1. Advantages of STR Analysis
Compares specific genetic markers, not whole genome.
Can be used for tiny samples or degraded samples because it makes many copies of sample.

2. STR Analysis Procedure
1. Isolate DNA
2. Polymerase Chain Reaction (PCR) Amplify (make copies) of specific loci
3. Electrophoresis - Sort fragments
4. Visualize fragments - can use various methods: Southern Blot, Stain Gel & UV Light, Fluorescent Tags

3. STEP 1. Extract DNA from cell
Break cell membrane – release nucleus
Open nucleus - release DNA
protect DNA from enzymes
Separate from other cell parts
DNA must be precipitated
un-dissolved from solution

4. STEP 2. PCR Procedure Heat DNA - strands unzip (denature) 2. Cool
3. Add
- Primers - “starter pieces” of DNA bind (anneal) - to complementary sequences at beginning of specific loci
- DNA polymerase - “replicator machine”
- Nucleotides (AGCT) building blocks
4. Heat to 75° C – DNA polymerization

5. DNA Amplification with Polymerase Chain Reaction (PCR)
Separate strands (denature) 5’ 3’
Starting DNA Template 5’ 3’
Add primers (anneal) 5’ 3’
Forward primer
Reverse primer 5’ 3’
Make copies (extend primers)

6. Polymerase Chain Reaction
GCTATTCTGGGAGTCCAGAGTGGACGT
CGATAAGACCCTCAGGTCTCACCTGCA
CAC
TGG A C G A T G C T

7. PCR Copies DNA Exponentially through Multiple Thermal Cycles
Original DNA target region
Thermal cycle
Thermal cycle
Thermal cycle
In 32 cycles at 100% efficiency, 1.07 billion copies of targeted DNA region are created
Each cycle takes less than 2 minutes from start to finish.

8. STEP Electrophoresis

9. STEP 4. Visualization of STR Alleles
Southern Blot & Autoradiography – with radioactive probes & X-ray.
Gel may be stained and visualized with UV light after electrophoresis is complete.
Fluorescent tags can be added to primer before running gel.
AATG
GCTCATA
CTAATGCCCGTA

10. Fluorescent dye labels
AATG
GCTCATA
CTAATGCCCGTA
GCTCATA
AATG
AATG
CTAATGCCCGTA
GCTCATA
AATG
AATG
AATG
CTAATGCCCGTA
GCTCATA
AATG
AATG
AATG
AATG
CTAATGCCCGTA
GCTCATA
AATG
AATG
CTAATGCCCGTA
For a specific loci there are many different alleles
Each allele varies in length due to the number of STR’s in the middle of the segment

11. How many loci were tested for each individual?
Each person has 2 alleles for a locus. M/D
The resulting pattern of an STR sequence for a single locus has either 1 or 2 bands
Can be homozygous or heterozygous for each locus.
1 band = homozygous – both fragments same length
2 bands = heterozygous – different fragment lengths
13 repeats
12 repeats
11 repeats
10 repeats
9 repeats
12 repeats
8 repeats
7 repeats
6 repeats
5 repeats
4 repeats
3 repeats
How many loci were tested for each individual?

12. An autoradadiogram has several lanes containing DNA ladders.
Each band in these lanes contains a known length of DNA -
# of base pairs
Used for comparison to determine the length of unknown DNA bands in other lanes.
Determine # bp -
Use to determine # of STR repeats

13. To be a match - every fragment must match up.

14. Should this person be convicted of rape?
Rape cases
separate male faction (sperm) DNA from female faction (epithelial) DNA
compare with DNA from blood
Should this person be convicted of rape?

15. How many loci were tested for each individual?
Paternity cases can be used to determine the parents of a child, to identify a child’s remains, in sibling analysis and inheritance right cases.
How many loci were tested for each individual?
Is each child the biological offspring of both parents?
Remember, each line represents one allele.
Each allele for each child is inherited from one of his/her parents.

16. FBI’s CODIS DNA Database
Combined DNA Index System
Links serial crimes & unsolved cases with repeat offenders
13 STR’s
TPOX
D3S1358
TH01
D8S1179
D5S818
VWA
FGA
D7S820
CSF1PO
AMEL
D13S317
AMEL
D16S539
D18S51
D21S11