DNA Evidence

You need to know 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?

What does DNA do?• DNA contains genetic information.• DNA codes for the proteins our

bodies make that are necessary for survival.

Deoxyribonucleic Acid

What is DNA?

DNA is a code for making proteins

AGC TAG CTT ATA CTC TAT CTC TTT

AminoAcid

AminoAcid

AminoAcid

AminoAcid

AminoAcid

AminoAcid

The order of amino acids determines what type of protein is made.

What is DNA?

Some common proteins are:

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.

• 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.

• 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?

• Blood• Semen• Saliva• Tissue

• Bone• Teeth• Hair• Maggot Crops

DNA can be recovered from any substance that contains cells.

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

• 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.

• 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

Short Tandem Repeats (STRs)• Individual identification possible• Samples: Blood stains, semen

Mitochondrial DNA• Used in cases of severely degraded DNA• Individual identification not possible• Samples: Bones, hair shafts

Two main types of analyses (90s - Present) :

Forensic DNA Analysis

Basic Steps in Analysis

Extraction:• Separates DNA from sample

Separation:• Separates amplified fragments according to size.

Amplification or PCR:• Amplifies small portions of DNA (STR regions)

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 Double Helix

One copy per cell Multiple copies in

each mitochondriaMultiple mitochondria in

each cell

One Ring 46 Chromosomes

MtDNA used for old or degraded samples

DNA and Statistics

The final result is presented as a statistic.

Do Say:“The chance that another person has this DNA

in the bloodstain is 1 in 300 billion.”

Do not say:“The DNA in the bloodstain is John Doe’s DNA.”

http://www.youtube.com/watch?v=dXYztbkMXwU

DNA usage