Notes 13.2

DNA typing

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

Tandem repeats cont….• Certain numbers of the repeats have

been chosen for DNA typing – Example: core sequence 15-35 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

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

RFLP

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

Polymerase Chain Reaction

• The newest technology to replace RFLP 1. Add a primer2. Then reduce temperature

allowing combination3. Add nucleotides

• Best used with sequences that are a couple of hundred bases

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

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

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

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

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

13.2 Questions1. What are tandem repeats and how are they used in

forensic science?

2. What are the dominant DNA typing procedures in the US until the mid 1990s?

3. What is short tandem repeat and why is it so attractive to forensic scientists?

4. How does the number of STRs characterized relate to the frequency of occurrence of the analyzed sample in the general population?

5. (p. 380) Name two process by which a forensic scientist can separate STRs for characterization. Which of the two processes is the preferred process?

6. 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?