes laboratory
blood tests which employ a specific
antigen and serum antibody reactions |
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Term
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Definition
• Three types (alleles) of blood type gene
– A, B, O
• Each individual inherits one blood type gene from
their mother & one from their father
– 6 possible combinations (genotypes)
• AA, BB, OO, AB, AO, and BO
• Genotype determines blood type
• Genotype: the particular combination of genes
present in the cells of an individual
• Phenotype: the physical manifestation of a
genetic trait such as shape, color, and blood type |
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Term
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Definition
• More than 15 blood antigen systems have been
identified, but the A-B-O and Rh systems are the
most important.
• An individual that is type A has A antigens on
his/her red blood cells, type B has B antigens, AB
has both A and B antigens, and type O has neither
A nor B antigens.
• Rh factor is determined by the presence of another
antigen, the D antigen.
• People having the D antigen are Rh positive; those
not having the antigen are Rh negative. |
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Term
| Antibodies or Agglutinins |
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Definition
• Proteins that are present in the serum
– Plasma: the fluid portion of unclotted blood
– Serum: the liquid that separates from the blood when a
clot is formed
• Responsible for ensuring that the only blood cells
that can survive in a person are cells of the correct
blood type
• For every antigen in the red blood cells
there is a specific antibody that will react
with it to form clumps known as
agglutination. |
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Term
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Definition
• Antibodies produced by the A alleles remove any
red blood cells containing B antigens by clumping
them together
• Antibodies produced by the B alleles remove any
red blood cells possessing A antigens
• Thus, if serum containing anti-B is added
to red blood cells carrying B antigen, they
will immediately react, normal red blood
cells will be destroyed. |
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Term
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Definition
• Possessed by people with genotype
– AA
– AO
• Has antigens A on the surface of red blood
cells
• Possesses antibodies B in the blood serum
– compatible with A or O red cells
– will destroy any Type B red cells |
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Term
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Definition
• Possessed by people with genotype
– BB
– BO
• Has antigens B on the surface of red blood
cells
• Possesses antibody A in the blood serum
– compatible with B or O red cells
– will destroy any Type A red cells |
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Term
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Definition
• Possessed by people with genotype
– AB
• Has antigens both A & B on the surface of
red blood cells
– can’t be added to any other blood type without
being destroyed by an antibody
• Possesses no antibody
– can have A, B, AB, or O red cells added |
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Term
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Definition
• Possessed by people whose genotype is OO
– both parents passed on the O gene
– have no antigens
• these cells can be introduced into people with Type
A, Type B or Type AB because these cells are not
attacked by the antibodies these people possess
– have both A & B antibodies
• can only have other O type red cells mixed with this
blood
15 |
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Term
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Definition
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Term
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Definition
• The identity of each of the four A-B-O
blood groups can be established by testing
the blood with anti-A and anti-B sera.
• The concept of specific antigen-antibody
reactions has been applied to immunoassay
techniques for the detection of drugs of
abuse in blood and urine. |
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Term
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Definition
• Blood typing is done by reacting whole
blood with anti-A serum (containing
antibody A) and anti-B serum (containing
antibody B). |
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Term
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Definition
Antibodies are proteins that bind very tightly to their targets(antigens).
They are produced in vertebrates as a defense against infection.
Each is made of two identical light chains and two identical heavy chains
The two antigen binding sites are identical |
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Term
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Definition
• A number of immunological assay techniques are
commercially available for detecting drugs through
antigen-antibody reaction.
• One such technique, the enzyme-multiplied
immunoassay technique (EMIT), is used by
toxicologists because of its speed and high
sensitivity for detecting drugs in urine.
• In a typical EMIT analysis, antibodies that will bind
to a specific drug are added to the subject’s urine.
• Other immunoassay procedures are also available,
such as radioimmunoassay (RIA) which uses drugs
labeled with radioactive tags. |
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Term
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Definition
• The criminalist must be prepared to answer
the following questions when examining
dried blood:
– (1) Is it blood?
