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Chapter 13 DNA : The Indispensable Forensic Science ToolDNA : The Indispensable Forensic Science Tool.

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Presentation on theme: "Chapter 13 DNA : The Indispensable Forensic Science ToolDNA : The Indispensable Forensic Science Tool."— Presentation transcript:

1 Chapter 13 DNA : The Indispensable Forensic Science ToolDNA : The Indispensable Forensic Science Tool

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3 Molecular Genetics’ Place in the History of Genetics 1900s: Classical (Mendelian) Genetics1900s: Classical (Mendelian) Genetics s: Biological Revolution (Period of change in our understanding of genetics at the molecular level) s: Biological Revolution (Period of change in our understanding of genetics at the molecular level) 1990s: Advances in biotechnology and cloning projects1990s: Advances in biotechnology and cloning projects

4 Watson & Crick (1953) Discovered the physical structure and chemical composition of DNADiscovered the physical structure and chemical composition of DNA

5 Nitrogenous Bases Bases which contain nitrogenBases which contain nitrogen Two Types of Nitrogenous Bases:Two Types of Nitrogenous Bases: PyrimidinesPyrimidines PurinesPurines Will have to know structures of these basesWill have to know structures of these bases

6 Pyrimidines Big Name…Small StructureBig Name…Small Structure Includes Cytosine, Thymine, and UracilIncludes Cytosine, Thymine, and Uracil Uracil is found in RNA only!Uracil is found in RNA only!

7 Pyrimidine Structures

8 Purines Small Name… Large StructureSmall Name… Large Structure Include Adenine and GuanineInclude Adenine and Guanine

9 Purine Structures

10 Deoxyribose The sugar component of DNAThe sugar component of DNA Deoxy– loses an OH- group on a Carbon atomDeoxy– loses an OH- group on a Carbon atom

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

12 Phosphoric Acid Charged oxygen atoms make the molecule acidic

13 Sugar + Phosphate= Phosphodiester Bond

14 Shorthand Notation Sugar + Phosphoric AcidSugar + Phosphoric Acid P

15 Deoxynucleotide Contains all 3 partsContains all 3 parts Phosphoric acidPhosphoric acid DeoxyriboseDeoxyribose Nitrogenous Base (C,T,G,A)Nitrogenous Base (C,T,G,A)

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

17 Video Questions on Worksheet Secrets of the Sequence

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

19 The Double Helix The Double Helix

20 Structure of DNA Complimentary Base Pairing C bonds with G A bonds with T

21 Edwin Chargaff ( ) Discovered the base composition and chemistry of DNADiscovered the base composition and chemistry of DNA

22 Chargaff’s Observations Rule 1:Rule 1: The number of As = the number of TsThe number of As = the number of Ts The number of Cs = the number of GsThe number of Cs = the number of Gs **Illustrates the concept of base pairing****Illustrates the concept of base pairing**

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

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

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

26 Chargaff’s Rules Example Problems If a dsDNA molecule has 10% G, how much T does it have?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 & GUse 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)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%80% is the amount of A & T together, so to get T, divide 80/2=40% The molecule has 40% Thymine!

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

28 Chargaff’s Rules Example Problems Which molecule is more stable?Which molecule is more stable? A T A C T G A T G T C A T A TA 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 AC G A T C G A T C G A T C G A Pair bases first, then look for C-G bonds 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!

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

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

31 Things they Knew… In the structure lies the mechanism for…In the structure lies the mechanism for… 1. Replication1. Replication 2. Transfer of info to future generations2. Transfer of info to future generations 3. Information Storage3. Information Storage 4. Molecular basis of mutation 4. Molecular basis of mutation

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

33 How It Works… Phosphodiester Bonds Link the DNA Backbone

34 How the strands make a helix…

35 The Story of Sam Sheppard The Story of Sam Sheppard

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

37 DNA Double Helix and Replication The nature of the Helix makes it easy to replicate the DNAThe 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 proteinsWhen 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 copyVery similar to making a carbon copy

38 DNA Typing Process of distinguishing one individual from anotherProcess 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 sceneUsing DNA, which is specific to every individual, Forensic Scientists are able to match suspects based on portions of DNA found at a crime scene

39 RFLPs - Restriction fragment length polymorphisms Segments of DNA that are used in DNA typingSegments 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.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.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 individualThe letters involved and the lengths of the tandem repeats are specific to every individual

40 Polymerase Chain Reaction (PCR) The main enzyme involved in DNA replication is called DNA Polymerase.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.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.The DNA is then used in several different lab tests to link a suspect to a crime or crime scene.

41 PCR used for DNA Typing PCR typing replaced RFLP DNA typing as the dominant method in the mid-1990s.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.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.Ex: Envelope seals, cigarette butts, soda cans, stains on clothes and bedding, etc.

42 Mitochondrial DNA Found inside the Mitochondria of the cell and is inherited solely from the mother.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.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.However, mtDNA analysis is more rigorous, time consuming, and costly when compared to nuclear DNA profiling.

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

44 DNA and the FBI Database DNA and the FBI Database

45 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.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.Allows investigators to compare new evidence to preexisting cases and/or convicted offenders and possibly solve the crime.

46 Collection and Preservation In order to properly collect and preserve DNA evidence, investigators must be sent to a crime scene immediately.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.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.

47 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.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.Each stained article should be packaged separately in a paper bag or in a well ventilated box.

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


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