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  1. DNA

  2. Understanding DNA DNA - Deoxyribonucleic Acid DNA are the molecules that carry the body’s genetic information

  3. Alec Jeffreys and his associates determined that a persons DNA was as unique as fingerprints

  4. DNA profiling or DNA typing now allows criminalists to link evidence to one person !! =

  5. Human body has 60 trillion cells Each cell contains chromosomes Each chromosome contains DNA with about 25,000 genes Chromosomes

  6. Structure of DNA Nucleotide • Sugar molecule • Phosphate molecule • Base – nitrogen containing molecule

  7. Bases 4 types of bases Adenine - A Cytosine - C Guanine - G Thymine - T

  8. Complementary Base Pairing Bases can only pair with certain bases A - T C - G Examples: T A T T A T A T A A T A G T A T C G C A T A G C

  9. DNA at work DNA directs the production ofproteins Proteins are made by linking a combination of amino acids The sequence of amino acids in a protein determines the function of the protein

  10. If any of the amino acids in the chain is incorrect a problem will appear Example: Sickle-cell hemoglobin Normal hemoglobin Valine Valine Histidine Histidine Leucine Leucine Threonine Threonine Proline Proline Glutamate Valine Glutamate Glutamate

  11. The code that determines the amino acid is a three base code C G T C T A A A A C G T Genetic Code Alanine Aspartate Phenylalanine Alanine

  12. Replication of DNA DNA Replication – synthesis of new DNA from existing DNA DNA strands unwind Letter by letter, the code is copied End with 2 identical copies The new strand is proofread for mistakes

  13. DNA Polymerases are the enzymes used to unwind, and produce new DNA

  14. DNA Typing with Tandem Repeats Tandem Repeats Repeating sequences in DNA that act as fillers or spacer Forensic Scientists can use tandem repeats to distinguish one individual from another

  15. Restriction Fragment Length Polymorphisms RFLPs - Repeating fragments that can be cut out of DNA by restriction enzymes - Criminalists have selected certain RFLPs to perform DNA typing - The number of times the RFLP occurs for each person is different

  16. Electrophoresis i. Once the DNA is cut into RFLP pieces the pieces are separated by electrophoresis ii. DNA is placed on gel plates iii. Electric potential is applied to the gel plate iv. The DNA fragment migrate across the plate

  17. v. Longer fragments move slower vi. Shorter fragments move quicker

  18. RFLP DNA typing was the first procedure accepted in the US for forensic science RFLP typing has been replaced RFLP typing was used to determine the source of the stain on Monica Lewinsky’s dress

  19. Polymerase Chain Reaction - PCR PCR Technique copies DNA strands Developed in the mid 1990s Because we know that DNA polymerases can unwind and replicate DNA…

  20. DNA polymerase can also be used to reproduce small or broken pieces of DNA found at a crime scene

  21. First the primer must be identified Primer - Short strand of DNA on each side of the piece of DNA being replicated G T C T C A G C T T C C A G C A G A G T C G A A G G T C

  22. Next the DNA is heated to 94° C to separate the strands G T C T C A G C T T C C A G C A G A C C A G C A G A G T C G A A G G T C Next primers are added and the temperature decreased to 60°C, the primers will combine with the DNA

  23. Then the DNA polymerase with a mixture of bases is added and the temperature is raised to 72°C to start replicating the DNA G T C T C A G C T T C C A G C A G A G T C G A A G G T C G T C T C A G C T T C C A G C A G A G T C G A A G G T C

  24. This completes the first cycle 28 -32 cycles are run to produce more than 1 billion copies of the original molecule Each cycle takes less than 2 minutes Advantages RFLP uses long strands which can break apart PCR uses smaller, more stable strands

  25. Short Tandem Repeats (STRs) Short, 3-7, repeating sequences in DNA that act as fillers or spacer STRs are very short, less susceptible to breaking and easily used for PCR STR is the latest, most successful method of DNA typing Human genomes have over 100 STRs that have been identified and named

  26. The more STRs identified the more likely the DNA belongs to a specific person Multiplexing Technique that detects more than one DNA STR at the same time 13 STRs are used in CODIS

  27. The number of times the STR repeats in a person will determine the distance traveled in the gel of electrophoresis The more repeats the less it will travel The less repeats the more it will travel

  28. Identifying an individual Out of the 13 CODIS STRs: If the first 3 STRs match the probability is1/5000 If the first 6 STRs match the probability is1/ 2 million If the first 9 STRs match the probability is1/ 1 billion If all 13 STRs match the probability is1/ 575 trillion for Caucasian-Americans and 1/ 900 trillion for African-Americans

  29. Flat gel-coated electrophoresis plate can be used Capillary electrophoresis was developed which is faster STR sample is injected The different size STRs travel through the detector at different speeds

  30. The STRs with less repeats will pass through the detector sooner. The DNA peaks are recorded by the detector on an electropherogram

  31. Identifying Sex using STRs Identifying the amelogenin gene Males will show two bands during separation Females will show one band during separation Identifying Y-STRs Found in males on the Y chromosome Used in sex crimes where there are multiple males involved

  32. Significance of DNA Typing 25% of DNA examination conducted by FBI since 1989 have excluded suspects identified by the police DNA has been used to exonerate people wrongly convicted and imprisoned DNA tests conducted by the government have had their accuracy questioned

  33. Mitochondrial DNA - mtDNA Cells in the human body have 2 types of DNA Nuclear DNA DNA located in the nucleus of the cell DNA from both parents Continuous linear strand Mitochondrial DNA DNA located in the mitochondria of the cell DNA from the mother Circular or loop configuration Contains 37 genes needed for energy

  34. Using mitochondrial DNA for profilingis tedious mitochondrial DNA has two regions that are very common Hypervariable region I HV1 HV2 Hypervariable region II

  35. Step 1: You make many copies of the HV1 and HV2 regions Step 2: You determine the pattern of A-T-G-C bases = sequencing Step 3: Then you determine how unique the amount of times these regions occur This will narrow you suspects!

  36. CODIS – Combined DNA Index System a. Computer Software b. Developed by FBI c. Contains DNA profiles from; Local offenders State and national offenders Unsolved Crime scene evidence Missing persons Allows criminalists to compare evidence with many suspects

  37. Collection and Preservation of biological Evidence DNA evidence can come from: Common evidence like Blood or semen Also from licked stamps, chewing gum, drinking cup, sweatband, etc.

  38. Collection First Record the evidence Photographs Notes/sketches Second Collect with care Minimal personal contact Assume infectious: Wear gloves, face masks, shoe covers

  39. Collect clothing from both the victim and the suspect Look for blood in less than obvious places Examples: Towels used to blood off hands Hidden objects Cracks and crevices

  40. Packaging DO NOT put in plastic or airtight containers Moisture can accumulate and allow DNA-destroying bacteria to grow Substrate control must be collected - Unstained surface Store at cool temperatures or refrigerate

  41. If blood is found in soil it must be frozen immediately Microbes in the soil can degrade DNA

  42. Obtaining DNA reference specimens Must have something to compare evidence to Blood sample Swab sample from inner cheek -buccal cells Samples can also come from other means Razor Hair brush Cigarette butts

  43. Contamination of DNA evidence Contamination can occur from: Sneezing Coughing Cross contamination of evidence Steps to prevent contamination Wear gloves Change gloves when necessary Collect substrate controls Use clean forceps Use well ventilated containers

  44. Even when you can not see blood, it may be there. Luminol can detect traces of blood that has been washed without damaging DNA