– (2) From what species did the blood originate?
– (3) If the blood is of human origin, how closely
can it be associated to a particular individual?
• The determination of blood is best made by
means of a preliminary color test. |
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Term
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Definition
• A positive result from the Kastle-Meyer color test
is highly indicative of blood.
– Hemoglobin causes a deep pink color.
• Alternatively, the luminol test is used to search
out trace amounts of blood located at crime
scenes.
– Produces light (luminescence) in a darkened area.
• Confirmatory testing of blood:
– Microcrystalline tests, such as the Takayama and
Teichmann tests, depend on the addition of specific
chemicals to the blood so that characteristic crystals
will be formed. |
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Term
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Definition
• Once the stain has been characterized as blood, the
precipitin test will determine whether the stain is
of human or animal origin.
• The precipitin test uses antisera normally derived
from rabbits that have been injected with the blood
of a known animal to determine the species origin
of a questioned bloodstain.
• Once it has been determined that the bloodstain is
of human origin, an effort must be made to
associate or dissociate the stain with a particular
individual. |
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Term
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Definition
• Prior to the advent of DNA typing,
bloodstains were linked to a source by A-BO
typing and the characterization of
polymorphic blood enzymes and proteins.
• This approach has now been supplanted by
the newer DNA technology.
• DNA analysis has allowed forensic
scientists to associate blood and semen
stains to a single individual. |
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Term
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Definition
• The transmission of hereditary material is
accomplished by means of microscopic units
called genes, located on chromosomes.
• Alternative forms of genes that influence a given
characteristic (such as eye color or blood type) are
known as alleles.
• Paternity testing has historically involved the
A-B-O blood typing system, along with blood
factors other than A-B-O.
• Currently, paternity testing has implemented DNA
test procedures that can raise the odds of
establishing paternity beyond 99 percent. |
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Term
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Definition
• The crime scene investigator must remember that
the location, distribution, and appearance of
bloodstains and spatters may be useful for
interpreting and reconstructing the events that
produced the bleeding.
• Surface texture and the stain’s shape, size, and
location must be considered when determining the
direction, dropping distance, and angle of impact
of a bloodstain. |
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Term
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Definition
• Surface texture is of paramount importance. In
general, the harder and less porous the surface, the
less spatter results.
• Dropping distance of a bloodstain can be
determined by analyzing the diameter of a
bloodstain. Longer distance of dropping results in
a larger diameter of a bloodstain.
• The direction of travel of blood striking an object
may be discerned because the pointed end (tail) of
a bloodstain always points to the direction of
travel of the blood drop. |
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Term
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Definition
• The impact angle of blood on a flat surface can be
determined by measuring the degree of circular
distortion. At right angles the blood drop is
circular, as the angle decreases, the stain becomes
elongated.
• The origin of a blood spatter in a two-dimensional
configuration can be established by drawing
straight lines through the long axis of several
individual bloodstains. The intersection or point
of convergence of the lines represents the origin
point. |
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Term
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Definition
The spherical shape of blood
in flight is important for the
calculation of the angle of
impact (AOI) of blood spatter
when it hits a surface |
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Term
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Definition
When a droplet of blood strikes a
surface perpendicular
(90 degrees) the resulting
bloodstain will be circular. That
being the length and width of the
stain will be equal. |
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Term
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Definition
Blood that strikes a surface at
an angle less than 90 degrees
will be elongated or have a tear
drop shape. |
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Term
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Definition
ANGLE of IMPACT is the
acute angle formed between
the direction of the blood
drop and the plane of the
surface it strikes. |
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Term
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Definition
By utilizing
trigonometric functions,
it’s possible to
determine the impact
angle for any given
blood droplet.
By accurately measuring
the length and width of a
bloodstain, the impact
angle can be calculated
using the SIN formula
below: |
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Term
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Definition
the
common point, on a 2
dimensional surface, over
which the directionality of
several bloodstains can be
retraced.
