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    DNA Profiling(DNA fingerprinting)

    A method of Forensicidentification

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    What is DNA Profiling?

    A technique used by scientists todistinguish between individuals ofthe same species using only samples

    of their DNA

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    Who Invented it?

    The process ofDNA fingerprintingwas invented by

    Alec Jeffreys at theUniversity ofLeicester in 1985.

    He was knighted in1994.

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    How do you figure out that

    someones DNA is more similar to

    anothers?

    The primary method of assessingsimilarities is by use of DNAfingerprinting or DNA restriction

    analysis.

    This process makes use of specialproteins called restriction enzymes

    and sections of the chromosomecalled tandem repeats

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    Tandem Repeats

    A region of the chromosome thatcontains multiple copies of a coreDNA sequence that are arranged in a

    repeating fashion

    Repeats act as fillers or spacersbetween coded sections of DNA

    All humans have the same type ofrepeats but there is tremendousvariation in the number of repeats

    that each of us has.

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    Stages of DNA Profiling

    Stage 1:

    Cells are broken down

    to release DNA

    If only a small amountof DNA is available itcan be amplified using

    the polymerase chainreaction (PCR)

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    Stages of DNA Profiling

    Step 2:The DNA is cut into fragments using restriction

    enzymes.

    Each restriction enzyme cuts DNA at a specific basesequence.

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    SSR= Simple sequence Repeatsor

    SSLP= Simple sequence length

    polymorphism

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    d

    D

    Probe binds repetitive

    sequences

    The number of repeated units in a tandem array is variable. Individuals

    heterozygous for different numbers of tandem repeats can be detected,

    and the heterozygous site (s) used as a marker (s) for mapping.

    This VNTR locus will form two bands on a Southern blot: one long and

    one short. Similar to an RFLP locus, this heterozygous site can be used

    for genetic mapping.

    At present, VNTR analysis is rapidly performed using PCR.

    Restriction target sites are outside the repetitive array.

    The basic unit of the array is indicated by the arrows.

    Simple-Sequence Length Polymorphisms (SSLPs)

    VNTRs :Variation in the

    Number of

    Tandem Repeats

    or Mini-satellite

    Molecular Markers

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    Restriction Fragment Length

    Polymorphisms (RFLP)

    RFLPs are different fragment lengthsof base pairs that result from cuttinga DNA molecule with a restriction

    enzyme

    It is the length differences associatedwith DNA strands or RFLPs that allow

    one to distinguish one person fromanother.

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    Stages of DNA Profiling

    The sections of DNA that are cut outare called restriction fragments.

    This yields thousands of restrictionfragments of all different sizesbecause the base sequences beingcut may be far apart (long fragment)or close together (short fragment).

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    Stages of DNA Profiling

    Stage 3:

    Fragments areseparated on the

    basis of size usinga process called gelelectrophoresis.

    DNA fragments are

    injected into wellsand an electriccurrent is appliedalong the gel.

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    Stages of DNA Profiling

    DNA is negativelycharged so it isattracted to thepositive end of thegel.

    The shorter DNAfragments movefaster than thelonger fragments.

    DNA is separated onbasis of size.

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    Stages of DNA Profiling

    A radioactivematerial is addedwhich combines

    with the DNAfragments toproduce afluorescent image.

    A photographiccopy of the DNAbands is obtained.

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    Stages of DNA Profiling

    Stage 4:

    The pattern of fragment distributionis then analysed.

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    Uses of DNA Profiling

    DNA profiling isused to solvecrimes and

    medicalproblems

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    Crime

    Forensic science is the use ofscientific knowledge in legalsituations.

    The DNA profile of each individual ishighly specific.

    The chances of two people having

    exactly the same DNA profile is30,000 million to 1 (except foridentical twins).

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    Biological materials used for DNA

    profiling Blood

    Hair

    Saliva

    Semen

    Body tissue cells

    DNA samples havebeen obtained fromvaginal cells

    transferred to theoutside of a condomduring sexualintercourse.

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    DNA Profiling can solve crimes

    The pattern of the DNA profile is thencompared with those of the victim and thesuspect.

    If the profile matches the suspect itprovides strong evidence that the suspectwas present at the crime scene (NB:itdoes not prove they committed thecrime).

    If the profile doesnt match the suspectthen that suspect may be eliminated fromthe enquiry.

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    Example

    A violent murder occurred.

    The forensics team retrieved a bloodsample from the crime scene.

    They prepared DNA profiles of theblood sample, the victim and asuspect as follows:

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    Was the suspect at the crime

    scene?

    SuspectsProfile

    Blood samplefrom crimescene

    Victimsprofile

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    Solving Medical Problems

    DNA profiles can be used to determinewhether a particular person is the parentof a child.

    A childs paternity (father) andmaternity(mother) can be determined.

    This information can be used in

    Paternity suits Inheritance cases

    Immigration cases

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    Example: A Paternity Test

    By comparing the DNA profile of a

    mother and her child it is possible to

    identify DNA fragments in the childwhich are absent from the mother and

    must therefore have been inherited

    from the biological father.

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    Is this man the father of the child?

    Mother Child Man

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    An Example Using EcoR I for a

    Question of Paternity

    EcoR I cuts asimilar section of

    DNA on Bob,Larry, and Mary

    After the cut how

    many fragmentsBob, Larry, andMary have?

    Answer: 2, 3, and

    none

    Recall: EcoR I cuts only at GAATTC

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    Example Cont

    After the DNA is cut with EcoR I, Bobs,Larrys and Marys fragments are placed indifferent lanes on an agarose gel

    The fragments are then subjected to anelectric field

    The smaller fragments move faster, thelarger ones move slower

    This process of separating the fragmentsby length is called electrophoresis.

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    Resulting Picture after

    Electrophoresis

    The biggerfragments arenear the top

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    Paternity Test

    In general the

    childs DNA mustbe a combinationof Marys DNA and

    one of the men.Which man is thefather?

    Answer: Larry

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    Famous cases

    In 2002 ElizabethHurley used DNAprofiling to prove

    that Steve Bingwas the father

    of her child Damien

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    Famous Cases

    Colin Pitchfork wasthe first criminalcaught based on

    DNA fingerprintingevidence.

    He was arrested in1986 for the rape

    and murder of twogirls and wassentenced in 1988.

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    Famous Cases

    O.J. Simpson wascleared of a doublemurder charge in1994 which relied

    heavily on DNAevidence.

    This casehighlighted labdifficulties.

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