1. DNA

2. A NYC Case Example

3. Sexual Assault in Building Hallway

4. Blood-soaked victim's shirt

8. Case Facts:
Person seen running from scene
ID of person made by tenant
Police arrest suspect
Suspect found to have prior history of sexual assault
Suspect confesses
Rape kit is negative for semen

9. DNA Testing Results
Blood stains on stairs do not match suspect
Local DNA database found a match with a semen stain from another case
Similar modus operandi and nearby location
Suspect identified in other case
DNA match with semen of other case

10. Did suspect have an accomplice?

11. Epilogue
Mixtures of DNA from victim and perpetrator found in bloodstain from victim’s clothing
Suspect’s DNA not found on clothing or crime scene
Charges dropped against suspect

12. What is DNA?

13. What does DNA look like?

15. Basic Components of Nucleic Acids
Phosphate
Base
5’end |
Phosphate
Sugar—Base…
3’end
Sugar

16. Base Pairing of DNA Strands
A = T
G  C
T = A
C  G T C A G 5’ 3’
denatured
strands
hybridized
Hydrogen bonds
Phosphate-sugar backbone

17. Sources of DNA Blood and blood stains Semen and semen stains Bones
Teeth
Hair with roots
Hair shaft
Saliva (containing nucleated cells)
Urine
Feces
Debris from fingernail scrapings
Muscle Tissue
Cigarette butts
Postage stamps
Envelope sealing flaps
Dandruff
Fingerprints
Personal Items:
razor blade, chewing gum, toothbrush, lipstick stains
Adopted from Butler, J.M. (2001) Forensic DNA Typing, Table 3.1, ©Academic Press

18. DNA BASICS Genomic DNA as hereditary material
-found in the nucleus of the cell (nuclear DNA)
Determines physical characteristics of people
Half the DNA is maternal/half the DNA is paternal

19. DNA BASICS Genomic DNA is found in every nucleic cell in the body
Genomic DNA is the same in all cells
Genomic DNA does not change throughout one’s lifetime

20. DNA BASICS
No two people (except identical twins) have exactly the same DNA
Mostly the same between people, but small regions vary between individuals
Less than 0.1% of DNA is unique

21. What we can’t tell from forensic DNA testing…..
Race- (now we can!)
Age
Diseases (unless genetic!)
How it got there

22. How is DNA Used in Forensic Analysis?
Entire genome is not examined
“Polymorphic” sequences of the genome are examined
Polymorphic areas are present on “loci”
- Two “alleles” at each loci

23. Types of DNA Polymorphisms
Length Polymorphism
----(AATG)(AATG)(AATG)-----
3 repeats
-----(AATG)(AATG)----
2 repeats

24. Variable Number of Tandem Repeats (VNTR)
Repetitive DNA in human genome (20-30%)
Satellite DNA
Minisatellite (extreme variability) DNA
base pair repeats
Polymorphism based on number of tandem repeats

25. STRs (microsatellite DNA)
Preferred genetic marker in forensic DNA testing
Tetranucleotide repeat sequence
CODIS (combined offender DNA indexing system) loci

26. Brief History of DNA Typing
Ray White describes first polymorphic RFLP marker
Alec Jeffreys discovers multilocus VNTR probes
first paper on PCR
FBI starts DNA casework
first STR paper
FSS starts UK DNA database
FBI launches CODIS database

27. DNA processing from scene to court

28. DNA Extraction Organic Method
INCUBATE (56 oC)
Centrifuge
Blood stain
SDS, DTT, EDTA and
proteinase K
Phenol,
chloroform,
isoamyl alcohol
VORTEX
Centrifuge
TRANSFER aqueous (upper) phase to new tube
TE buffer
CONCENTRATE sample (Centricon/Microcon-100 or ethanol precipitation)
Centrifuge
QUANTITATE DNA
PERFORM PCR

29. PCR
Polymerase Chain Reaction=Simulated Natural DNA replication

30. DNA Replication

31. PCR

32. Thermal Cycling Temperatures
94 oC
94 oC
94 oC
94 oC
Single Cycle
72 oC
72 oC
72 oC
Temperature
60 oC
60 oC
60 oC
Time
Typically cycles performed during PCR
94oC - Denaturation
60oC - Annealing
72oC - Extension

33. PCR

34. PCR

35. Number of Target Molecules Created1 2 3 4 5 8 6 16 7 32 64 9
128 10
256 11
512 12
1024 13
2048 14
4096 15
8192
16,384 17
32,768 18
65,536 19
131,072 20
262,144 21
524,288 22
1,048,576 23
2,097,152 24
4,194,304 25
8,388,608 26
16,777,216 27
33,544,432 28
67,108,864 29
134,217,728 30
268,435,456 31
536,870,912
1,073,741,824
Cycle Number
Number of Double-stranded Target Molecules

36. Advantages of PCR Need very small amounts of biological material
Minute amounts of DNA template may be used (i.e., 3 uL whole blood, 2-3 mm bloodstain cutting)
Can be used with forensic samples
DNA degraded to fragments only a few hundred base pairs in length can serve as effective templates for amplification.
Contaminant DNA, such as fungal and bacterial sources, will not amplify because human-specific primers are used.

37. DNA Replicated via PCR, Now what?
Electrophoresis and Southern Blotting
DNA to convict the guilty and exonerate the innocent