Transcript Forensic Science: An Introduction

Forensic Science:
An Introduction
Blood and DNA
Blood Components
• Formed Elements (cells) – 45%
– Erythrocytes - Red Blood Cells
carry oxygen
– Leukocytes – White Blood Cells
fight infections; have nuclear DNA
– Thrombocytes – platelets for clotting
• Plasma – 55%; Fluid portion of unclotted
blood
– Serum – liquid that separates from blood when
clotted
Antigens & Antibodies
• Antigens - proteins found on surface of
RBCs
• Antibodies - proteins found in serum
– Destroys or inactivates a specific antigen
– Binds to two different antigens at a time and
causes agglutination (clumping)
• Serology – the study of
antigen-antibody reactions
Blood Type
Blood type Antigens
Antibodies
A
A
Anti-B
B
B
Anti-A
AB
Both A & B
Neither Anti-A or Anti-B
O
Neither A or B
Both Anti-A or Anti-B
Rh factor – have it (+) or not (-)
Immunoassay
• Animals can be exposed to drug-protein
complexes to produce antibodies against
the drug. The antibodies can then be
used to as a presumptive test for drugs
• Enzyme-multiplied immunoassay
technique (EMIT) – labels antibody with
enzyme that can cause a color change
• Radioimmunoassays- antibodies labeled
with radioactive tag
Making Antibodies
• Polyclonal antibodies – produced by
injecting animals with a specific antigen, A
series of antibodies are produced
responding to a variety of different sites on
the antigen.
Monoclonal Antibodies
• A collection of identical antibodies that interact
with a single antigen site
• Hybridoma – fused
plasma cell (from
spleen) and tumor
cells to produce a
limitless supply of I
dentical monoclonal
antibodies
Forensic Characterization
of Blood Stains
• Kastle-Myer – tests for hemoglobin to presume
that sample is blood
• Luminol – binds with blood and emits light; very
sensitive
• Microcrystalline tests- chemicals added to blood
cause crystals to form; less specific
• Precipitin test – antihuman antibodies react with
human blood to identify as such
• Gel Diffusion – antigens and antibodies placed
in holes in an agar plate. If they react they leave
a line of precipitation that is visible.
Blood Stain Patterns
• Surface texture is important; the harder and less
porous, the less splatter
• The direction of travel of the blood can be
determined by its shape
• The angle of the impact can be determined by
the circular distortion of the drop.
• The origin of blood splatter can be determined
by converging the long axes of several drops to
create a 2 D origin
Heredity
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Zygote
Sperm/egg
Chromosome
Gene
Allele
Locus
DNA
Homozygous
Heterozygous
Genotype
Phenotype
Paternity testing
• Used to identify is a male is the father of a
specific child
• ABO system
• HLA antigens – 90%
• DNA – 99% +
Semen
• Acid phosphatase – enzyme found in high
concentration is semen
• Can be visualize when reacted with certain
chemicals and exposed to UV light
• Microscopy – located spermatozoa
• Prostate Specific Antigen (PSA) – antigenantibody test for p30 protein found in
semen; very sensitive
Rape Evidence
From the victim
From the suspect
• Pubic combings
• Pubic hair standard/reference
sample
• External genital dry-skin areas
• Vaginal swabs and smears
• Cervix swabs
• Rectal swabs and smears (
• Oral swabs and smears
• Head hairs
• Blood sample
• Fingernail scrapings
• All clothing
• Urine specimen
• All clothing
• Pubic hair combings
• Pulled head and pubic hair
standard/reference samples
• Penile swab
• Blood or buccal swab
DNA
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DeoxyriboNucleic Acid
Found in the nucleus
46 chromosomes
25,000 genes
Structure determined by Crick and Watson
DNA fingerprinting by Alec Jeffreys
Structure of DNA
• A polymer made of repeating nucleotides
• Nucleotide consists of a deoxyribose
sugar, a phosphate, and a nitrogen base
(Adenine, Thymine, Cytosine, or Guanine)
• Double stranded, helical
• Complementary base
pairing, A=T, G=C
DNA at Work
• DNA in nucleus is copied into a strand of
RNA (transcription)
• RNA is read at the ribosome to make
assemble amino acids into proteins
(translation)
• Every 3 bases on DNA
codes for a different
amino acid
Replication of DNA
• Replication – the synthesis of new DNA
from existing DNA in the nucleus
• DNA polymerase assembles
new DNA strand and proofreads it
• Replication occurs in nucleus prior to cell
division
Polymerase Chain Reaction
• A technique for replication, or amplifying, a
portion of DNA outside
the cell
• Each cycle doubles
the number of copies
• 1 1x107 in 30 cycles
DNA Typing with Tandem Repeats
• Region of chromosome that contains multiple
copies of a core DNA sequence arranging in a
repeating fashion between the coding regions
(genes)
• Restriction Fragment Length Polymorphisms
used enzymes to cut the DNA around these
tandem repeat sites and then run them on a gel
electrophoresis
• A Southern blot was then performed and
radioactive probes were hybridized to help
visualize the RFLPs
RFLPs
PCR
PCR has the following advantages:
1. PCR can use shorter sequences
2. shorter pieces more stable
3. smaller amounts of DNA can
be used (10-9 gram)
Short Tandem Repeats (STRs)
• A region of a DNA molecule that contains
short segments of 3-7 repeating base
pairs.
• Generally less than 450 bp long
• Less degradation
• Can be PCR’d
• Can multiplex a large number of these
STRs at once
• US uses 13 STRs for tests
• Capillary electrophoresis
• Sex Identification by focusing on the
amelogenin gene
Mitochondrial DNA
• Cell organelle responsible for supplying
energy to the cell by producing ATP from
glucose
• Mitochondria has its own DNA
• Inherited solely from mother
• Used for identification when nuclear DNA
is degraded
• Reference sample can be maternal
relative
CODIS
• A computer software program developed
by the FBI that contains local, state and
national databases of DNA profiles from
convicted offenders, unsolved crime-scene
evidence, and profiles of missing people
Collection and Preservation
• Can use low copy samples
• Avoid contamination; change gloves
before each new sample
• Collect substrate controls/reference
samples (buccal swabs)
• Package in air tight containers