1. Chapter 13 DNA: The Indispensable Forensic Science Tool

2. DNA Deoxyribonucleic acid A polymer
In 1985 it was discovered that certain genes are as unique as fingerprints
DNA markers
Discovered in 1868
In the 1950s, James Watson and Francis Crick figured out the structure of DNA

3. DNA base pairing There are four DNA bases
Adenine, Cytosine, Guanine, and Thymine
They link together to form DNA, with pairs spanning the double-helix
A and T always pair
C and G always pair
The sequence is different for each individual

4. Proteins
Play basic roles in the structures and functions of living things
Made of amino acids
There are 20
They link to form proteins
Different sequences make different proteins

5. Abnormalities
A difference of one amino acid in a protein can cause big differences

6. Human Genome Total DNA content About 3 billion base pairs
Has been mapped

7. DNA replication Making new DNA from existing DNA Two strands unwind
Each recreates the double helix

8. PCR Polymerase chain reaction
Used to copy and fill in small pieces of DNA from crime scenes
Sample size is no longer a limit

9. Recombinant DNA
Cutting DNA into fragments that can later be incorporated into another DNA strand
Uses restriction enzymes
Chemicals that act as scissors to cut DNA at specific locations
Used for genetic engineering
Making fish grow faster
Growing insulin through bacteria
Making plants grow faster and/or produce higher yields

10. Tandem Repeats
A region of a chromosome that contains a repeating DNA sequence
About 30% of human genome
Origin and significance not known
Possibly fillers
Helpful in DNA typing
Distinguish individuals

11. RFLP Restriction Fragment Length Polymorphisms
Repeat segments that are cut from DNA molecule with restriction enzymes
Used for DNA typing

12. Hybridization
Joining two complementary strands of DNA to form a double-stranded molecule

13. DNA technology over time
1985 to mid 1990s
RFLP (Restriction Fragment Length Polymorphisms)
RFLP Portions of DNA are repeated numerous times (over 30%)
Act as fillers or spacers
Restriction enzymes are used to cut the fillers
Everyone has different numbers and order of these fillers
Mid 1990s on
PCR (Polymerase Chain Reaction)
Copies or multiplies DNA strands

14. Short Tandem Repeat (STR)
Latest method of DNA typing
Locations on DNA that contain short segments of 3 – 7 repeating base pairs
Abundant in human genome
Easier to multiply using PCR
13 STRs used by CODIS

15. Multiplexing
Simultaneously detecting more than one DNA marker in a single analysis

16. Amelogenin Gene Used to determine gender
Not one of the 13 CODIS STRS, but can also be useful

17. Y-STRs STRs on the Y chromosome
Useful for separating DNA when multiple males are involved in a sexual assault

18. Sequencing
A procedure used to determine the order of the base pairs that make up DNA

19. Mitochondria Small structures in the cell Supply energy to the cell
Contain maternal DNA
Mitochondrial DNA or mtDNA
More difficult, time-consuming, and costly to analyze than nuclear DNA

20. mtDNA Circular or loop configuration Contains 37 genes
Found in hair that lacks nuclear DNA
Found in skeletons
Reference samples can be taken from maternal relatives

21. mtDNA First used in State of Tennessee v. Paul Ware in 1996
Used to identify the individual buried in the tomb of the Vietnam War’s unknown soldier

22. CODIS Combined DNA Index System
Local, Regional, and National DNA databases
DNA is collected from convicted offenders of particular crimes (especially sexual assault)
Also contains DNA from unknown suspects and missing persons

23. DNA evidence Only about 36 cells are needed to get an STR profile
Low copy number – fewer than 36 cells
Epithelial cells
Outer layer of skin cells
Easily rubbed off
Can yield DNA

24. Collecting evidence Must be photographed first
Don’t disturb blood pattern until after it has been analyzed for shape and location
Minimal personal contact
Assume all fluids are infectious
Each stained article should be packaged separately in a paper bag or a well-ventilated box

25. Substrate control Unstained material from near the bloodstain
Confirm test results
Swabs must be dried before being packaged
Takes 5 – 10 minutes
Biologicals should be stored in a cool place, refrigerated, or frozen until testing

26. Collecting reference samples
Blood samples
Buccal swabs
Inner cheek lining
Hairbrushes, toothbrushes, etc.

27. Contamination Coughing or sneezing on sample Incorrect packaging
Avoiding
Change gloves for each piece of evidence
Substrate controls
Use disposable forceps (tweezers) to pick up evidence
Separate containers