Once the directionality of a
group of stains (one or two
stains is not sufficient) has
been determined, it's
possible to determine a 2D
point (or area)
for the group of stains.
By drawing a line through
the long axis of a group of
bloodstains, the point of
convergence can be
determined. |
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Term
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Definition
Experiments with blood have shown that
a drop of blood tends to form into a
sphere in flight rather than the artistic
teardrop shape.
The formation of the sphere is a result of
surface tension that binds the molecules
together.
This elastic like property of the surface
of the liquid makes it tend to contract. |
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Term
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Definition
More rapid bleeding may
result in slightly larger
drops.
BUT, on the contrary,
slower bleeding does not
result in smaller drops.
Blood cast from a moving
source will tend to consist
of smaller droplets.
Blood drop behaves as a
projectile in motion and
obeys the laws of physics
and mathematics. |
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Term
| Categories of Blood Stains |
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Definition
•PASSIVE (dropping)
•TRANSFER
•PROJECTED (impacted)49 |
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Term
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Definition
drops
created or formed by
the force of gravity
acting alone. |
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Term
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Definition
created
when a wet, bloody
surface comes in
contact with a
secondary surface. |
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Term
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Definition
created
when an exposed blood
source is subjected to an
action or force, greater
than the force of gravity |
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Term
| categories of impact spatter |
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Definition
Low Velocity
Medium Velocity
High Velocity |
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Term
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Definition
Impact from
a hand or
hand-held
object
Relatively large stains 4mm in diameter and greater.
Gravitational pull less than 5 feet/sec. |
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Term
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Definition
Impact from
a hand or
hand-held
object
Preponderant stain size 1 to 4mm in diameter.
Force of 5 to 25 feet/sec. |
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Term
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Definition
Impact from
gunshots
Preponderant stain size 1mm in diameter or less.
Force of 100 feet/sec. or less |
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Term
Chapter 13
DNA: The indispensible forensic science tool |
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Definition
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Term
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Definition
DNA is the chemical substance which makes
up our chromosomes and controls all
inheritable traits (eye, hair and skin color)
DNA is different for every individual except
identical twins
DNA is found in all cells with a nucleus (white
blood cells, soft tissue cells, bone cells, hair
root cells and spermatozoa)
Half of a individual’s DNA/chromosomes come
from the father & the other half from the
mother. |
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Term
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Definition
DNA is a double-stranded molecule.
An individual’s DNA remains the same
throughout life.
In specific regions on a DNA strand
each person has a unique sequence of
DNA or genetic code. |
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Term
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Definition
The smallest unit of
life
The nucleus is the
“brain” of the cell
contains all the
genetic info the cell
needs to exist & to
reproduce
In most types of cells,
genetic information is
organized into
structures called
chromosomes |
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Term
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Definition
In most types of
cells, genetic
information is
organized into
structures called
chromosomes
usually X shaped
Y chromosome in
males
23 pairs in humans
one set from mother
& one set from father |
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Term
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Definition
Each chromosome
contains hundreds to
thousands information
blocks called genes
Each gene is actually
composed of DNA
specifically designed to
carry out a single body
function.
only identical twins will have
all the genes identical |
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Term
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Definition
DNA is a very large,
double stranded
molecule.
if fully extended the
molecule would be
about 1.7 meters
long
unwrapping all the
DNA in all your cells
cover the distance
from earth to moon
6,000 times |
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Term
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Definition
DNA is a polymer
built from
monomers called
nucleotides
Each nucleotide
consists of
deoxyribose
pentose sugar
phosphoric acid
a nitrogenous base |
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Term
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Definition
The Purines
Adenine (A)
Guanine (G) |
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Term
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Definition
The Pyrimidines
Cytosine (C)
Thymine (T) |
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Term
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Definition
DNA is normally a
double stranded
macromolecule
Two polynucleotide
chains are held
together by H-bonding
A always pairs with T
C always pairs with G |
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Term
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Definition
5’ T-T-G-A-C-T-A-T-C-C-A-G-A-T-C 3’
3’ A-A-C-T-G-A-T-A-G-G-T-C-T-A-G 5’
In a double helix the strands go in
opposite directions |
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Term
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Definition
Two Functions
To transmit information from one
generation of cells to the next
To provide the information for the
synthesis of components (proteins)
necessary for cellular function |
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Term
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Definition
DNA directs the production of proteins, which are made by
combining amino acids.
