1. DNA Fingerprinting and Forensic Analysis

2. What is a DNA fingerprint?
Every cell of an individual carries a copy of the DNA
a cell collected from a person’s skin or hair follicle contains the same DNA as from that persons heart tissue or white blood cells
Order of base pairs in the DNA of every individual is different except identical twins

3. How do we distinguish one person’s DNA from another?
Don’t need to sequence all 3 billion base pairs of a person’s DNA to distinguish it from another person’s DNA
Intron regions of DNA (junk DNA) contain sequences that are bp in length that are repeated at different locations (loci) along the chromosome. CGGCTACGGCTACGGCTA (repeated 3 times at this location; at another location, it may be repeated 9 times)
These sequences are called Short Tandem Repeats (STRs) or VNTRs

4. STRs
Each person has some STRs that were inherited from mother and some from father
No person has STRs that are identical to those of either parent
The number of repeats at each loci on chromosome is highly variable in the population, ranging from 4 to 40.
The length of the DNA after cutting the chromosome with a restriction enzyme, and its position after electrophoresis will depend on the exact number of repeats at the locus

5. The uniqueness of an individual’s STRs provides the scientific marker of identity known as a DNA fingerprint.
In the United States the FBI has standardized a set of 13 STR assays (13 different locations on the chromosomes) for DNA typing, and has organized the CODIS database for forensic identification in criminal cases.
The United States maintains the largest DNA database in the world: The Combined DNA Index System, with over 60 million records as of 2007.

6. Preparation of a DNA fingerprint Step 1
Specimen collection
blood, semen, etc
easy to contaminate a DNA sample with DNA from other sources (bacteria, DNA of person collecting sample)
DNA is not stable for very long-it degrades
sunlight
heat
moisture

7. Step 2 DNA fingerprinting is a comparative process: DNA extraction
DNA from crime scene is compared with DNA of a suspect
So minimum of two samples must be prepared
Step 2
DNA extraction
standardized methods have been developed
need to separate DNA from other cell material and debris from crime scene.

8. Step 3 PCR using primers targeting STRs at different loci
PCR amplify STRs using target sites on chromosome

9. Step 3 PCR amplification of DNA
1 strand
of DNA
Heat to
denature
double-
stranded
DNA
STR locus
Design primers that anneal to STR locus
Amplify all the regions of the chromosome
where the STRs exist.
STR locus

10. PCR allows you to make millions of copies of the STR region from a single copy of DNA you recovered from crime scene.

11. Since the # of times sequence is repeated is different for each person, fragment size will be different.
This is done for 13 different STR sequences at this one locus
Differences occur among individuals at each of the 13 loci on the chromosome where the STRs occur
This allows for a lot of variation

12. Restriction Fragment Length Polymorphism
For 1 STR sequence at 1 locus
G-G-C-C-X-X-X-G-G-C-C-X-X..
G-G-G-C-C-X-X-G-G-C-C-X-X…..
STR
C-C-X-X-X-G-G
C-C-X-X-G-G
PCR amplify
STR region
well
Gel
electrophoresis
Person A
Forensic sample

13. If you do this for 13 different repeat sequences at 13 different loci on the chromosome, each person produces a different band pattern when the fragments are separated by gel electrophoresis

14. Different STRs at other loci
Do any of the individuals compare with forensic sample?