1. Notes 13.2
DNA typing

2. Tandem repeats
Portions of the DNA molecule that contain sequences of letters that are repeated numerous times that act to fill the space between coding regions of DNA
All humans have the repeats
There is variation in the number of repeats that each of us have
Restriction fragment length polymorphisms (RFLPs) – repeat segments cut from a strand of DNA

3. Tandem repeats cont….
Certain numbers of the repeats have been chosen for DNA typing
Example: core sequence bases which repeats itself about 1000 times
Remember, you inherit 1 chromosome of each pair from each parent
RFLP’s are created, which contain repeating sequences.
Length difference allows a scientist to distinguish the individual
Length difference determined using electrophoresis gel

4. Tandem repeats cont…. Southern Blotting
Used to transfer double stranded DNA from electrophoresis which have been separated and transferred to a nylon membrane for visualization of the RFLP’s.
Nylon membrane is treated with radioactive probes containing one base sequence complementary to the RFLP (hybridization)
Nylon sheet is place against X ray film, exposed and processed which causes bands to appear which can be compared to test specimens

5. RFLP
DNA typing is the first scientifically accepted protocol in the US for forensic characterization of DNA
Read Paragraph 3 on page 372

6. Polymerase Chain Reaction
The newest technology to replace RFLP
Add a primer
Then reduce temperature allowing combination
Add nucleotides
Best used with sequences that are a couple of hundred bases

7. PCR continued
Has the advantage because it is short and less vulnerable to degeneration
Can amplify small amount of DNA
Can be used to ID saliva residues on envelopes, stamps, soda cans and cigarette butts

8. Short Tandem Repeats (STR)
Latest method of DNA analysis
Locations on the chromosome that contain short sequence elements that repeat
Less than 400 base pairs
Shorter than RFLP
Less susceptible to degradation
Can be recovered from bodies or stains that have been subject to decomposition

9. STR cont… Can be easily multiplied using PCR
Only 1/1,000,000,000th of a gram is needed
ex: AATG (known as STR TH01)
Every person has two STR types for TH01
Hundreds of different types are found in human genes

10. STR cont…
The more STR one can ID, the smaller the percentage of the population that have them
Currently there are 13 STRs in the national data base known as CODIS
Combined DNA Index System
See page 379
Sequence of bases flanking the repeats must also be known

11. STR cont…
Commercial STR kits contain the capacity to identify the sex of the individual by the amelogenin gene
Found on the X and Y chromosome coding for tooth pulp
6 bases shorter on the X chromosome than the Y chromosome
There are more than 20 Y-STR markers

12. 13.2 Questions
What are tandem repeats and how are they used in forensic science?
What are the dominant DNA typing procedures in the US until the mid 1990s?
What is short tandem repeat and why is it so attractive to forensic scientists?
How does the number of STRs characterized relate to the frequency of occurrence of the analyzed sample in the general population?
(p. 380) Name two process by which a forensic scientist can separate STRs for characterization. Which of the two processes is the preferred process?
What gene is often used to determine the sex of a DNA contributor? What characteristic of the gene allows the forensic scientist to make this determination?