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Chapter 13

Chapter 13. DNA : The Indispensable Forensic Science Tool. Molecular Genetics’ Place in the History of Genetics. 1900s: Classical ( Mendelian ) Genetics 1940-1960s: Biological Revolution (Period of change in our understanding of genetics at the molecular level)

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Chapter 13

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  1. Chapter 13 • DNA : The Indispensable Forensic Science Tool

  2. Molecular Genetics’ Place in the History of Genetics • 1900s: Classical (Mendelian) Genetics • 1940-1960s: Biological Revolution (Period of change in our understanding of genetics at the molecular level) • 1990s: Advances in biotechnology and cloning projects

  3. Watson & Crick (1953) • Discovered the physical structure and chemical composition of DNA

  4. Nitrogenous Bases • Bases which contain nitrogen • Two Types of Nitrogenous Bases: • Pyrimidines • Purines • Will have to know structures of these bases

  5. Pyrimidines • Big Name…Small Structure • Includes Cytosine, Thymine, and Uracil • Uracil is found in RNA only!

  6. Pyrimidine Structures

  7. Purines • Small Name… Large Structure • Include Adenine and Guanine

  8. Purine Structures

  9. Deoxyribose • The sugar component of DNA • Deoxy– loses an OH- group on a Carbon atom

  10. What is the difference in sugars? • Ribonucleic acids (RNA) and deoxyribonucleic acids (DNA) contain 5-carbon sugars (ribose) • RNA contains a ribose molecule • DNA contains a deoxy-ribose molecule (2'-deoxy-ribose)

  11. Phosphoric Acid Charged oxygen atoms make the molecule acidic

  12. Sugar + Phosphate= Phosphodiester Bond

  13. Shorthand Notation • Sugar + Phosphoric Acid P

  14. Deoxynucleotide • Contains all 3 parts • Phosphoric acid • Deoxyribose • Nitrogenous Base (C,T,G,A)

  15. Shorthand Notation Phosphoric Acid Purine or Pyrimidine P Pu/Py Deoxyribose

  16. Video Questions on Worksheet Secrets of the Sequence

  17. Just to Review • Deoxyribonucleic Acid (DNA) • Chemical unit of heritable information • Capable of transformation • Unit structure is called a nucleotide • Base composition (C,T,G,A)

  18. The Double Helix

  19. Structure of DNA • Complimentary Base Pairing • C bonds with G • A bonds with T

  20. Edwin Chargaff (1949-53) • Discovered the base composition and chemistry of DNA

  21. Chargaff’s Observations • Rule 1: • The number of As = the number of Ts • The number of Cs = the number of Gs • **Illustrates the concept of base pairing**

  22. Chargaff’s Observations • Rule 2: • In double strand DNA, • The sum of purines (A+G) = The sum of pyrimidines (C+T)

  23. Chargaff’s Observations • Rule 3: • % of (G+C) WILL NOT EQUAL % of (A+T)

  24. Chargaff’s Observations • Rule 4: • G-C Content Increases Stability of Molecule : WHY?? • A-T (2 Hydrogen Bonds) • C-G (3 Hydrogen Bonds) • More stable in higher temperatures • Info was used by Watson & Crick to model the structure of DNA

  25. Chargaff’s Rules Example Problems • If a dsDNA molecule has 10% G, how much T does it have? • Use Rule 1: 10% G then 10% C because they must equal so 20% is C & G • Assuming 100%, subtract C & G amounts from 100. (100-20=80) • 80% is the amount of A & T together, so to get T, divide 80/2=40% The molecule has 40% Thymine!

  26. Chargaff’s Rules Example Problems • Given the following: Is the molecule ds or sstranded? • A=18% T=26% C=26% G=30% • Rule 2: A + G = C + T if dstranded • Add percentages: • 18 + 30 = 26 + 26 • 48 = 52 so the molecule is sstranded

  27. Chargaff’s Rules Example Problems • Which molecule is more stable? • A T A C T G A T G T C A T A T • C G A T C G A T C G A T C G A • Pair bases first, then look for C-G bonds Molecule 1 has 4 C-G bonds Molecule 2 has 8 C-G bonds Molecule 2 is more stable!

