DNA: A New Forensic Science Tool

DNA: A New Forensic Science Tool
Chapter 13
What is DNA?
Deoxyribonucleic Acid
Double Helix
Basis of all inheritance
Used as a diagnostic tool for genetic disorders
DNA “fingerprinting” “profiling” or “typing”
Link evidence to person with relative certainty
What is DNA?
Individual unit called NUCLEOTIDE
Sugar
Phosphate
Base
DNA bases are
Adenine
Cytosine
Guanine
Thymine

These four letters make up the DNA alphabet that is the language of the cell.
What is DNA?
Double Helix






A always pairs with T
C always pairs with G
What is DNA?
DNA is the ‘recipe’ for who we are:
HEIGHT
EYE COLOR
FINGERS & TOES
PROTEINS
ORGANS
LIVING FUNCTIONS
Found in virtually all 60 trillion cells in the human body
DNA in each cell is identical
Similar to ‘fingerprints’
DNA in the Cell
chromosome
cell nucleus
Double stranded DNA molecule
Individual nucleotides
HISTORY OF DNA
1954 - Watson & Crick describe the structure (Nobel Prize)
1985 - Alec Jeffreys discovers multilocus VNTR probes
1985 - Mullis develops protocol for polymerase chain reaction (PCR) (Nobel Prize)
1988 - FBI starts DNA casework
1991 - first STR paper
1998 - FBI launches CODIS database


What type of cases are investigated?
Forensic cases -- matching suspect with evidence
Paternity testing -- identifying father
Historical investigations
Missing persons investigations
Mass disasters -- putting pieces back together
Military DNA “dog tag”
Convicted felon DNA databases
What type of samples are submitted?
Blood
Semen
Saliva
Urine
Hair
Teeth
Bone
Tissue
Buccal (CHEEK) swabs primarily
used as DNA knowns today.
What type of samples are submitted?
Blood
Semen
Saliva
Urine
Hair
Teeth
Bone
Tissue
Buccal (CHEEK) swabs primarily
used as DNA knowns today.
Probative Items To Look For
Glasses
Hat
Belt
Gum
Beer / coke can
Toothpick
Phone
Cigarette
RESTRICTION
FRAGMENT LENGTH POLYMORPHISM
(RFLP)

RFLP
Historical DNA
Restriction Enzyme cuts DNA into fragments
Repeated segments make fragments different lengths
DNA is separated according to size
Separated DNA is transferred to a membrane
Membrane is “probed” with complimentary strand
of DNA that is “labeled” to visualize
Exposure to X-ray film produces “bar code” image
POLYMERASE CHAIN REACTION (PCR)
DNA TESTING
EXTRACTION
Purify DNA
QUANTITATION
How much is there?
AMPLIFICATION
Copy
DETECTION
What alleles are present?
ANALYSIS OF DATA
Who are you?
DNA Amplification with the Polymerase Chain Reaction (PCR)
In 32 cycles at 100% efficiency, 1.07 billion copies of targeted DNA region are created
PCR Copies DNA Exponentially through Multiple Thermal Cycles
PCR
THERMAL CYCLER is used to amplify
CYCLES TEMPERATURES to
Separate strands
Anneal primers
Extend strand copy
Repeat
PCR ADVANTAGES
Small sample size required
Short turn around time
Automation possible
Fluorescent tags instead of radioactivity
Precise allele size
Short Tandem Repeats (STRs)
the repeat region is variable between samples while the flanking regions where PCR primers bind are constant
7 repeats
8 repeats
Homozygote = both alleles are the same length
Heterozygote = alleles differ and can be resolved from one another
ABI Prism 310 Genetic Analyzer
Close-up of ABI Prism 310 Sample Loading Area
FBI’s CODIS DNA Database
Combined DNA Index System
Used for linking serial crimes and unsolved cases with repeat offenders
Launched October 1998
Links all 50 states
Requires >4 RFLP markers
and/or 13 core STR markers
Current backlog of >600,000 samples

13 CODIS Core STR Loci with Chromosomal Positions
CSF1PO
D5S818
D21S11
TH01
TPOX
D13S317
D7S820
D16S539
D18S51
D8S1179
D3S1358
FGA
VWA
AMEL
AMEL
13 CODIS Core STR Loci with Chromosomal Positions
CSF1PO
D5S818
D21S11
TH01
TPOX
D13S317
D7S820
D16S539
D18S51
D8S1179
D3S1358
FGA
VWA
AMEL
AMEL
CODIS Does Work
Fort Worth Police Department was the first agency in Texas to get a ‘cold hit’
1993 two young females, ages 10 and 11, were sexually assaulted. Semen was the only evidence from the assault and the suspect was excluded.
1998 a man was arrested in Abilene and charged with indecency with a child. Once convicted, his DNA profile went into the CODIS system and the FW Crime Lab was notified of a ‘hit’
Subsequently linked to 5 similar offenses
MITOCHONDRIAL DNA (mtDNA)
Found in the cytoplasm of cells and hair shafts
Hundreds of copies per cell
Only two copies of nuclear DNA per cell
Less likely to be totally degraded
Inherited maternally (MOM ONLY)
Same type as Mom, Siblings, etc.
MITOCHONDRIAL DNA (mtDNA)
Less Individualizing
Can use the same instruments
ABI 310
Sequence DNA (spell out the words)
TTCGGCCATTGCGCGCTTACGCTTAAA
PCR / STR DNA analyzes sizes
words identical and repeated (AACT AACT)