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• 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) – antigen-antibody test for p30 protein found in semen; very sensitive

Rape EvidenceFrom the victim• 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

From the suspect• All clothing• Pubic hair combings• Pulled head and pubic hair

standard/reference samples• Penile swab• Blood or buccal swab

DNA

• 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 proof-

reads 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