The sequence of amino acids in a protein chain determines
the shape and function of the protein.
Each group of three nucleotides in a DNA sequence codes
for a particular amino acid.
Example: G-A-G codes for the amino acid glutamine,
while C-G-T codes for alanine.
If a nucleotide is “changed”, for example a T is substituted
for A and G-A-G becomes G-T-G, the “wrong” amino acid is
placed in the protein (in this case glutamine is replaced with
valine).
As a result, the protein may not function correctly and this
is the basis for many diseases and health issues. |
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Term
| Conventional Blood Typing |
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Definition
Used for more than 50 years
Utilized ABO blood typing groups
Identified genetic variations in blood
proteins, tissue specific proteins and
serum protein types
Major problem is that the conventional
blood protein markers are not
found in semen
. |
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Term
| Development of DNA Analysis |
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Definition
RFLP (Restriction Fragment Length
Polymorphism)
Southern Blot |
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Term
| Restriction Fragment Length Poiymorphism |
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Definition
Restriction Enzymes (biological
catalysts) cut DNA whenever they
encounter a specific DNA sequence.
Gel electrophoresis separates the
fragments of DNA according to their
length. |
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Term
| Cutting Variable DNA Sections |
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Definition
Restriction Enzymes are proteins that cut
DNA molecules at specific cut |
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Term
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Definition
A short segment of DNA that is
complementary to a portion of the
desired DNA fragments is labeled with a
radioactive atom.
This probe binds to the fragment of
interest on the gel electrophoresis.
Visualized using X-ray film. |
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Term
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Definition
In 1980 David Botstein and others used
RFLP to construct a human gene map.
Used genetic variations as markers
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Term
| Kary Mullis Invented PCR Methods(1984) |
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Definition
Polymerase Chain Reaction (PCR)
amplifies short specific regions of DNA
PCR is a technique that can yield
millions of copies of desired DNA
Does not use radioactivity
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Term
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Definition
Was searching for disease
markers
Applied the technique to
personal identification
Demonstrated that the DNA
could be retrieved from old
dried blood stains
Applied the technique to highprofile
forensic tests |
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Term
| Commercial Dev of DNA Testing |
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Definition
Lifecodes Corporation—Founded in
1982 in Valhalla, NY as diagnostic
company. Began performing forensic
DNA testing in 1987
Cellmark—The US branch of Britain’s
Imperial Chemical Industries opened in
Germantown, MD in 1987. |
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Term
| FBI and RCMP Set up DNA Labs |
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Definition
1987 FBI with NIH began collaborative
research to establish DNA identification
techniques
In late 1988 FBI set up their own
laboratory at their Pennsylvania Avenue
headquarters
RCMP also set up their own DNA
laboratory. |
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Term
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Definition
They used four different DNA probes
GeneLex
Dr. Raymond White of Howard Hughes
Medical Center
Lifecodes
Cellmark
DNA probes and primers are the key
patented biomolecules used to identify
the individual genetic variability. |
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Term
| Standardization to the industry |
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Definition
Established detailed laboratory
protocols
Performed validation studies
Cut through the competitive nature that
clouded the environment of the testing
methods and tools |
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Term
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Definition
A technique used by scientists to
distinguish between individuals of the
same species using only samples of
their DNA |
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Term
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Definition
Humans carry 2 copies of the DNA in their cells (diploid). The
exception is sperm and eggs which contain one copy (haploid)
The DNA is organized into chromosomes – long strands of DNA
On the chromosomes, genes (sequences of short DNA that code
for a protein) are found. The location of the gene on the
chromosome is its locus (plural: loci).
