Chapter 8 DNA Fingerprinting and Forensic Analysis by pptfiles


									DNA Fingerprinting and Forensic
    What is a DNA fingerprint?
• Every cell of an individual carries a copy of
  the DNA
  – a cell collected from a person’s skin or hair
    follicle contains the same DNA as from that
    persons heart tissue or white blood cells
• Order of base pairs in the DNA of every
  individual is different except identical twins
    How do we distinguish one person’s
          DNA from another?
• Don’t need to sequence all 3 billion base pairs of a
  person’s DNA to distinguish it from another person’s
• Intron regions of DNA (junk DNA) contain sequences
  that are 20-100 bp in length that are repeated at
  different locations (loci) along the chromosome.
  CGGCTACGGCTACGGCTA (repeated 3 times at this
  location; at another location, it may be repeated 9
• These sequences are called Short Tandem Repeats
  (STRs) or VNTRs
• Each person has some STRs that were inherited
  from mother and some from father
• No person has STRs that are identical to those of
  either parent
• The number of repeats at each loci on
  chromosome is highly variable in the population,
  ranging from 4 to 40.
• The length of the DNA after cutting the
  chromosome with a restriction enzyme, and its
  position after electrophoresis will depend on the
  exact number of repeats at the locus
• The uniqueness of an individual’s STRs provides
  the scientific marker of identity known as a DNA
• In the United States the FBI has standardized a
  set of 13 STR assays (13 different locations on
  the chromosomes) for DNA typing, and has
  organized the CODIS database for forensic
  identification in criminal cases.
• The United States maintains the largest DNA
  database in the world: The Combined DNA Index
  System, with over 60 million records as of 2007.
   Preparation of a DNA fingerprint
                Step 1
• Specimen collection
  – blood, semen, etc
  – easy to contaminate a DNA sample with DNA from
    other sources (bacteria, DNA of person collecting
  – DNA is not stable for very long-it degrades
     • sunlight
     • heat
     • moisture
• DNA fingerprinting is a comparative
  – DNA from crime scene is compared with DNA
    of a suspect
  – So minimum of two samples must be prepared

                     Step 2
• DNA extraction
  – standardized methods have been developed
  – need to separate DNA from other cell material
    and debris from crime scene.
                    Step 3
     PCR using primers targeting STRs at
                different loci

• PCR amplify STRs using target sites on
                   Step 3
           PCR amplification of DNA

1 strand        Heat to
of DNA          denature                     STR locus

 Design primers that anneal to STR locus     STR locus

 Amplify all the regions of the chromosome
 where the STRs exist.
PCR allows you to make
millions of copies of the STR
region from a single copy of
DNA you recovered from crime
• Since the # of times sequence is repeated is
  different for each person, fragment size will be
• This is done for 13 different STR sequences at
  this one locus
• Differences occur among individuals at each of
  the 13 loci on the chromosome where the STRs
• This allows for a lot of variation
 Restriction Fragment Length Polymorphism
             For 1 STR sequence at 1 locus
       Person A                         Forensic sample
        STR                                      STR

G-G-C-C-X-X-X-G-G-C-C-X-X.. G-G-G-C-C-X-X-G-G-C-C-X-X…..
                         PCR amplify
                          STR region

                 well                     well

• If you do this for 13 different repeat sequences
  at 13 different loci on the chromosome, each
  person produces a different band pattern when
  the fragments are separated by gel
 STRs at
other loci


Do any of the
compare with
forensic sample?

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