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Forensic DNA Analysis

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  1. Forensic DNA Analysis Harris County Medical Examiner’s Office Joseph A. Jachimczyk Forensic Center Houston, TX *Presentation is automatic and will begin shortly.

  2. Short Tandem Repeat analysis, or STR analysis Uses nuclear DNA Forensic DNA technology used at the Harris County Medical Examiner’s DNA Laboratory

  3. Target Region for PCR DNA in the Cell Nucleus chromosome cell nucleus Double stranded DNA molecule Individual nucleotides Source:

  4. DNA Extraction • DNA analysis begins with DNA extraction from biological evidence • DNA extraction is: • A chemical process by which DNA is released from cell(s) • Other cellular components are removed leaving a pure sample of DNA in an aqueous (liquid) solution

  5. Sources of Biological Evidence Blood Semen Saliva Urine Hair Teeth Bone Tissue Perspiration Vaginal secretions Source:

  6. DNA Amplification • After DNA extraction DNA is amplified using the Polymerase Chain Reaction (PCR) • PCR amplification makes many copies of the DNA to allow for detection (think photocopies) PCR DNA strand Amplified DNA

  7. DNA Detection • Amplified DNA fragments are labeled with a fluorescent tag • DNA fragments are separated by electrophoresis (electric current) • The size of the fluorescent DNA fragment is determined by a machine which displays the information graphically. • Analysts read the graph to determine the DNA type

  8. DNA Analysis • Short Tandem Repeats (STRs) are short stretches of DNA that are repeated several times at a particular location on a chromosome • The number of repeats and, therefore, the length of the Short Tandem Repeat DNA varies from person to person • Each individual has two different (or two equal) length repeats at the same location on a chromosome • STR DNA testing determines the length of the STR DNA at 13 different chromosomal locations • STR DNA testing analysis is the most commonly used forensic DNA analysis method

  9. 13 CODIS Core STR Loci with Chromosomal Positions A listing of all lengths at each location (loci) is called a DNA profile TPOX D3S1358 TH01 D8S1179 D5S818 VWA FGA D7S820 CSF1PO AMEL D13S317 AMEL D16S539 D18S51 D21S11 Source:

  10. (4) (5) CAG CAG CAG CAG CAG CAG CAG CAG CAG (6) (7) CAG CAG CAG CAG CAG CAG CAG CAG CAG CAG CAG CAG CAG Father’s Type: 5, 6 Mother’s Type: 4, 7 (6) CAG CAG CAG CAG CAG CAG (4) CAG CAG CAG CAG Baby’s Type: 4, 6 At a single STR locus: *(Other possibilities: 4, 5 or 5, 7 or 6, 7)

  11. Statistics • The 13 STR DNA tests produce exceedingly rare DNA profiles • A STR DNA profile might be found in 1 person out of a quadrillion (that is, a billion million) people • A match between two DNA samples, say, a crime scene stain and the DNA of a suspect, can link a subject to a crime • A non-match may shift the focus away from a particular suspect