1 Forensic Science Chapter 13 DNA: The Indispensable Forensic Science Tool
Jan 13, 2016
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Forensic ScienceChapter 13
DNA: The Indispensable Forensic Science Tool
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DNA
• Deoxyribonucleic acid• A polymer• In 1985 it was discovered that certain genes
are as unique as fingerprints• DNA markers
• Discovered in 1868• In the 1950s, James Watson and Francis
Crick figured out the structure of DNA
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DNA base pairing
• There are four DNA bases• Adenine, Cytosine, Guanine, and Thymine
• They link together to form DNA, with pairs spanning the double-helix
• A and T always pair• C and G always pair• The sequence is different for each
individual
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Proteins
• Play basic roles in the structures and functions of living things
• Made of amino acids• There are 20• They link to form proteins• Different sequences make different proteins
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Abnormalities
• A difference of one amino acid in a protein can cause big differences
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Human Genome
• Total DNA content
• About 3 billion base pairs
• Has been mapped
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DNA replication
• Making new DNA from existing DNA
• Two strands unwind
• Each recreates the double helix
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PCR
• Polymerase chain reaction
• Used to copy and fill in small pieces of DNA from crime scenes
• Sample size is no longer a limit
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Recombinant DNA• Cutting DNA into fragments that can later
be incorporated into another DNA strand• Uses restriction enzymes
• Chemicals that act as scissors to cut DNA at specific locations
• Used for genetic engineering• Making fish grow faster• Growing insulin through bacteria• Making plants grow faster and/or produce
higher yields
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Tandem Repeats
• A region of a chromosome that contains a repeating DNA sequence
• About 30% of human genome
• Origin and significance not known• Possibly fillers
• Helpful in DNA typing• Distinguish individuals
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RFLP
• Restriction Fragment Length Polymorphisms
• Repeat segments that are cut from DNA molecule with restriction enzymes
• Used for DNA typing
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Hybridization
• Joining two complementary strands of DNA to form a double-stranded molecule
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DNA technology over time
• 1985 to mid 1990s• RFLP (Restriction Fragment Length Polymorphisms)• RFLP Portions of DNA are repeated numerous times
(over 30%)• Act as fillers or spacers
• Restriction enzymes are used to cut the fillers• Everyone has different numbers and order of these
fillers• Mid 1990s on
• PCR (Polymerase Chain Reaction)• Copies or multiplies DNA strands
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Short Tandem Repeat (STR)
• Latest method of DNA typing
• Locations on DNA that contain short segments of 3 – 7 repeating base pairs
• Abundant in human genome
• Easier to multiply using PCR
• 13 STRs used by CODIS
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Multiplexing
• Simultaneously detecting more than one DNA marker in a single analysis
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Amelogenin Gene
• Used to determine gender
• Not one of the 13 CODIS STRS, but can also be useful
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Y-STRs
• STRs on the Y chromosome
• Useful for separating DNA when multiple males are involved in a sexual assault
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Sequencing
• A procedure used to determine the order of the base pairs that make up DNA
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Mitochondria
• Small structures in the cell
• Supply energy to the cell
• Contain maternal DNA• Mitochondrial DNA or mtDNA• More difficult, time-consuming, and costly to
analyze than nuclear DNA
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mtDNA
• Circular or loop configuration
• Contains 37 genes
• Found in hair that lacks nuclear DNA
• Found in skeletons
• Reference samples can be taken from maternal relatives
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mtDNA
• First used in State of Tennessee v. Paul Ware in 1996
• Used to identify the individual buried in the tomb of the Vietnam War’s unknown soldier
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CODIS
• Combined DNA Index System• Local, Regional, and National DNA
databases• DNA is collected from convicted offenders
of particular crimes (especially sexual assault)
• Also contains DNA from unknown suspects and missing persons
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DNA evidence
• Only about 36 cells are needed to get an STR profile• Low copy number – fewer than 36 cells
• Epithelial cells• Outer layer of skin cells• Easily rubbed off• Can yield DNA
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Collecting evidence
• Must be photographed first• Don’t disturb blood pattern until after it has
been analyzed for shape and location• Minimal personal contact
• Assume all fluids are infectious
• Each stained article should be packaged separately in a paper bag or a well-ventilated box
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Substrate control
• Unstained material from near the bloodstain• Confirm test results
• Swabs must be dried before being packaged• Takes 5 – 10 minutes
• Biologicals should be stored in a cool place, refrigerated, or frozen until testing
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Collecting reference samples
• Blood samples
• Buccal swabs• Inner cheek lining
• Hairbrushes, toothbrushes, etc.
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Contamination
• Coughing or sneezing on sample• Incorrect packaging• Avoiding
• Change gloves for each piece of evidence• Substrate controls• Use disposable forceps (tweezers) to pick up
evidence• Separate containers