Much of the DNA is non-coding (junk DNA) and even in protein
coding genes, there may be sequences that are cut out (introns)
before they are used to make a protein. The remaining
sequences are the exons.
Genes are sequences of short DNA – there are only 4 base
building blocks of DNA monomer the nucleotide (A,T,G and C),
so the genes are actually sequences of these bases. The length
and order of bases determines the type of protein that is
produced by that gene.
Differences in the bases exist between individuals largely in the
non-coding DNA (introns and junk DNA). DNA profiles detect
and exploit these differences |
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Term
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Definition
DNA is unique to each individual: even though we
share 99.9% of our genome in common with other
humans, 1 out of every 1000 nucleotides is a
significant and detectable level of difference
The basic structure of everyone’s DNA is the same
the difference between people is the ordering and length of
the base pairs
Every person can be distinguished by the sequence
of their base pairs
millions of base pairs make this impractical
a shorter method uses repeating patterns that are present in
DNA |
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Term
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Definition
Portions of the DNA molecule contain sequences
of bases that are repeated numerous times,
known as tandem repeats.
To a forensic scientist, these tandem repeats offer
a means of distinguishing one individual from
another through DNA typing.
Tandem repeats seem to 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. |
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Term
| 2 types of DNA typing techniques |
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Definition
Restriction Fragment Length Polymorphisms (RFLPs)
Restriction from the enzymes that cut the DNA (restriction
enzymes)
Fragment for the fragments produced by the cutting
Length and Polymorphisms for the different sized fragments
produced (polymorphic = many forms)
Short Tandem Repeats (STRs)
Short because one sequence usually contains 3-7 bases in
length
Tandem because they occur one after the other
Repeats because they are repeats of the same DNA
sequence
e.g. ACTG-GCC-GCC-GCC-GCC-ATCGACC = 4 tandem
repeats of GCC |
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Term
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Definition
Length differences associated with relatively long
repeating DNA strands are called restriction
fragment length polymorphisms (RFLP) and form
the basis for one of the first DNA typing
procedures.
Typically, a core sequence consists of 15 to 35
bases in length and repeats itself up to a
thousand times.
The key to understanding DNA typing lies in the
knowledge that numerous possibilities exist for
the number of times a particular sequence of base
letters can repeat itself on a DNA strand. |
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Term
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Definition
DNA is cut by molecular “scissors” – enzymes
which recognize particular sequences of
nucleotides
These enzymes identify short sequences of
DNA, then snip it
Because everyone’s DNA is different,
enzymes cut in different places
The resulting samples contain DNA fragments
of different size (Restriction Fragment Length
Polymorphisms) |
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Term
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Definition
Once the DNA molecules have been cut up by a
restriction enzyme, the resulting fragments are
sorted out by electrophoresis.
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 in a process called Southern blotting.
To visualize the RFLPs, the nylon sheet is treated
with radioactive probes containing a base
sequence complementary to the RFLPs being
identified (a process called hybridization). |
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Term
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Definition
Next, the nylon sheet is placed against X-ray film
and exposed for several days.
When the film is processed, bands appear where
radioactive probes stuck to fragments on the nylon
sheet.
A typical DNA fragment pattern will show two
bands (one RFLP from each chromosome).
When comparing the DNA fragment patterns of
two or more specimens, one merely looks for a
match between the band sets.
A high degree of discrimination can be achieved by
using a number of different radioactive probes and
combining their frequencies. |
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Term
| Stages of DNA RFLP Typing |
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Definition
Stage 1: Cells are broken down to releasr DNA
Stage 2: The DNA is cut into Fragments using restriction enzymes. The sections of DNA RFLP Typing is called restriction fragments.
Stage 3: Electrophoresis.
