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Notes 13.2

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Notes 13.2

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  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 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

  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?