Download DNA Evidence

DNA Evidence
You need to know
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What is DNA.
What is the purpose of DNA.
Where is DNA located in the cell.
What are some sources of DNA at a crime scene.
What is DNA?
What does DNA stand for?
Deoxyribonucleic Acid
What does DNA do?
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DNA contains genetic information.
DNA codes for the proteins our
bodies make that are necessary for
survival.
What is DNA?
DNA is a code for making proteins
AGC TAG CTT ATA CTC TAT CTC TTT
Amino Amino Amino Amino Amino Amino
Acid
Acid
Acid
Acid
Acid
Acid
The order of amino acids determines
what type of protein is made.
What is DNA?
Some common proteins are:
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Hemoglobin - carries oxygen from lungs to cells
Insulin - regulates metabolism
Many types of enzymes - catalyze reactions in the
body, such as the breakdown of sugar for energy
DNA also determines how much of these
proteins each cell makes.
What is DNA?
Among humans, most of the 3 billion bases in the DNA
sequence are exactly the same.
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Our Human DNA is 99.8% similar to each
other, but the 0.2% difference is more than
enough to distinguish us from one another.
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NO TWO PEOPLE HAVE IDENTICAL DNA*
*except identical twins
Where is DNA?
Where is DNA?
DNA is found in the cells in our body.
Nucleus
(Brain of the cell)
Mitochondria
(more later)
Where is DNA?
All types of cells in our body contain a copy of the same
DNA.
Some cells important to forensic science are:
White Blood Cell
Sperm Cell
Cheek Cell
Where is DNA?
What are sources of DNA at a crime scene?
DNA can be recovered from any
substance that contains cells.
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Blood
Semen
Saliva
Tissue
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Bone
Teeth
Hair
Maggot Crops
Maggot Crop
Forensic DNA Analysis
Forensic DNA Analysis
Collection of Evidence
Types of Unknown Samples:
• Blood, Semen, Stains, Saliva
• Hair, Tissue, Bones, Teeth
Types of Known Samples:
• Blood or buccal swabs from suspect
or victim or other known person
Forensic DNA Analysis
Packaging Evidence
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Package each item individually.
• Put evidence into paper bags, not plastic.
• Moisture degrades DNA; air dry samples.
• Keep samples at room temperature and out
of sun.
Forensic DNA Analysis
Beware of Contamination
Contamination occurs when DNA from another source gets
mixed in with the sample being collected.
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An investigator touches, sneezes, bleeds on a sample.
Wear gloves and use disposable instruments
Package items separately.
Especially, do not mix known samples (from victim or
suspect) with unknown samples.
Forensic DNA Analysis
Two main types of analyses (90s - Present) :
Short Tandem Repeats (STRs)
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Individual identification possible
Samples: Blood stains, semen
Mitochondrial DNA
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Used in cases of severely degraded DNA
• Individual identification not possible
• Samples: Bones, hair shafts
Forensic DNA Analysis
Basic Steps in Analysis
Extraction:
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Separates DNA from sample
Amplification or PCR:
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Amplifies small portions of DNA (STR regions)
Separation:
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Separates amplified fragments according to size.
The Thermal Cycler
Amplifies DNA
FMBio
Separates Amplified DNA
Forensic DNA Analysis
Color image of
gel
Forensic DNA (mitochondria)
Mitochondria - The powerhouse of the cell.
Mitochondria
Mitochondria have
their own DNA
Forensic DNA (mitochondria)
Nuclear DNA
vs. Mitochondrial DNA
Double Helix
46 Chromosomes
One copy per cell
Double Helix
One Ring
Multiple copies in
each mitochondria
Multiple mitochondria in
each cell
MtDNA used for old or degraded samples
DNA and Statistics
The final result is presented as a statistic.
Do not say:
“The DNA in the bloodstain is John Doe’s DNA.”
Do Say:
“The chance that another person has this DNA
in the bloodstain is 1 in 300 billion.”
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http://www.youtube.com/watch?v=dXYztbkMXwU
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DNA usage