  28. More DNA Discoveries Rosalind Franklin (1950-1953) • Xray Diffraction of dsDNA Molecule • Physical evidence of what DNA looked like • Regular structure, specific width • X-shape suggested a helical structure

  29. In 1953, Watson & Crick asked… • How do chemical components come together to make a molecular form? • How does structure relate to the biological properties of DNA?

  30. Things they Knew… • In the structure lies the mechanism for… • 1.Replication • 2.Transfer of info to future generations • 3.Information Storage 4. Molecular basis of mutation

  31. What they did… • Watson & Crick used the chemical info from Chargaff and physical info from Franklin to find DNA was a helix. • Chemistry + Physical = Helix

  32. How It Works… Phosphodiester Bonds Link the DNA Backbone

  33. How the strands make a helix…

  34. The Story of Sam Sheppard

  35. DNA at Work • DNA is the fundamental unit of heredity. • DNA codes for proteins which control all aspects of physical appearance, from eye color to height. • Changes in or absences of these proteins result in genetic disorders such as hemophilia, muscular dystrophy, and Huntington’s disease.

  36. DNA Double Helix and Replication • The nature of the Helix makes it easy to replicate the DNA • When the Helix unwinds, both strands can be copied at the same time to make 2 brand new, identical DNA Helices with the help of certain enzymes and proteins • Very similar to making a carbon copy

  37. DNA Typing • Process of distinguishing one individual from another • Using DNA, which is specific to every individual, Forensic Scientists are able to match suspects based on portions of DNA found at a crime scene

  38. RFLPs - Restriction fragment length polymorphisms • Segments of DNA that are used in DNA typing • A large portion of DNA in the human genome seems to act as filler DNA and does not code for any proteins. • These gaps in coding DNA are called tandem repeats - sequences of letters that are repeated several times. • The letters involved and the lengths of the tandem repeats are specific to every individual

  39. Polymerase Chain Reaction (PCR) • The main enzyme involved in DNA replication is called DNA Polymerase. • Scientists can use DNA Polymerase to replicate and amplify small pieces of DNA found at a crime scene into workable samples. • The DNA is then used in several different lab tests to link a suspect to a crime or crime scene.

  40. PCR used for DNA Typing • PCR typing replaced RFLP DNA typing as the dominant method in the mid-1990s. • PCR uses a much smaller sample size which means that it can characterize DNA extracted from small amounts of blood, semen, and saliva. • Ex: Envelope seals, cigarette butts, soda cans, stains on clothes and bedding, etc.

  41. Mitochondrial DNA • Found inside the Mitochondria of the cell and is inherited solely from the mother. • Can be used when nuclear DNA is not available due to charred remains, small quantities like hair shafts, etc. • However, mtDNA analysis is more rigorous, time consuming, and costly when compared to nuclear DNA profiling.

  42. Mitochondrial DNA cont’d • There are hundreds of thousands of copies of mtDNA in each cell • mtDNA is in a loop instead of a strand or double helix • Reference samples for lab testing can be obtained from any maternal relative

  43. DNA and the FBI Database

  44. CODIS - Combined DNA Index System • A computer software program that stores local, state, and national databases of DNA from convicted offenders, unsolved crime scene evidence, and profiles of missing persons. • Allows investigators to compare new evidence to preexisting cases and/or convicted offenders and possibly solve the crime.

  45. Collection and Preservation • In order to properly collect and preserve DNA evidence, investigators must be sent to a crime scene immediately. • Care must be taken so that the investigator does not make personal contact with the evidence; latex gloves, shoe covers, and face masks must be used.

  46. Collection and Preservation cont’d • All clothing from the victim and suspect(s) needs to be sent to a lab to test for saliva, semen, and blood samples, along with other fabrics in and around the crime scene. • Each stained article should be packaged separately in a paper bag or in a well ventilated box.

  47. Interesting cases with DNA evidence • Bill Clinton impeachment trial • DNA taken from Monica Lewinsky’s dress. • Anna Nichole Smith’s child • DNA tests to determine paternity • Phillip Spector – movie producer • DNA found on breast of breast of victim • Yale Student disappearance • Bloody clothes found in tiles of the wall matched the suspect

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