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DNA

Honors Forensic Science. DNA. I. DNA is identical in every cell in body. A. DNA can be left at a crime scene even if there is no blood B. DNA can survive longer than a fingerprint. Has even been performed on Egyptian mummies. c. Can indicate familial relationships.

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DNA

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  1. Honors Forensic Science DNA

  2. I. DNA is identical in every cell in body • A. DNA can be left at a crime scene even if there is no blood • B. DNA can survive longer than a fingerprint. Has even been performed on Egyptian mummies

  3. c. Can indicate familial relationships • i. Example – a Philippine case involved a murder conducted by two individuals. One was identified by an eyewitness, but the second one was not. DNA from spit at the scene helped to ID the second man. He was a brother to the first.

  4. D. DNA evidence does not combine like blood evidence does.

  5. III. What is DNA? • A. Gene = fundamental unit of heredity • i. They instruct the body cells to make protein that determine everything from hair color to our susceptibility to disease • Ii. Each gene is composed of DNA specifically designed to carry out a single body function • Iii. Each gene has its own location

  6. B. DNA is arranged in a double helix and on long strands called chromosomes • C. Humans have 46 chromosomes in 23 pairs

  7. D. Structure deduced by Watson and Crick • E. Is a polymer; smaller units are called monomers • F. Units are called nucleotides

  8. i. Composed of sugar – deoxyribose • Ii. Phosphate group • Iii. Nitrogen bases • 1. adenine • 2. cytosine • 3. thymine • 4. guanine

  9. 5. adenine always pairs with thymine • 6. cytosine always pairs with guanine

  10. G. Sugar and phosphates bond together to form a backbone • H. Bases form rungs of ladder • I. Double helix

  11. III. DNA at work • A. Each of us has approximately 3 billion base pairs in our DNA • B. Possible base sequences for any given DNA strand are astronomical • C. DNA directs the production of proteins

  12. D. Egg and Sperm – each have 23 paired chromosomes • I. When egg and sperm join there are more than 8 trillion possible combinations

  13. E. All of our cells have DNA • i. EXCEPT red blood cells – have no nuclei; white blood cells have nuclei so DNA can be extracted from blood • ii. Hair follicles have cells • Iii. Saliva pick up cells from mouth

  14. F. DNA directs the production of proteins

  15. i. Proteins are made of amino acids • Ii. There are 20 common amino acids • Iii. Each amino acid is coded for by 3 bases • Iv. This code is not restricted to humans

  16. G. Only about 5% of DNA directly carry out work of life = encoded • i. Remaining 95% of DNA regulates how genes function (non-encoded)

  17. IV. Human genome • A. Scientists are working to unravel the human genome • B. Hope it will lead to cures for genetic diseases • C. Better understanding of role and implications of evolution

  18. V. Forensic Investigators look for 2 types of DNA sequences • a. VNTRs – Variable Number Tandem Repeats • i. Same base sequence repeated throughout a specific locus within the strand

  19. B. STRs – Short Tandem Repeats • i. Usually 3-7 base pairs long that repeat throughout portions of DNA chains • Ii. STRs repeat over segments of DNA strand as long as 400 bases so good for scientists can use even severely degraded samples

  20. Iii. In working to match DNA investigators look at repeats on particular loci of DNA • 1. Ex. May find 8 repeated of a particular STR from one parent and 14 repeats from another. Another individual may be 15 repeats of the same STR from one parent and 23 from another

  21. 2. OR Locus 1 12 9 • 3. 6 14 • i. 23 16 • Ii. 5 18 • Iii. 8 19 • 4. The probability of having someone else match this exact sequence of STR repeats is 48 out of 10 billion. This probability will decrease with more loci checked.

