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

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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