- Fragments are separated on the basis of size using a process called gel electrophoresis.
- DNA fragments are injected into wells and an electric current is applied along the gel
- DNA is negatively charged so it is attracted to the positive end of the gel.
- The shorter DNA fragments move faster than the longer fragments.
- DNA is separated on basis of size.
Stage 4: Southern Blotting
- The DNA fragments are then transferred to a nylon membrane in much the same way as the one would transfer process is called southern blotting.
Stage 5: Hybridization
- A hybridization solution w. a radioactive probe is added which combines w/ the specific DNA fragments to produce a flurescent image.
Stage 6: A photographic copy of the DNA bands is obtained. The pattern of fragment distribution is then analysed.
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Term
| How unique are these profiles |
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Definition
The probability of 2 people except
identical twins having exactly the same
DNA profile is between
1 in 5 million to
1 in 100 billion
(greater than the population of humans on
earth)
This number becomes even larger if you
consider more regions of DNA
Thus, the odds that the DNA evidence
from a crime scene will match your DNA
profile is astronomically small (unless
you have an evil identical twin) |
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Term
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Definition
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 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. |
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Term
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Definition
Much of the process of collecting STR data
has been automated, including gel
electrophoresis
To collect and analyze STR evidence, copies
of the variables regions of the DNA are
amplified (millions of copies are made)
The DNA is then fed through a machine that
reads the DNA by size – a laser scans and
detects the stained DNA samples as they
electrophorese through the machine |
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Term
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Definition
STRs normally consist of repeating sequences
of 3 to 7 bases in length, and the entire
strand of an STR is also very short, less than
450 bases in length.
This means that STRs are much less
susceptible to degradation and may often be
recovered from bodies or stains that have
been subjected to extreme decomposition. |
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Term
| PCR: Amplifying the DNA of Interest |
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Definition
Because most tissue samples from a crime scene
contain very little DNA, the goal is amplify, or
make many copies of the DNA of interest
In STR analysis, you want to amplify the DNA
containing the tandem repeats and only this
DNA
The process used is called Polymerase Chain
Reaction (PCR)
PCR Machines, or thermocyclers, use repeated
cycles of heating and cooling to replicate the
DNA using many of the same enzymes found in
cells which facilitate DNA replication |
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Term
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Definition
For the forensic scientist, PCR offers a distinct
advantage in that it can amplify minute
quantities of DNA many millions of times.
First, the DNA is heated to separate it.
Second, primers (short strands of DNA used to
target specific regions of DNA for replication)
are added which hybridize with the strands.
Third, DNA polymerase and free nucleotides are
added to rebuild each of the separated strands.
This process is repeated 25 to 30 times. |
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Term
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Definition
One advantage in moving to shorter DNA strands
is that they would be expected to be more stable
and less subject to degradation brought about by
adverse environmental conditions.
The long RFLP strands tend to readily break apart
under the adverse conditions not uncommon at
crime scenes.
PCR also offers the advantage in that it can amplify
minute quantities of DNA, thus overcoming the
limited sample size problem often associated with
crime scene evidence. |
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Term
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Definition
PCR technology cannot be applied to
RFLP DNA typing.
The RFLP strands are too long, often
numbering in the thousands of bases.
PCR is best used with DNA strands that
are no longer than a couple of hundred
bases. |
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Term
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Definition
In STR PCR, the procedure is the same for STR
analysis, but several different STR primers
amplifying several areas of interest
simultaneously.
Because of their shortness, STRs are ideal
candidates for multiplication by PCR, thus
overcoming the limited-sample-size problem
often associated with crime-scene evidence. |
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Term
| Variable Number Tandem Repeats |
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Definition
Another method of DNA typing is variable number
tandem repeat (VNTR) analysis
DNA strands contain information which directs an
organism’s development
Exons (a nucleotide that codes information for protein
synthesis)
Also contain stretches which appear to provide no
relevant genetic information
Introns (a nucleotide that does not code information for
protein sysnthesis)
VNTR: repeated sequences of base pairs on these introns
All humans have some VNTRs |
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Term
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Definition
What makes STRs so attractive to forensic
scientists is that hundreds of different types
of STRs are found in human genes.