  22. VII. Replication of DNA • A. Each strand in the double helix has the same information • B. DNA replication begins with the unwinding of the DNA strands in the double helix • C. Free nucleotides bind with single strand templates to form new double strand

  23. VII. Polymerase Chain Reaction • A. A technique for replicating or copying a portion of a DNA strand outside a living cell • B. This technique leads to millions of copies of the DNA strand

  24. C. Means that more testing can be done on the original DNA because not limited by sample size

  25. VIII. Recombinant DNA • A. Relationship between the base letters on a DNA strand and the type of protein specified by the sequence of these letters is called the genetic code • B. Understanding what is produced by different sequence of bases has given rise to recombinant DNA technology

  26. C. Relies on ability of certain chemicals (restriction enzymes) to cut DNA into fragments • D. Highly specialized “scissors” • E. Once DNA is cut other DNA can be inserted (usually bacterium)

  27. F. Altered DNA is then passed on to descendants • G. Has enormous commercial potential • H. allows us to manipulate DNA • i. Possible treatments for disease • J. Plant genetic engineering

  28. IX. DNA Typing • A. 1985 – Dr. Alec Jeffreysand associates discovered that each persons DNA is actually unique • i. They found certain areas exhibit polymorphism (different forms)

  29. Ii. Analyzing these areas allows scientists to make distinctions between one individual and the next • Iii. He also discovered a process for isolating and analyzing these areas = DNA Fingerprinting and Typing

  30. B. Restriction Fragment Length Polymorphisms (RFLP) • i. Portions of DNA has sequences of letters repeated numerous times • Ii. These “tandem repeats” offer a means to distinguish one individual from another

  31. Iii. Within the world’s population there are numerous possibilities for the number of times a particular sequence of base letters can repeat themselves • Iv. The number of possibilities increases when consider two chromosomes

  32. V. Restriction enzymes may be used to cut chromosomes into hundreds of fragments, some containing repeating sequences • Vi. Length differences that result from this process are Restriction Fragment Length Polymorphisms • Vii. Are several thousands of bases long

  33. Viii. Once DNA is cut, it is sorted using electrophoresis • 1. fragments migrate across gel plate different distances depending on their length • 2. shorter fragments go farther, longer ones go less far

  34. Ix. Once electrophoresis is complete, fragments are transferred to nylon sheet and treated with radioactively labeled probes containing complementary base sequences (this process is called hybridization) • X. Next nylon sheet is placed against x-ray film and exposed for several days

  35. Xi. This test by itself is not enough to individualize DNA • Xii. Can use other probes to search for different repeating segments of DNA to get higher degree of differentiation or even individualization

  36. Xiii. First technique accepted to characterize DNA • Xiv. New technology making this process more or less obsolete

  37. c. Polymerase Chain Reaction (PCR) • i. More viable method • Ii. Increased sensitivity • Iii. Can yield information from degraded samples • Iv. First, heat DNA to about degrees C, strands begin to separate

  38. V. Second, add primers to separated strands and lower temp, so will hybridize • Vi. Primers = short DNA segments in pure form • Viii. Third, add DNA polymerase and free nucleotides, rebuilds double strands

  39. Viii. Each cycle doubles the amount of DNA present • Ix. Usually go through 25-30 cycles • X. First validated PCR-based genetic marker system available for forensic science was DQA1 • 1. DQA1 gene has lots of variants

  40. Xi. DNA is extracted from sample • Xii. Primer, DNA polymerase and nucleotides added • Xiii. Amplification process • Xiv. DNA is added to select areas of nylon strip • 1. nylon strips pre-affixed with probes • 2. blue dots will appear if DNA type is present

  41. Xv. Generally, not a discriminating technique • Xvi. Polymarker (PM) = frequency of occurrence in range of 1/5000 • Xvii. Advantage of PCR = can use very small amounts of DNA (ex. saliva residues on stamps, cigarette butts)

  42. d. Short Tandem Repeats • i. Latest method • Ii. Most successful and widely used • Iii. Used to identify bodies of victims of TWA Flight 800, Branch Davidian compound and September 11, 2001 • Iv. Higher discrimination than RFLP • V. Can use small sample size

  43. Vi. STRs are locations on chromosome that contain in short sequence elements that repeat themselves within DNA molecule • Vii. Are useful because are found in abundance in human genome • Viii. STR is extracted and amplified • Ix. Are separated on electrophoretic gel

  44. X. By examining the distance the STR migrated on the gel, one can determine the number of repeats that exist in the STR • Xi. Are hundreds of types of STRs • Xii. The more you can characterize the smaller the % of the population from which they can emanate

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