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DNA Fingerprinting

DNA Fingerprinting. A method of developing a person’s DNA “profile,” similar to a fingerprint. Pioneered in England in 1984 by Dr. Alec Jeffreys. Dr. Alec Jeffreys. First Forensic Use. First used by law enforcement in England in the mid-1980’s.

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DNA Fingerprinting

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  1. DNA Fingerprinting • A method of developing a person’s DNA “profile,” similar to a fingerprint. • Pioneered in England in 1984 by Dr. Alec Jeffreys Dr. Alec Jeffreys

  2. First Forensic Use • First used by law enforcement in England in the mid-1980’s. • DNA evidence exonerated one man, and convicted another. • Described in The Blooding, by Joseph Wambaugh

  3. How does it work? • 99.9% of your DNA is the same as everyone else’s. • The 0.1% that differs are a combination of: • Gene differences (Differences in the genes themselves) • Differences in “polymorphic regions” between the genes on the DNA.

  4. How does it work? • Certain points between the genes on the DNA have repeating base sequences. • For example:ATTACGCGCGCGCGCGCGCTAGC • These are called short tandem repeats (STRs for short)

  5. How does it work? • Everyone has STRs at the same place in their DNA, but they are different lengths for different people. • For example:Person 1: ATTACGCGCGCGCGCGCGTAGC (7 repeats)Person 2: ATTACGCGCGCGCGTAGC (5 repeats)

  6. To Make a DNA Fingerprint… • First, we use restriction enzymes to chop the DNA up into millions of fragments of various lengths. • Some of the fragments contain STRs; some do not. The ones that do are different lengths for different people.

  7. Restriction Fragment Length Polymorphisms (RFLPs) • Polymorphisms are slight differences in DNA sequences as seen in individuals of the same species

  8. To Make a DNA Fingerprint… • Next, we use gel electrophoresis to sort the DNA fragments by size.

  9. Gel Electrophoresis • Method for sorting proteins or nucleic acids on the basis of their electric charge and size

  10. Gel Electrophoresis • Electrical current carries negatively-charged DNA through gel towards positive electrode • Agarose gel sieves DNA fragments according to size • Small fragments move farther than large fragments

  11. Gel Electrophoresis

  12. Restriction fragmentpreparation 1 Restrictionfragments Gel electrophoresis 2 Blotting 3 Radioactive probe 4 Probe Detection of radioactivity(autoradiography) 5 Film Figure 12.11C To Make a DNA Fingerprint… • Finally, a radioactive probe attaches to our STRs. Only the fragments with our STRs will show up on the gel.

  13. To Make a DNA Fingerprint… • Since STRS are different lengths in different people, this creates a DNA Fingerprint.

  14. Two uses for DNA Fingerprints... • ForensicsDNA taken from crime scenes (blood, semen, hair, etc.) can be compared to the DNA of suspects.Real-life CSI!

  15. Two uses for DNA Fingerprints... • ForensicsThis is an example of a gel that might be used to convict a rape suspect. Compare the “Sperm DNA” to the “Suspect DNA.” Which suspect committed the rape?

  16. Two uses for DNA Fingerprints... • Paternity TestingSince all of our DNA markers came from either mommy or daddy, we can use DNA fingerprints to determine whether a child and alleged father are related…just like on Maury Povich!

  17. Two uses for DNA Fingerprints... • Look at the two “Child” markers on this gel. Can they both be matched up to either the mother or the “alleged father?” • Yes. This is a “positive” test for paternity.

  18. Two uses for DNA Fingerprints... • How about this gel? Do both of the child’s markers match either the mother or the “alleged father.” • No! The “alleged father” is not this child’s biological parent.

  19. Interpreting DNA Fingerprints • Which child is not related to the mother? • Son 2 • Which children are not related to the father? • Daughter 2 and Son 2

  20. Interpreting DNA Fingerprints • A blood stain was found at a murder scene. The blood belongs to which of the seven possible suspects? Suspect 3

  21. Interpreting DNA Fingerprints • These DNA fingerprints are from a mother, a child, and two possible biological fathers. Which one is the daddy? 2nd alleged father

  22. The Polymerase Chain Reaction (PCR) • The polymerase chain reaction, PCR, can produce many copies of a specific target segment of DNA • A three-step cycle—heating, cooling, and replication—brings about a chain reaction that produces an exponentially growing population of identical DNA molecules

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