The more STRs one can characterize, the
smaller will be the percentage of the
population from which a particular
combination of STRs can exist.
This gives rise to the concept of multiplexing.
Using the technology of PCR, one can
simultaneously extract and amplify a
combination of different STRs. |
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Term
| Standardizing STR Testing |
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Definition
With STR, as little as 125 picograms of
DNA is required for analysis.
This is 100 times less than that normally
required for RFLP analysis. |
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Term
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Definition
DNA Analysis is useful because:
The DNA contains “detectable” patterns unique to each
individual
DNA is a robust molecule, and is stable under most (but
not all) environmental conditions
DNA can be isolated from a wide range of biological
samples likely present at a crime scene
The source of DNA doesn’t matter – it is the same in all
sources (blood, semen, skin etc)
Rapid advances in technology allow the precise patterns
to be detected even with very small samples (a blood
spot, single hair follicle, lip-prints on a glass, physical
fingerprints, saliva/skin on a cigarette butt etc)
Methods are fast
Data are complied in databases, and are easily searched |
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Term
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Definition
Blood (White blood cells)
Semen (Sperm cells)
Hair with roots (Hair follicle cells)
Skin, dandruff (Skin cells)
Sweat stains (Skin cells sloughed off)
Saliva (Mucosal surfaces)
Vaginal fluids (Mucosal surfaces)
Nasal secretions (Mucosal surfaces)
Urine (Mucosal surfaces)
Feces (Digestive system cells) |
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Term
| Biological materials used for DNA profiling |
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Definition
Blood
Hair
Saliva
Semen
Body tissue cells
DNA samples have been
obtained from vaginal
cells transferred to the
outside of a condom
during sexual
intercourse. |
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Term
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Definition
Another type of DNA used for individual
characterization is mitochondrial DNA.
Mitochondrial DNA (mDNA) is located
outside the cell’s nucleus and is
inherited from the mother.
Mitochondria are structures found in all
our cells used to provide energy that
our bodies need to function. |
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Term
| Mitochondrial DNA Testing |
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Definition
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 mDNA is more rigorous, time
consuming, and costly when compared to nuclear
DNA analysis.
Also, all individuals of the same maternal linkage will
be indistinguishable by mDNA analysis.
Two regions of mDNA have been found to be highly
variable and a procedure known as sequencing is
used to determine the order of base pairs. |
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Term
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Definition
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. |
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Term
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Definition
Uses 13 loci
All forensic laboratories that use the CODIS system
can contribute to a national database.
Only Mississippi doesn’t participate
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: 119,782
Convicted Offender Profiles in NDIS: 2,643,409 |
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Term
| Collecting and Preserving the DNA |
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Definition
Although DNA is relatively stable, it does
denature or get destroyed through enzyme
action, from bacteria or through oxidation
Therefore, samples should be collected soon
and preserved (usually in a buffer and by
freezing) if possible
Care should also be taken not to cross
contaminate during collection
Blood is also a potential pathogen, so care
must be taken to avoid exposing yourself to
blood borne viruses like Hep B or HIV |
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Term
| Packing Biological Evidence |
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Definition
Before the collection of biological evidence
begins, it is important that it be
photographed and recorded on sketches.
Wearing disposable latex gloves while
handling the evidence is required.
Clothing from victim and suspect with blood
evidence must be collected.
The packaging of biological evidence in
plastic or airtight containers must be avoided
because the accumulation of residual
moisture could contribute to the growth of
DNA-destroying bacteria and fungi. |
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Term
| Packing Biological Evidence |
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Definition
Each stained article should be packaged
separately in a paper bag or in a well-ventilated
box.
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 swabbing the mouth
and cheek. |
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